Introduction
Hash tables are extremely popular
computer data structures and can be found under one form or another in virtually any programming
language. Whereas other associative structures such as rbtrees (used in C++ by std::set
and std::map
)
have logarithmictime complexity for insertion and lookup, hash tables, if configured properly,
perform these operations in constant time on average, and are generally much faster.
C++ introduced unordered associative containers std::unordered_set
, std::unordered_map
,
std::unordered_multiset
and std::unordered_multimap
in C++11, but research on hash tables
hasn’t stopped since: advances in CPU architectures such as
more powerful caches, SIMD operations
and increasingly available multicore processors
open up possibilities for improved hashbased data structures and new use cases that
are simply beyond reach of unordered associative containers as specified in 2011.
Boost.Unordered offers a catalog of hash containers with different standards compliance levels, performances and intented usage scenarios:
Nodebased 
Flat 


Closed addressing 


Open addressing 


Concurrent 


Closedaddressing containers are fully compliant with the C++ specification for unordered associative containers and feature one of the fastest implementations in the market within the technical constraints imposed by the required standard interface.

Openaddressing containers rely on much faster data structures and algorithms (more than 2 times faster in typical scenarios) while slightly diverging from the standard interface to accommodate the implementation. There are two variants: flat (the fastest) and nodebased, which provide pointer stability under rehashing at the expense of being slower.

Finally, concurrent containers are designed and implemented to be used in highperformance multithreaded scenarios. Their interface is radically different from that of regular C++ containers.
All sets and maps in Boost.Unordered are instantiatied similarly as
std::unordered_set
and std::unordered_map
, respectively:
namespace boost {
template <
class Key,
class Hash = boost::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<Key> >
class unordered_set;
// same for unordered_multiset, unordered_flat_set, unordered_node_set
// and concurrent_flat_set
template <
class Key, class Mapped,
class Hash = boost::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<std::pair<Key const, Mapped> > >
class unordered_map;
// same for unordered_multimap, unordered_flat_map, unordered_node_map
// and concurrent_flat_map
}
Storing an object in an unordered associative container requires both a key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses boost::hash it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the Equality Predicates and Hash Functions section for more details.
Basics of Hash Tables
The containers are made up of a number of buckets, each of which can contain
any number of elements. For example, the following diagram shows a boost::unordered_set
with 7 buckets containing 5 elements, A
,
B
, C
, D
and E
(this is just for illustration, containers will typically
have more buckets).
In order to decide which bucket to place an element in, the container applies
the hash function, Hash
, to the element’s key (for sets the key is the whole element, but is referred to as the key
so that the same terminology can be used for sets and maps). This returns a
value of type std::size_t
. std::size_t
has a much greater range of values
then the number of buckets, so the container applies another transformation to
that value to choose a bucket to place the element in.
Retrieving the elements for a given key is simple. The same process is applied
to the key to find the correct bucket. Then the key is compared with the
elements in the bucket to find any elements that match (using the equality
predicate Pred
). If the hash function has worked well the elements will be
evenly distributed amongst the buckets so only a small number of elements will
need to be examined.
You can see in the diagram that A
& D
have been placed in the same bucket.
When looking for elements in this bucket up to 2 comparisons are made, making
the search slower. This is known as a collision. To keep things fast we try to
keep collisions to a minimum.
If instead of boost::unordered_set
we had used boost::unordered_flat_set
, the
diagram would look as follows:
In openaddressing containers, buckets can hold at most one element; if a collision happens
(like is the case of D
in the example), the element uses some other available bucket in
the vicinity of the original position. Given this simpler scenario, Boost.Unordered
openaddressing containers offer a very limited API for accessing buckets.
All containers 


Method 
Description 

The number of buckets. 
Closedaddressing containers only 

Method 
Description 

An upper bound on the number of buckets. 

The number of elements in bucket 

Returns the index of the bucket which would contain 

Return begin and end iterators for bucket 









Controlling the Number of Buckets
As more elements are added to an unordered associative container, the number
of collisions will increase causing performance to degrade.
To combat this the containers increase the bucket count as elements are inserted.
You can also tell the container to change the bucket count (if required) by
calling rehash
.
The standard leaves a lot of freedom to the implementer to decide how the number of buckets is chosen, but it does make some requirements based on the container’s load factor, the number of elements divided by the number of buckets. Containers also have a maximum load factor which they should try to keep the load factor below.
You can’t control the bucket count directly but there are two ways to influence it:

Specify the minimum number of buckets when constructing a container or when calling
rehash
. 
Suggest a maximum load factor by calling
max_load_factor
.
max_load_factor
doesn’t let you set the maximum load factor yourself, it just
lets you give a hint. And even then, the standard doesn’t actually
require the container to pay much attention to this value. The only time the
load factor is required to be less than the maximum is following a call to
rehash
. But most implementations will try to keep the number of elements
below the max load factor, and set the maximum load factor to be the same as
or close to the hint  unless your hint is unreasonably small or large.
All containers 


Method 
Description 

Construct an empty container with at least 

Construct an empty container with at least 

The average number of elements per bucket. 

Returns the current maximum load factor. 

Changes the container’s maximum load factor, using 

Changes the number of buckets so that there at least 
Openaddressing and concurrent containers only 

Method 
Description 

Returns the maximum number of allowed elements in the container before rehash. 
A note on max_load
for openaddressing and concurrent containers: the maximum load will be
(max_load_factor() * bucket_count()
) right after rehash
or on container creation, but may
slightly decrease when erasing elements in highload situations. For instance, if we
have a boost::unordered_flat_map
with size()
almost
at max_load()
level and then erase 1,000 elements, max_load()
may decrease by around a
few dozen elements. This is done internally by Boost.Unordered in order
to keep its performance stable, and must be taken into account when planning for rehashfree insertions.
Equality Predicates and Hash Functions
While the associative containers use an ordering relation to specify how the elements are stored, the unordered associative containers use an equality predicate and a hash function. For example, boost::unordered_map is declared as:
template <
class Key, class Mapped,
class Hash = boost::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<std::pair<Key const, Mapped> > >
class unordered_map;
The hash function comes first as you might want to change the hash function but not the equality predicate. For example, if you wanted to use the FNV1a hash you could write:
boost::unordered_map<std::string, int, hash::fnv_1a>
dictionary;
There is an implementation of FNV1a in the examples directory.
If you wish to use a different equality function, you will also need to use a matching hash function. For example, to implement a case insensitive dictionary you need to define a case insensitive equality predicate and hash function:
struct iequal_to
{
bool operator()(std::string const& x,
std::string const& y) const
{
return boost::algorithm::iequals(x, y, std::locale());
}
};
struct ihash
{
std::size_t operator()(std::string const& x) const
{
std::size_t seed = 0;
std::locale locale;
for(std::string::const_iterator it = x.begin();
it != x.end(); ++it)
{
boost::hash_combine(seed, std::toupper(*it, locale));
}
return seed;
}
};
Which you can then use in a case insensitive dictionary:
boost::unordered_map<std::string, int, ihash, iequal_to>
idictionary;
This is a simplified version of the example at /libs/unordered/examples/case_insensitive.hpp which supports other locales and string types.
Caution

Be careful when using the equality (== ) operator with custom equality
predicates, especially if you’re using a function pointer. If you compare two
containers with different equality predicates then the result is undefined.
For most stateless function objects this is impossible  since you can only
compare objects with the same equality predicate you know the equality
predicates must be equal. But if you’re using function pointers or a stateful
equality predicate (e.g. boost::function ) then you can get into trouble.

Custom Types
Similarly, a custom hash function can be used for custom types:
struct point {
int x;
int y;
};
bool operator==(point const& p1, point const& p2)
{
return p1.x == p2.x && p1.y == p2.y;
}
struct point_hash
{
std::size_t operator()(point const& p) const
{
std::size_t seed = 0;
boost::hash_combine(seed, p.x);
boost::hash_combine(seed, p.y);
return seed;
}
};
boost::unordered_multiset<point, point_hash> points;
Since the default hash function is Boost.Hash, we can extend it to support the type so that the hash function doesn’t need to be explicitly given:
struct point {
int x;
int y;
};
bool operator==(point const& p1, point const& p2)
{
return p1.x == p2.x && p1.y == p2.y;
}
std::size_t hash_value(point const& p) {
std::size_t seed = 0;
boost::hash_combine(seed, p.x);
boost::hash_combine(seed, p.y);
return seed;
}
// Now the default function objects work.
boost::unordered_multiset<point> points;
See the Boost.Hash documentation for more detail on how to do this. Remember that it relies on extensions to the standard  so it won’t work for other implementations of the unordered associative containers, you’ll need to explicitly use Boost.Hash.
Method  Description 


Returns the container’s hash function. 

Returns the container’s key equality function.. 
Regular Containers
Boost.Unordered closedaddressing containers (boost::unordered_set
, boost::unordered_map
,
boost::unordered_multiset
and boost::unordered_multimap
) are fully conformant with the
C++ specification for unordered associative containers, so for those who know how to use
std::unordered_set
, std::unordered_map
, etc., their homonyms in Boost.Unordered are
dropin replacements. The interface of openaddressing containers (boost::unordered_node_set
,
boost::unordered_node_map
, boost::unordered_flat_set
and boost::unordered_flat_map
)
is very similar, but they present some minor differences listed in the dedicated
standard compliance section.
For readers without previous experience with hash containers but familiar
with normal associative containers (std::set
, std::map
,
std::multiset
and std::multimap
), Boost.Unordered containers are used in a similar manner:
typedef boost::unordered_map<std::string, int> map;
map x;
x["one"] = 1;
x["two"] = 2;
x["three"] = 3;
assert(x.at("one") == 1);
assert(x.find("missing") == x.end());
But since the elements aren’t ordered, the output of:
for(const map::value_type& i: x) {
std::cout<<i.first<<","<<i.second<<"\n";
}
can be in any order. For example, it might be:
two,2
one,1
three,3
There are other differences, which are listed in the Comparison with Associative Containers section.
Iterator Invalidation
It is not specified how member functions other than rehash
and reserve
affect
the bucket count, although insert
can only invalidate iterators
when the insertion causes the container’s load to be greater than the maximum allowed.
For most implementations this means that insert
will only
change the number of buckets when this happens. Iterators can be
invalidated by calls to insert
, rehash
and reserve
.
As for pointers and references,
they are never invalidated for nodebased containers
(boost::unordered_[multi]set
, boost::unordered_[multi]map
, boost::unordered_node_set
, boost::unordered_node_map
),
but they will be when rehashing occurs for
boost::unordered_flat_set
and boost::unordered_flat_map
: this is because
these containers store elements directly into their holding buckets, so
when allocating a new bucket array the elements must be transferred by means of move construction.
In a similar manner to using reserve
for vector
s, it can be a good idea
to call reserve
before inserting a large number of elements. This will get
the expensive rehashing out of the way and let you store iterators, safe in
the knowledge that they won’t be invalidated. If you are inserting n
elements into container x
, you could first call:
x.reserve(n);
 Note

reserve(n)
reserves space for at leastn
elements, allocating enough buckets so as to not exceed the maximum load factor.Because the maximum load factor is defined as the number of elements divided by the total number of available buckets, this function is logically equivalent to:
x.rehash(std::ceil(n / x.max_load_factor()))
See the reference for more details on the
rehash
function.
Comparison with Associative Containers
Associative Containers  Unordered Associative Containers 

Parameterized by an ordering relation 
Parameterized by a function object 
Keys can be compared using 
Keys can be hashed using 
Constructors have optional extra parameters for the comparison object. 
Constructors have optional extra parameters for the initial minimum number of buckets, a hash function and an equality object. 
Keys 
Keys 
Member function 
No equivalent. Since the elements aren’t ordered 




Iterators, pointers and references to the container’s elements are never invalidated. 
Iterators can be invalidated by calls to insert or rehash. 
Iterators iterate through the container in the order defined by the comparison object. 
Iterators iterate through the container in an arbitrary order, that can change as elements are inserted, although equivalent elements are always adjacent. 
No equivalent 
Closedaddressing containers: Local iterators can be used to iterate through individual buckets. (The order of local iterators and iterators aren’t required to have any correspondence.) 
Can be compared using the 
Can be compared using the 
When inserting with a hint, implementations are permitted to ignore the hint. 
Operation  Associative Containers  Unordered Associative Containers 

Construction of empty container 
constant 
O(n) where n is the minimum number of buckets. 
Construction of container from a range of N elements 
O(N log N), O(N) if the range is sorted with 
Average case O(N), worst case O(N^{2}) 
Insert a single element 
logarithmic 
Average case constant, worst case linear 
Insert a single element with a hint 
Amortized constant if 
Average case constant, worst case linear (ie. the same as a normal insert). 
Inserting a range of N elements 
N log( 
Average case O(N), worst case O(N * 
Erase by key, 
O(log( 
Average case: O( 
Erase a single element by iterator 
Amortized constant 
Average case: O(1), Worst case: O( 
Erase a range of N elements 
O(log( 
Average case: O(N), Worst case: O( 
Clearing the container 
O( 
O( 
Find 
logarithmic 
Average case: O(1), Worst case: O( 
Count 
O(log( 
Average case: O(1), Worst case: O( 

logarithmic 
Average case: O( 

logarithmic 
n/a 
Concurrent Containers
Boost.Unordered provides boost::concurrent_flat_set
and boost::concurrent_flat_map
,
hash tables that allow concurrent write/read access from
different threads without having to implement any synchronzation mechanism on the user’s side.
std::vector<int> input;
boost::concurrent_flat_map<int,int> m;
...
// process input in parallel
const int num_threads = 8;
std::vector<std::jthread> threads;
std::size_t chunk = input.size() / num_threads; // how many elements per thread
for (int i = 0; i < num_threads; ++i) {
threads.emplace_back([&,i] {
// calculate the portion of input this thread takes care of
std::size_t start = i * chunk;
std::size_t end = (i == num_threads  1)? input.size(): (i + 1) * chunk;
for (std::size_t n = start; n < end; ++n) {
m.emplace(input[n], calculation(input[n]));
}
});
}
In the example above, threads access m
without synchronization, just as we’d do in a
singlethreaded scenario. In an ideal setting, if a given workload is distributed among
N threads, execution is N times faster than with one thread —this limit is
never attained in practice due to synchronization overheads and contention (one thread
waiting for another to leave a locked portion of the map), but Boost.Unordered concurrent containers
are designed to perform with very little overhead and typically achieve linear scaling
(that is, performance is proportional to the number of threads up to the number of
logical cores in the CPU).
Visitationbased API
The first thing a new user of boost::concurrent_flat_set
or boost::concurrent_flat_map
will notice is that these classes do not provide iterators (which makes them technically
not Containers
in the C++ standard sense). The reason for this is that iterators are inherently
threadunsafe. Consider this hypothetical code:
auto it = m.find(k); // A: get an iterator pointing to the element with key k
if (it != m.end() ) {
some_function(*it); // B: use the value of the element
}
In a multithreaded scenario, the iterator it
may be invalid at point B if some other
thread issues an m.erase(k)
operation between A and B. There are designs that
can remedy this by making iterators lock the element they point to, but this
approach lends itself to high contention and can easily produce deadlocks in a program.
operator[]
has similar concurrency issues, and is not provided by
boost::concurrent_flat_map
either. Instead, element access is done through
socalled visitation functions:
m.visit(k, [](const auto& x) { // x is the element with key k (if it exists)
some_function(x); // use it
});
The visitation function passed by the user (in this case, a lambda function) is executed internally by Boost.Unordered in a threadsafe manner, so it can access the element without worrying about other threads interfering in the process.
On the other hand, a visitation function can not access the container itself:
m.visit(k, [&](const auto& x) {
some_function(x, m.size()); // forbidden: m can't be accessed inside visitation
});
Access to a different container is allowed, though:
m.visit(k, [&](const auto& x) {
if (some_function(x)) {
m2.insert(x); // OK, m2 is a different boost::concurrent_flat_map
}
});
But, in general, visitation functions should be as lightweight as possible to reduce contention and increase parallelization. In some cases, moving heavy work outside of visitation may be beneficial:
std::optional<value_type> o;
bool found = m.visit(k, [&](const auto& x) {
o = x;
});
if (found) {
some_heavy_duty_function(*o);
}
Visitation is prominent in the API provided by boost::concurrent_flat_set
and boost::concurrent_flat_map
, and
many classical operations have visitationenabled variations:
m.insert_or_visit(x, [](auto& y) {
// if insertion failed because of an equivalent element y,
// do something with it, for instance:
++y.second; // increment the mapped part of the element
});
Note that in this last example the visitation function could actually modify
the element: as a general rule, operations on a boost::concurrent_flat_map
m
will grant visitation functions const/nonconst access to the element depending on whether
m
is const/nonconst. Const access can be always be explicitly requested
by using cvisit
overloads (for instance, insert_or_cvisit
) and may result
in higher parallelization. For boost::concurrent_flat_set
, on the other hand,
visitation is always const access.
Consult the references of
boost::concurrent_flat_set
and
boost::concurrent_flat_map
for the complete list of visitationenabled operations.
WholeTable Visitation
In the absence of iterators, visit_all
is provided
as an alternative way to process all the elements in the container:
m.visit_all([](auto& x) {
x.second = 0; // reset the mapped part of the element
});
In C++17 compilers implementing standard parallel algorithms, wholetable visitation can be parallelized:
m.visit_all(std::execution::par, [](auto& x) { // run in parallel
x.second = 0; // reset the mapped part of the element
});
Traversal can be interrupted midway:
// finds the key to a given (unique) value
int key = 0;
int value = ...;
bool found = !m.visit_while([&](const auto& x) {
if(x.second == value) {
key = x.first;
return false; // finish
}
else {
return true; // keep on visiting
}
});
if(found) { ... }
There is one last wholetable visitation operation, erase_if
:
m.erase_if([](auto& x) {
return x.second == 0; // erase the elements whose mapped value is zero
});
visit_while
and erase_if
can also be parallelized. Note that, in order to increase efficiency,
wholetable visitation operations do not block the table during execution: this implies that elements
may be inserted, modified or erased by other threads during visitation. It is
advisable not to assume too much about the exact global state of a concurrent container
at any point in your program.
Bulk visitation
Suppose you have an std::array
of keys you want to look up for in a concurrent map:
std::array<int, N> keys;
...
for(const auto& key: keys) {
m.visit(key, [](auto& x) { ++x.second; });
}
Bulk visitation allows us to pass all the keys in one operation:
m.visit(keys.begin(), keys.end(), [](auto& x) { ++x.second; });
This functionality is not provided for mere syntactic convenience, though: by processing all the keys at once, some internal optimizations can be applied that increase performance over the regular, oneatatime case (consult the benchmarks). In fact, it may be beneficial to buffer incoming keys so that they can be bulk visited in chunks:
static constexpr auto bulk_visit_size = boost::concurrent_flat_map<int,int>::bulk_visit_size;
std::array<int, bulk_visit_size> buffer;
std::size_t i=0;
while(...) { // processing loop
...
buffer[i++] = k;
if(i == bulk_visit_size) {
map.visit(buffer.begin(), buffer.end(), [](auto& x) { ++x.second; });
i = 0;
}
...
}
// flush remaining keys
map.visit(buffer.begin(), buffer.begin() + i, [](auto& x) { ++x.second; });
There’s a latency/throughput tradeoff here: it will take longer for incoming keys to
be processed (since they are buffered), but the number of processed keys per second
is higher. bulk_visit_size
is the recommended chunk size —smaller buffers
may yield worse performance.
Blocking Operations
boost::concurrent_flat_set
s and boost::concurrent_flat_map
s can be copied, assigned, cleared and merged just like any
Boost.Unordered container. Unlike most other operations, these are blocking,
that is, all other threads are prevented from accesing the tables involved while a copy, assignment,
clear or merge operation is in progress. Blocking is taken care of automatically by the library
and the user need not take any special precaution, but overall performance may be affected.
Another blocking operation is rehashing, which happens explicitly via rehash
/reserve
or during insertion when the table’s load hits max_load()
. As with nonconcurrent containers,
reserving space in advance of bulk insertions will generally speed up the process.
Interoperability with nonconcurrent containers
As openaddressing and concurrent containers are based on the same internal data structure,
boost::unordered_flat_set
and boost::unordered_flat_map
can
be efficiently moveconstructed from boost::concurrent_flat_set
and boost::concurrent_flat_map
,
respectively, and vice versa.
This interoperability comes handy in multistage scenarios where parts of the data processing happen
in parallel whereas other steps are nonconcurrent (or nonmodifying). In the following example,
we want to construct a histogram from a huge input vector of words:
the population phase can be done in parallel with boost::concurrent_flat_map
and results
then transferred to the final container.
std::vector<std::string> words = ...;
// Insert words in parallel
boost::concurrent_flat_map<std::string_view, std::size_t> m0;
std::for_each(
std::execution::par, words.begin(), words.end(),
[&](const auto& word) {
m0.try_emplace_or_visit(word, 1, [](auto& x) { ++x.second; });
});
// Transfer to a regular unordered_flat_map
boost::unordered_flat_map m=std::move(m0);
Hash Quality
In order to work properly, hash tables require that the supplied hash function
be of good quality, roughly meaning that it uses its std::size_t
output
space as uniformly as possible, much like a random number generator would do
—except, of course, that the value of a hash function is not random but strictly determined
by its input argument.
Closedaddressing containers in Boost.Unordered are fairly robust against hash functions with lessthanideal quality, but openaddressing and concurrent containers are much more sensitive to this factor, and their performance can degrade dramatically if the hash function is not appropriate. In general, if you’re using functions provided by or generated with Boost.Hash, the quality will be adequate, but you have to be careful when using alternative hash algorithms.
The rest of this section applies only to openaddressing and concurrent containers.
Hash Postmixing and the Avalanching Property
Even if your supplied hash function does not conform to the uniform behavior required by open addressing, chances are that the performance of Boost.Unordered containers will be acceptable, because the library executes an internal postmixing step that improves the statistical properties of the calculated hash values. This comes with an extra computational cost; if you’d like to opt out of postmixing, annotate your hash function as follows:
struct my_string_hash_function
{
using is_avalanching = std::true_type; // instruct Boost.Unordered to not use postmixing
std::size_t operator()(const std::string& x) const
{
...
}
};
By setting the
hash_is_avalanching trait, we inform Boost.Unordered
that my_string_hash_function
is of sufficient quality to be used directly without
any postmixing safety net. This comes at the risk of degraded performance in the
cases where the hash function is not as wellbehaved as we’ve declared.
Container Statistics
If we globally define the macro BOOST_UNORDERED_ENABLE_STATS
, openaddressing and
concurrent containers will calculate some internal statistics directly correlated to the
quality of the hash function:
#define BOOST_UNORDERED_ENABLE_STATS
#include <boost/unordered/unordered_map.hpp>
...
int main()
{
boost::unordered_flat_map<std::string, int, my_string_hash> m;
... // use m
auto stats = m.get_stats();
... // inspect stats
}
The stats
object provides the following information:
stats
.insertion // Insertion operations
.count // Number of operations
.probe_length // Probe length per operation
.average
.variance
.deviation
.successful_lookup // Lookup operations (element found)
.count // Number of operations
.probe_length // Probe length per operation
.average
.variance
.deviation
.num_comparisons // Elements compared per operation
.average
.variance
.deviation
.unsuccessful_lookup // Lookup operations (element not found)
.count // Number of operations
.probe_length // Probe length per operation
.average
.variance
.deviation
.num_comparisons // Elements compared per operation
.average
.variance
.deviation
Statistics for three internal operations are maintained: insertions (without considering the previous lookup to determine that the key is not present yet), successful lookups, and unsuccessful lookups (including those issued internally when inserting elements). Probe length is the number of bucket groups accessed per operation. If the hash function behaves properly:

Average probe lengths should be close to 1.0.

The average number of comparisons per successful lookup should be close to 1.0 (that is, just the element found is checked).

The average number of comparisons per unsuccessful lookup should be close to 0.0.
An example is provided that displays container
statistics for boost::hash<std::string>
, an implementation of the
FNV1a hash
and two illbehaved custom hash functions that have been incorrectly marked as avalanching:
boost::unordered_flat_map: 319 ms insertion: probe length 1.08771 successful lookup: probe length 1.06206, num comparisons 1.02121 unsuccessful lookup: probe length 1.12301, num comparisons 0.0388251 boost::unordered_flat_map, FNV1a: 301 ms insertion: probe length 1.09567 successful lookup: probe length 1.06202, num comparisons 1.0227 unsuccessful lookup: probe length 1.12195, num comparisons 0.040527 boost::unordered_flat_map, slightly_bad_hash: 654 ms insertion: probe length 1.03443 successful lookup: probe length 1.04137, num comparisons 6.22152 unsuccessful lookup: probe length 1.29334, num comparisons 11.0335 boost::unordered_flat_map, bad_hash: 12216 ms insertion: probe length 699.218 successful lookup: probe length 590.183, num comparisons 43.4886 unsuccessful lookup: probe length 1361.65, num comparisons 75.238
Standard Compliance
Closedaddressing Containers
boost::unordered_[multi]set
and boost::unordered_[multi]map
provide a conformant
implementation for C++11 (or later) compilers of the latest standard revision of
C++ unordered associative containers, with very minor deviations as noted.
The containers are fully AllocatorAware
and support fancy pointers.
Deduction Guides
Deduction guides for class template argument deduction (CTAD) are only available on C++17 (or later) compilers.
Piecewise Pair Emplacement
In accordance with the standard specification,
boost::unordered_[multi]map::emplace
supports piecewise pair construction:
boost::unordered_multimap<std::string, std::complex> x;
x.emplace(
std::piecewise_construct,
std::make_tuple("key"), std::make_tuple(1, 2));
Additionally, the same
functionality is provided via nonstandard boost::unordered::piecewise_construct
and Boost.Tuple:
x.emplace(
boost::unordered::piecewise_construct,
boost::make_tuple("key"), boost::make_tuple(1, 2));
This feature has been retained for backwards compatibility with
previous versions of Boost.Unordered: users are encouraged to
update their code to use std::piecewise_construct
and
std::tuple
s instead.
Swap
When swapping, Pred
and Hash
are not currently swapped by calling
swap
, their copy constructors are used. As a consequence, when swapping
an exception may be thrown from their copy constructor.
Openaddressing Containers
The C++ standard does not currently provide any openaddressing container
specification to adhere to, so boost::unordered_flat_set
/unordered_node_set
and
boost::unordered_flat_map
/unordered_node_map
take inspiration from std::unordered_set
and
std::unordered_map
, respectively, and depart from their interface where
convenient or as dictated by their internal data structure, which is
radically different from that imposed by the standard (closed addressing).
Openaddressing containers provided by Boost.Unordered only work with reasonably compliant C++11 (or later) compilers. Languagelevel features such as move semantics and variadic template parameters are then not emulated. The containers are fully AllocatorAware and support fancy pointers.
The main differences with C++ unordered associative containers are:

In general:

begin()
is not constanttime. 
erase(iterator)
does not return an iterator to the following element, but a proxy object that converts to that iterator if requested; this avoids a potentially costly iterator increment operation when not needed. 
There is no API for bucket handling (except
bucket_count
). 
The maximum load factor of the container is managed internally and can’t be set by the user. The maximum load, exposed through the public function
max_load
, may decrease on erasure under highload conditions.


Flat containers (
boost::unordered_flat_set
andboost::unordered_flat_map
):
value_type
must be moveconstructible. 
Pointer stability is not kept under rehashing.

There is no API for node extraction/insertion.

Concurrent Containers
There is currently no specification in the C++ standard for this or any other type of concurrent
data structure. The APIs of boost::concurrent_flat_set
and boost::concurrent_flat_map
are modelled after std::unordered_flat_set
and std::unordered_flat_map
, respectively,
with the crucial difference that iterators are not provided
due to their inherent problems in concurrent scenarios (high contention, prone to deadlocking):
so, Boost.Unordered concurrent containers are technically not models of
Container, although
they meet all the requirements of AllocatorAware
containers (including
fancy pointer support)
except those implying iterators.
In a nonconcurrent unordered container, iterators serve two main purposes:

Access to an element previously located via lookup.

Container traversal.
In place of iterators, boost::concurrent_flat_set
and boost::concurrent_flat_map
use internal visitation
facilities as a threadsafe substitute. Classical operations returning an iterator to an
element already existing in the container, like for instance:
iterator find(const key_type& k);
std::pair<iterator, bool> insert(const value_type& obj);
are transformed to accept a visitation function that is passed such element:
template<class F> size_t visit(const key_type& k, F f);
template<class F> bool insert_or_visit(const value_type& obj, F f);
(In the second case f
is only invoked if there’s an equivalent element
to obj
in the table, not if insertion is successful). Container traversal
is served by:
template<class F> size_t visit_all(F f);
of which there are parallelized versions in C++17 compilers with parallel
algorithm support. In general, the interface of concurrent containers
is derived from that of their nonconcurrent counterparts by a fairly straightforward
process of replacing iterators with visitation where applicable. If for
regular maps iterator
and const_iterator
provide mutable and const access to elements,
respectively, here visitation is granted mutable or const access depending on
the constness of the member function used (there are also *cvisit
overloads for
explicit const visitation); In the case of boost::concurrent_flat_set
, visitation is always const.
One notable operation not provided by boost::concurrent_flat_map
is operator[]
/at
, which can be
replaced, if in a more convoluted manner, by
try_emplace_or_visit
.
Data Structures
Closedaddressing Containers
Boost.Unordered sports one of the fastest implementations of closed addressing, also commonly known as separate chaining. An example figure representing the data structure is below:
An array of "buckets" is allocated and each bucket in turn points to its own individual linked list. This makes meeting the standard requirements of bucket iteration straightforward. Unfortunately, iteration of the entire container is often times slow using this layout as each bucket must be examined for occupancy, yielding a time complexity of O(bucket_count() + size())
when the standard requires complexity to be O(size())
.
Canonical standard implementations will wind up looking like the diagram below:
It’s worth noting that this approach is only used by libc++ and libstdc++; the MSVC Dinkumware implementation uses a different one. A more detailed analysis of the standard containers can be found here.
This unusually laid out data structure is chosen to make iteration of the entire container efficient by interconnecting all of the nodes into a singlylinked list. One might also notice that buckets point to the node before the start of the bucket’s elements. This is done so that removing elements from the list can be done efficiently without introducing the need for a doublylinked list. Unfortunately, this data structure introduces a guaranteed extra indirection. For example, to access the first element of a bucket, something like this must be done:
auto const idx = get_bucket_idx(hash_function(key));
node* p = buckets[idx]; // first load
node* n = p>next; // second load
if (n && is_in_bucket(n, idx)) {
value_type const& v = *n; // third load
// ...
}
With a simple bucket group layout, this is all that must be done:
auto const idx = get_bucket_idx(hash_function(key));
node* n = buckets[idx]; // first load
if (n) {
value_type const& v = *n; // second load
// ...
}
In practice, the extra indirection can have a dramatic performance impact to common operations such as insert
, find
and erase
. But to keep iteration of the container fast, Boost.Unordered introduces a novel data structure, a "bucket group". A bucket group is a fixedwidth view of a subsection of the buckets array. It contains a bitmask (a std::size_t
) which it uses to track occupancy of buckets and contains two pointers so that it can form a doublylinked list with nonempty groups. An example diagram is below:
Thus containerwide iteration is turned into traversing the nonempty bucket groups (an operation with constant time complexity) which reduces the time complexity back to O(size())
. In total, a bucket group is only 4 words in size and it views sizeof(std::size_t) * CHAR_BIT
buckets meaning that for all common implementations, there’s only 4 bits of space overhead per bucket introduced by the bucket groups.
A more detailed description of Boost.Unordered’s closedaddressing implementation is given in an external article. For more information on implementation rationale, read the corresponding section.
Openaddressing Containers
The diagram shows the basic internal layout of boost::unordered_flat_set
/unordered_node_set
and
boost:unordered_flat_map
/unordered_node_map
.
As with all openaddressing containers, elements (or pointers to the element nodes in the case of
boost::unordered_node_set
and boost::unordered_node_map
) are stored directly in the bucket array.
This array is logically divided into 2^{n} groups of 15 elements each.
In addition to the bucket array, there is an associated metadata array with 2^{n}
16byte words.
A metadata word is divided into 15 h_{i} bytes (one for each associated bucket), and an overflow byte (ofw in the diagram). The value of h_{i} is:

0 if the corresponding bucket is empty.

1 to encode a special empty bucket called a sentinel, which is used internally to stop iteration when the container has been fully traversed.

If the bucket is occupied, a reduced hash value obtained from the hash value of the element.
When looking for an element with hash value h, SIMD technologies such as SSE2 and Neon allow us to very quickly inspect the full metadata word and look for the reduced value of h among all the 15 buckets with just a handful of CPU instructions: nonmatching buckets can be readily discarded, and those whose reduced hash value matches need be inspected via full comparison with the corresponding element. If the lookedfor element is not present, the overflow byte is inspected:

If the bit in the position h mod 8 is zero, lookup terminates (and the element is not present).

If the bit is set to 1 (the group has been overflowed), further groups are checked using quadratic probing, and the process is repeated.
Insertion is algorithmically similar: empty buckets are located using SIMD, and when going past a full group its corresponding overflow bit is set to 1.
In architectures without SIMD support, the logical layout stays the same, but the metadata word is codified using a technique we call bit interleaving: this layout allows us to emulate SIMD with reasonably good performance using only standard arithmetic and logical operations.
A more detailed description of Boost.Unordered’s openaddressing implementation is given in an external article. For more information on implementation rationale, read the corresponding section.
Concurrent Containers
boost::concurrent_flat_set
and boost::concurrent_flat_map
use the basic
openaddressing layout described above
augmented with synchronization mechanisms.
Two levels of synchronization are used:

Container level: A readwrite mutex is used to control access from any operation to the container. Typically, such access is in read mode (that is, concurrent) even for modifying operations, so for most practical purposes there is no thread contention at this level. Access is only in write mode (blocking) when rehashing or performing containerwide operations such as swapping or assignment.

Group level: Each 15slot group is equipped with an 8byte word containing:

A readwrite spinlock for synchronized access to any element in the group.

An atomic insertion counter used for optimistic insertion as described below.

By using atomic operations to access the group metadata, lookup is (grouplevel) lockfree up to the point where an actual comparison needs to be done with an element that has been previously SIMDmatched: only then is the group’s spinlock used.
Insertion uses the following optimistic algorithm:

The value of the insertion counter for the initial group in the probe sequence is locally recorded (let’s call this value
c0
). 
Lookup is as described above. If lookup finds no equivalent element, search for an available slot for insertion successively locks/unlocks each group in the probing sequence.

When an available slot is located, it is preemptively occupied (its reduced hash value is set) and the insertion counter is atomically incremented: if no other thread has incremented the counter during the whole operation (which is checked by comparing with
c0
), then we’re good to go and complete the insertion, otherwise we roll back and start over.
This algorithm has very low contention both at the lookup and actual insertion phases in exchange for the possibility that computations have to be started over if some other thread interferes in the process by performing a succesful insertion beginning at the same group. In practice, the startover frequency is extremely small, measured in the range of parts per million for some of our benchmarks.
For more information on implementation rationale, read the corresponding section.
Debuggability
Visual Studio Natvis
All containers and iterators have custom visualizations in the Natvis framework, as long as their allocator uses regular raw pointers. Any container or iterator with an allocator using fancy pointers does not have a custom visualization right now.
The visualizations mirror those for the standard unordered containers. A container has a maximum of 100 elements displayed at once. Each set element has its item name listed as [i]
, where i
is the index in the display, starting at 0
. Each map element has its item name listed as [{keydisplay}]
by default. For example, if the first element is the pair ("abc", 1)
, the item name will be ["abc"]
. This behaviour can be overridden by using the view "ShowElementsByIndex", which switches the map display behaviour to name the elements by index. This same view name is used in the standard unordered containers.
By default, the closedaddressing containers will show the [hash_function]
and [key_eq]
, the [spare_hash_function]
and [spare_key_eq]
if applicable, the [allocator]
, and the elements. Using the view "detailed" adds the [bucket_count]
and [max_load_factor]
. Conversely, using the view "simple" shows only the elements, with no other items present.
By default, the openaddressing containers will show the [hash_function]
, [key_eq]
, [allocator]
, and the elements. Using the view "simple" shows only the elements, with no other items present. Both the SIMD and the nonSIMD implementations are viewable through the Natvis framework.
Iterators are displayed similarly to their standard counterparts. An iterator is displayed as though it were the element that it points to. An end iterator is simply displayed as \{ end iterator }
.
Benchmarks
boost::unordered_[multi]set
All benchmarks were created using unordered_set<unsigned int>
(nonduplicate) and unordered_multiset<unsigned int>
(duplicate). The source code can be found here.
The insertion benchmarks insert n
random values, where n
is between 10,000 and 3 million. For the duplicated benchmarks, the same random values are repeated an average of 5 times.
The erasure benchmarks erase all n
elements randomly until the container is empty. Erasure by key uses erase(const key_type&)
to remove entire groups of equivalent elements in each operation.
The successful lookup benchmarks are done by looking up all n
values, in their original insertion order.
The unsuccessful lookup benchmarks use n
randomly generated integers but using a different seed value.
GCC 12 + libstdc++v3, x64
Insertion
nonduplicate elements 
duplicate elements 
duplicate elements, 

nonduplicate elements, 
duplicate elements, 
duplicate elements, 

Erasure
nonduplicate elements 
duplicate elements 
duplicate elements, 

by key, duplicate elements 
by key, duplicate elements, 
Successful Lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

Unsuccessful lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

Clang 15 + libc++, x64
Insertion
nonduplicate elements 
duplicate elements 
duplicate elements, 

nonduplicate elements, 
duplicate elements, 
duplicate elements, 

Erasure
nonduplicate elements 
duplicate elements 
duplicate elements, 

by key, duplicate elements 
by key, duplicate elements, 
Successful lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

Unsuccessful lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

Visual Studio 2022 + Dinkumware, x64
Insertion
nonduplicate elements 
duplicate elements 
duplicate elements, 

nonduplicate elements, 
duplicate elements, 
duplicate elements, 

Erasure
nonduplicate elements 
duplicate elements 
duplicate elements, 

by key, duplicate elements 
by key, duplicate elements, 
Successful lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

Unsuccessful lookup
nonduplicate elements 
duplicate elements 
duplicate elements, 

boost::unordered_(flatnode)_map
All benchmarks were created using:

absl::flat_hash_map<uint64_t, uint64_t>

boost::unordered_map<uint64_t, uint64_t>

boost::unordered_flat_map<uint64_t, uint64_t>

boost::unordered_node_map<uint64_t, uint64_t>
The source code can be found here.
The insertion benchmarks insert n
random values, where n
is between 10,000 and 10 million.
The erasure benchmarks erase traverse the n
elements and erase those with odd key (50% on average).
The successful lookup benchmarks are done by looking up all n
values, in their original insertion order.
The unsuccessful lookup benchmarks use n
randomly generated integers but using a different seed value.
GCC 12, x64
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

Clang 15, x64
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

Visual Studio 2022, x64
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

Clang 12, ARM64
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

GCC 12, x86
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

Clang 15, x86
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

Visual Studio 2022, x86
running insertion 
running erasure 
successful lookup 
unsuccessful lookup 

boost::concurrent_flat_map
All benchmarks were created using:

oneapi::tbb::concurrent_hash_map<int, int>

gtl::parallel_flat_hash_map<int, int>
with 64 submaps 
boost::concurrent_flat_map<int, int>
The source code can be found here.
The benchmarks exercise a number of threads T (between 1 and 16) concurrently performing operations randomly chosen among update, successful lookup and unsuccessful lookup. The keys used in the operations follow a Zipf distribution with different skew parameters: the higher the skew, the more concentrated are the keys in the lower values of the covered range.
boost::concurrent_flat_map
is exercised using both regular and bulk visitation:
in the latter case, lookup keys are buffered in a local array and then processed at
once each time the buffer reaches bulk_visit_size
.
GCC 12, x64
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Clang 15, x64
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Visual Studio 2022, x64
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Clang 12, ARM64
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

GCC 12, x86
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Clang 15, x86
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Visual Studio 2022, x86
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 
500k updates, 4.5M lookups 

5M updates, 45M lookups 
5M updates, 45M lookups 
5M updates, 45M lookups 

Implementation Rationale
Closedaddressing Containers
boost::unordered_[multi]set
and boost::unordered_[multi]map
adhere to the standard requirements for unordered associative
containers, so the interface was fixed. But there are
still some implementation decisions to make. The priorities are
conformance to the standard and portability.
The Wikipedia article on hash tables has a good summary of the implementation issues for hash tables in general.
Data Structure
By specifying an interface for accessing the buckets of the container the standard pretty much requires that the hash table uses closed addressing.
It would be conceivable to write a hash table that uses another method. For example, it could use open addressing, and use the lookup chain to act as a bucket but there are some serious problems with this:

The standard requires that pointers to elements aren’t invalidated, so the elements can’t be stored in one array, but will need a layer of indirection instead  losing the efficiency and most of the memory gain, the main advantages of open addressing.

Local iterators would be very inefficient and may not be able to meet the complexity requirements.

There are also the restrictions on when iterators can be invalidated. Since open addressing degrades badly when there are a high number of collisions the restrictions could prevent a rehash when it’s really needed. The maximum load factor could be set to a fairly low value to work around this  but the standard requires that it is initially set to 1.0.

And since the standard is written with a eye towards closed addressing, users will be surprised if the performance doesn’t reflect that.
So closed addressing is used.
Number of Buckets
There are two popular methods for choosing the number of buckets in a hash table. One is to have a prime number of buckets, another is to use a power of 2.
Using a prime number of buckets, and choosing a bucket by using the modulus of the hash function’s result will usually give a good result. The downside is that the required modulus operation is fairly expensive. This is what the containers used to do in most cases.
Using a power of 2 allows for much quicker selection of the bucket to use, but at the expense of losing the upper bits of the hash value. For some specially designed hash functions it is possible to do this and still get a good result but as the containers can take arbitrary hash functions this can’t be relied on.
To avoid this a transformation could be applied to the hash function, for an
example see
Thomas Wang’s article on integer hash functions.
Unfortunately, a transformation like Wang’s requires knowledge of the number
of bits in the hash value, so it was only used when size_t
was 64 bit.
Since release 1.79.0, Fibonacci hashing
is used instead. With this implementation, the bucket number is determined
by using (h * m) >> (w  k)
, where h
is the hash value, m
is 2^w
divided
by the golden ratio, w
is the word size (32 or 64), and 2^k
is the
number of buckets. This provides a good compromise between speed and
distribution.
Since release 1.80.0, prime numbers are chosen for the number of buckets in tandem with sophisticated modulo arithmetic. This removes the need for "mixing" the result of the user’s hash function as was used for release 1.79.0.
Openaddresing Containers
The C++ standard specification of unordered associative containers impose severe limitations on permissible implementations, the most important being that closed addressing is implicitly assumed. Slightly relaxing this specification opens up the possibility of providing container variations taking full advantage of openaddressing techniques.
The design of boost::unordered_flat_set
/unordered_node_set
and boost::unordered_flat_map
/unordered_node_map
has been
guided by Peter Dimov’s Development Plan for Boost.Unordered.
We discuss here the most relevant principles.
Hash Function
Given its rich functionality and crossplatform interoperability,
boost::hash
remains the default hash function of openaddressing containers.
As it happens, boost::hash
for integral and other basic types does not possess
the statistical properties required by open addressing; to cope with this,
we implement a postmixing stage:
a ← h mulx C,
h ← high(a) xor low(a),
where mulx is an extended multiplication (128 bits in 64bit architectures, 64 bits in 32bit environments), and high and low are the upper and lower halves of an extended word, respectively. In 64bit architectures, C is the integer part of 2^{64}∕φ, whereas in 32 bits C = 0xE817FB2Du has been obtained from Steele and Vigna (2021).
When using a hash function directly suitable for open addressing, postmixing can be opted out of via a dedicated hash_is_avalanching
trait.
boost::hash
specializations for string types are marked as avalanching.
Platform Interoperability
The observable behavior of boost::unordered_flat_set
/unordered_node_set
and boost::unordered_flat_map
/unordered_node_map
is deterministically
identical across different compilers as long as their std::size_t
s are the same size and the userprovided
hash function and equality predicate are also interoperable
—this includes elements being ordered in exactly the same way for the same sequence of
operations.
Concurrent Containers
The same data structure used by Boost.Unordered openaddressing containers has been chosen
also as the foundation of boost::concurrent_flat_set
and boost::concurrent_flat_map
:

Openaddressing is faster than closedaddressing alternatives, both in nonconcurrent and concurrent scenarios.

Openaddressing layouts are eminently suitable for concurrent access and modification with minimal locking. In particular, the metadata array can be used for implementations of lookup that are lockfree up to the last step of actual element comparison.

Layout compatibility with Boost.Unordered flat containers allows for fast transfer of all elements between
boost::concurrent_flat_map
andboost::unordered_flat_map
, and vice versa.
Hash Function and Platform Interoperability
Concurrent containers make the same decisions and provide the same guarantees as Boost.Unordered openaddressing containers with regards to hash function defaults and platform interoperability.
Reference
Class Template unordered_map
boost::unordered_map
— An unordered associative container that associates unique keys with another value.
Synopsis
// #include <boost/unordered/unordered_map.hpp> namespace boost { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> class unordered_map { public: // types using key_type = Key; using mapped_type = T; using value_type = std::pair<const Key, T>; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename std::allocator_traits<Allocator>::pointer; using const_pointer = typename std::allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = std::size_t; using difference_type = std::ptrdiff_t; using iterator = implementationdefined; using const_iterator = implementationdefined; using local_iterator = implementationdefined; using const_local_iterator = implementationdefined; using node_type = implementationdefined; using insert_return_type = implementationdefined; // construct/copy/destroy unordered_map(); explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(const unordered_map& other); unordered_map(unordered_map&& other); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, const allocator_type& a); explicit unordered_map(const Allocator& a); unordered_map(const unordered_map& other, const Allocator& a); unordered_map(unordered_map&& other, const Allocator& a); unordered_map(std::initializer_list<value_type> il, size_type n = implementationdefined const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(size_type n, const allocator_type& a); unordered_map(size_type n, const hasher& hf, const allocator_type& a); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a); unordered_map(std::initializer_list<value_type> il, const allocator_type& a); unordered_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a); unordered_map(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ~unordered_map(); unordered_map& operator=(const unordered_map& other); unordered_map& operator=(unordered_map&& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_move_assignable_v<Hash> && boost::is_nothrow_move_assignable_v<Pred>); unordered_map& operator=(std::initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> std::pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); std::pair<iterator, bool> insert(const value_type& obj); std::pair<iterator, bool> insert(value_type&& obj); template<class P> std::pair<iterator, bool> insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class P> iterator insert(const_iterator hint, P&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); void insert(std::initializer_list<value_type>); template<class... Args> std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> std::pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M> std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M> std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M> std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); node_type extract(const_iterator position); node_type extract(const key_type& k); template<class K> node_type extract(K&& k); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& k); iterator erase(const_iterator first, const_iterator last); void quick_erase(const_iterator position); void erase_return_void(const_iterator position); void swap(unordered_map& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_swappable_v<Hash> && boost::is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // map operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq); template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> const_iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; // element access mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k); template<class K> mapped_type& operator[](K&& k); mapped_type& at(const key_type& k); const mapped_type& at(const key_type& k) const; template<class K> mapped_type& at(const K& k); template<class K> const mapped_type& at(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; // Deduction Guides template<class InputIterator, class Hash = boost::hash<iterkeytype<InputIterator>>, class Pred = std::equal_to<iterkeytype<InputIterator>>, class Allocator = std::allocator<itertoalloctype<InputIterator>>> unordered_map(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> unordered_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_map<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_map(InputIterator, InputIterator, typename see below::size_type, Allocator) > unordered_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_map(InputIterator, InputIterator, Allocator) > unordered_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) > unordered_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class Key, class T, class Allocator> unordered_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Allocator) > unordered_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_map(std::initializer_list<std::pair<Key, T>>, Allocator) > unordered_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Hash, Allocator) > unordered_map<Key, T, Hash, std::equal_to<Key>, Allocator>; // Equality Comparisons template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& x, const unordered_map<Key, T, Hash, Pred, Alloc>& y); template<class Key, class T, class Hash, class Pred, class Alloc> bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& x, const unordered_map<Key, T, Hash, Pred, Alloc>& y); // swap template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x, unordered_map<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // Erasure template<class K, class T, class H, class P, class A, class Predicate> typename unordered_map<K, T, H, P, A>::size_type erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred); // Pmr aliases (C++17 and up) namespace unordered::pmr { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>> using unordered_map = boost::unordered_map<Key, T, Hash, Pred, std::pmr::polymorphic_allocator<std::pair<const Key, T>>>; } }
Description
Template Parameters
Key 

T 

Hash 
A unary function object type that acts a hash function for a 
Pred 
A binary function object that implements an equivalence relation on values of type 
Allocator 
An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported. 
The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.
The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.
Configuration macros
BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0
Globally define this macro to support loading of unordered_map
s saved to
a Boost.Serialization archive with a version of Boost prior to Boost 1.84.
Typedefs
typedef implementationdefined iterator;
An iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Convertible to const_iterator
.
typedef implementationdefined const_iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
typedef implementationdefined local_iterator;
An iterator with the same value type, difference type and pointer and reference type as iterator.
A local_iterator
object can be used to iterate through a single bucket.
typedef implementationdefined const_local_iterator;
A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.
A const_local_iterator object can be used to iterate through a single bucket.
typedef implementationdefined node_type;
A class for holding extracted container elements, modelling NodeHandle.
typedef implementationdefined insert_return_type;
A specialization of an internal class template:
template<class Iterator, class NodeType>
struct insert_return_type // name is exposition only
{
Iterator position;
bool inserted;
NodeType node;
};
with Iterator
= iterator
and NodeType
= node_type
.
Constructors
Default Constructor
unordered_map();
Constructs an empty container using hasher()
as the hash function,
key_equal()
as the key equality predicate, allocator_type()
as the allocator
and a maximum load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Bucket Count Constructor
explicit unordered_map(size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash
function, eql
as the key equality predicate, a
as the allocator and a maximum
load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Iterator Range Constructor
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 
If the defaults are used, 
Copy Constructor
unordered_map(unordered_map const& other);
The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.
If Allocator::select_on_container_copy_construction
exists and has the right signature, the allocator will be constructed from its result.
Requires: 

Move Constructor
unordered_map(unordered_map&& other);
The move constructor.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Iterator Range Constructor with Allocator
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, const allocator_type& a);
Constructs an empty container using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Allocator Constructor
explicit unordered_map(Allocator const& a);
Constructs an empty container, using allocator a
.
Copy Constructor with Allocator
unordered_map(unordered_map const& other, Allocator const& a);
Constructs an container, copying other
's contained elements, hash function, predicate, maximum load factor, but using allocator a
.
Move Constructor with Allocator
unordered_map(unordered_map&& other, Allocator const& a);
Construct a container moving other
's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a
.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Initializer List Constructor
unordered_map(std::initializer_list<value_type> il,
size_type n = implementationdefined
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from il
into it.
Requires: 
If the defaults are used, 
Bucket Count Constructor with Allocator
unordered_map(size_type n, allocator_type const& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default hash function and key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Bucket Count Constructor with Hasher and Allocator
unordered_map(size_type n, hasher const& hf, allocator_type const& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator, with the default key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

initializer_list Constructor with Allocator
unordered_map(std::initializer_list<value_type> il, const allocator_type& a);
Constructs an empty container using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Allocator
unordered_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

Destructor
~unordered_map();
Note: 
The destructor is applied to every element, and all memory is deallocated 
Assignment
Copy Assignment
unordered_map& operator=(unordered_map const& other);
The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.
If Alloc::propagate_on_container_copy_assignment
exists and Alloc::propagate_on_container_copy_assignment::value
is true
, the allocator is overwritten, if not the copied elements are created using the existing allocator.
Requires: 

Move Assignment
unordered_map& operator=(unordered_map&& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_move_assignable_v<Hash> &&
boost::is_nothrow_move_assignable_v<Pred>);
The move assignment operator.
If Alloc::propagate_on_container_move_assignment
exists and Alloc::propagate_on_container_move_assignment::value
is true
, the allocator is overwritten, if not the moved elements are created using the existing allocator.
Requires: 

Initializer List Assignment
unordered_map& operator=(std::initializer_list<value_type> il);
Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.
Requires: 

Iterators
begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns: 
An iterator referring to the first element of the container, or if the container is empty the pasttheend value for the container. 
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns: 
An iterator which refers to the pasttheend value for the container. 
cbegin
const_iterator cbegin() const noexcept;
Returns: 
A 
cend
const_iterator cend() const noexcept;
Returns: 
A 
Modifiers
emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. If 
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent key.
position
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. If 
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Emplace Insert
template<class P> std::pair<iterator, bool> insert(P&& obj);
Inserts an element into the container by performing emplace(std::forward<P>(value))
.
Only participates in overload resolution if std::is_constructible<value_type, P&&>::value
is true
.
Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Emplace Insert with Hint
template<class P> iterator insert(const_iterator hint, P&& obj);
Inserts an element into the container by performing emplace_hint(hint, std::forward<P>(value))
.
Only participates in overload resolution if std::is_constructible<value_type, P&&>::value
is true
.
hint
is a suggestion to where the element should be inserted.
Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Initializer List
void insert(std::initializer_list<value_type>);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
try_emplace
template<class... Args>
std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class K, class... Args>
std::pair<iterator, bool> try_emplace(K&& k, Args&&... args)
Inserts a new element into the container if there is no existing element with key k
contained within it.
If there is an existing element with key k
this function does nothing.
Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
This function is similiar to emplace except the
instead of emplace which simply forwards all arguments to Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. The 
try_emplace with Hint
template<class... Args>
iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
template<class K, class... Args>
iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
Inserts a new element into the container if there is no existing element with key k
contained within it.
If there is an existing element with key k
this function does nothing.
hint
is a suggestion to where the element should be inserted.
Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
This function is similiar to emplace_hint except the
instead of emplace_hint which simply forwards all arguments to The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. The 
insert_or_assign
template<class M>
std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class K, class M>
std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
Inserts a new element into the container or updates an existing one by assigning to the contained value.
If there is an element with key k
, then it is updated by assigning std::forward<M>(obj)
.
If there is no such element, it is added to the container as:
// first two overloads
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<Key>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
// third overload
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<K>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. The 
insert_or_assign with Hint
template<class M>
iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
template<class K, class M>
iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);
Inserts a new element into the container or updates an existing one by assigning to the contained value.
If there is an element with key k
, then it is updated by assigning std::forward<M>(obj)
.
If there is no such element, it is added to the container as:
// first two overloads
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<Key>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
// third overload
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<K>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
hint
is a suggestion to where the element should be inserted.
Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. The 
Extract by Iterator
node_type extract(const_iterator position);
Removes the element pointed to by position
.
Returns: 
A 
Notes: 
A node extracted using this method can be inserted into a compatible 
Extract by Key
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);
Removes an element with key equivalent to k
.
Returns: 
A 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
A node extracted using this method can be inserted into a compatible The 
Insert with node_handle
insert_return_type insert(node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
If Otherwise if there was already an element with an equivalent key, returns an Otherwise if the insertion succeeded, returns an 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
if and only if there is no element in the container with an equivalent key.
If there is already an element in the container with an equivalent key has no effect on nh
(i.e. nh
still contains the node.)
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If If there was already an element in the container with an equivalent key returns an iterator pointing to that. Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to hasher the function has no effect. 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);
Erase the element pointed to by position
.
Returns: 
The iterator following 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated. 
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);
Erase all elements with key equivalent to k
.
Returns: 
The number of elements erased. 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
The 
Erase Range
iterator erase(const_iterator first, const_iterator last);
Erases the elements in the range from first
to last
.
Returns: 
The iterator following the erased elements  i.e. 
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
quick_erase
void quick_erase(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
erase_return_void
void erase_return_void(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
swap
void swap(unordered_map& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_swappable_v<Hash> &&
boost::is_nothrow_swappable_v<Pred>);
Swaps the contents of the container with the parameter.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
clear
void clear();
Erases all elements in the container.
Postconditions: 

Throws: 
Never throws an exception. 
merge
template<class H2, class P2>
void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
template<class H2, class P2>
void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
Attempt to "merge" two containers by iterating source
and extracting any node in source
that is not contained
in *this
and then inserting it into *this
.
Because source
can have a different hash function and key equality predicate, the key of each node in
source
is rehashed using this>hash_function()
and then, if required, compared using this>key_eq()
.
The behavior of this function is undefined if this>get_allocator() != source.get_allocator()
.
This function does not copy or move any elements and instead simply relocates the nodes from source
into *this
.
Notes: 

Lookup
find
iterator find(const key_type& k);
const_iterator find(const key_type& k) const;
template<class K>
iterator find(const K& k);
template<class K>
const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq) const;
Returns: 
An iterator pointing to an element with key equivalent to 
Notes: 
The templated overloads containing The 
count
size_type count(const key_type& k) const;
template<class K>
size_type count(const K& k) const;
Returns: 
The number of elements with key equivalent to 
Notes: 
The 
contains
bool contains(const key_type& k) const;
template<class K>
bool contains(const K& k) const;
Returns: 
A boolean indicating whether or not there is an element with key equal to 
Notes: 
The 
equal_range
std::pair<iterator, iterator> equal_range(const key_type& k);
std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
template<class K>
std::pair<iterator, iterator> equal_range(const K& k);
template<class K>
std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns: 
A range containing all elements with key equivalent to 
Notes: 
The 
operator[]
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
template<class K> mapped_type& operator[](K&& k);
Effects: 
If the container does not already contain an elements with a key equivalent to 
Returns: 
A reference to 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. The 
at
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
template<class K> mapped_type& at(const K& k);
template<class K> const mapped_type& at(const K& k) const;
Returns: 
A reference to 
Throws: 
An exception object of type 
Notes: 
The 
Bucket Interface
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns: 
An upper bound on the number of buckets. 
bucket_size
size_type bucket_size(size_type n) const;
Requires: 

Returns: 
The number of elements in bucket 
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns: 
The index of the bucket which would contain an element with key 
Postconditions: 
The return value is less than 
Notes: 
The 
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the first element in the bucket with index 
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the 'one past the end' element in the bucket with index 
cbegin
const_local_iterator cbegin(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the first element in the bucket with index 
cend
const_local_iterator cend(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the 'one past the end' element in the bucket with index 
Hash Policy
max_load_factor
float max_load_factor() const noexcept;
Returns: 
Returns the current maximum load factor. 
Set max_load_factor
void max_load_factor(float z);
Effects: 
Changes the container’s maximum load factor, using 
rehash
void rehash(size_type n);
Changes the number of buckets so that there are at least n
buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count()
associated with the container.
When size() == 0
, rehash(0)
will deallocate the underlying buckets array.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
reserve
void reserve(size_type n);
Equivalent to a.rehash(ceil(n / a.max_load_factor()))
, or a.rehash(1)
if n > 0
and a.max_load_factor() == std::numeric_limits<float>::infinity()
.
Similar to rehash
, this function can be used to grow or shrink the number of buckets in the container.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
Deduction Guides
A deduction guide will not participate in overload resolution if any of the following are true:

It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter. 
It has an
Allocator
template parameter and a type that does not qualify as an allocator is deduced for that parameter. 
It has a
Hash
template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. 
It has a
Pred
template parameter and a type that qualifies as an allocator is deduced for that parameter.
A size_type
parameter type in a deduction guide refers to the size_type
member type of the
container type deduced by the deduction guide. Its default value coincides with the default value
of the constructor selected.
itervaluetype
template<class InputIterator> using itervaluetype = typename std::iterator_traits<InputIterator>::value_type; // exposition only
iterkeytype
template<class InputIterator> using iterkeytype = std::remove_const_t< std::tuple_element_t<0, itervaluetype<InputIterator>>>; // exposition only
itermappedtype
template<class InputIterator> using itermappedtype = std::tuple_element_t<1, itervaluetype<InputIterator>>; // exposition only
itertoalloctype
template<class InputIterator> using itertoalloctype = std::pair< std::add_const_t<std::tuple_element_t<0, itervaluetype<InputIterator>>>, std::tuple_element_t<1, itervaluetype<InputIterator>>>; // exposition only
Equality Comparisons
operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
const unordered_map<Key, T, Hash, Pred, Alloc>& y);
Return true
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& x,
const unordered_map<Key, T, Hash, Pred, Alloc>& y);
Return false
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
Swap
template<class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
unordered_map<Key, T, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Swaps the contents of x
and y
.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Effects: 

Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
erase_if
template<class K, class T, class H, class P, class A, class Predicate>
typename unordered_map<K, T, H, P, A>::size_type
erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);
Traverses the container c
and removes all elements for which the supplied predicate returns true
.
Returns: 
The number of erased elements. 
Notes: 
Equivalent to:

Serialization
unordered_map
s can be archived/retrieved by means of
Boost.Serialization using the API provided
by this library. Both regular and XML archives are supported.
Saving an unordered_map to an archive
Saves all the elements of an unordered_map
x
to an archive (XML archive) ar
.
Requires: 

Loading an unordered_map from an archive
Deletes all preexisting elements of an unordered_map
x
and inserts
from an archive (XML archive) ar
restored copies of the elements of the
original unordered_map
other
saved to the storage read by ar
.
Requires: 

Note: 
If the archive was saved using a release of Boost prior to Boost 1.84,
the configuration macro 
Saving an iterator/const_iterator to an archive
Saves the positional information of an iterator
(const_iterator
) it
to an archive (XML archive) ar
. it
can be and end()
iterator.
Requires: 
The 
Loading an iterator/const_iterator from an archive
Makes an iterator
(const_iterator
) it
point to the restored position of
the original iterator
(const_iterator
) saved to the storage read by
an archive (XML archive) ar
.
Requires: 
If 
Class Template unordered_multimap
boost::unordered_multimap
— An unordered associative container that associates keys with another value. The same key can be stored multiple times.
Synopsis
// #include <boost/unordered/unordered_map.hpp> namespace boost { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> class unordered_multimap { public: // types using key_type = Key; using mapped_type = T; using value_type = std::pair<const Key, T>; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename std::allocator_traits<Allocator>::pointer; using const_pointer = typename std::allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = std::size_t; using difference_type = std::ptrdiff_t; using iterator = implementationdefined; using const_iterator = implementationdefined; using local_iterator = implementationdefined; using const_local_iterator = implementationdefined; using node_type = implementationdefined; // construct/copy/destroy unordered_multimap(); explicit unordered_multimap(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(const unordered_multimap& other); unordered_multimap(unordered_multimap&& other); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a); explicit unordered_multimap(const Allocator& a); unordered_multimap(const unordered_multimap& other, const Allocator& a); unordered_multimap(unordered_multimap&& other, const Allocator& a); unordered_multimap(std::initializer_list<value_type> il, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(size_type n, const allocator_type& a); unordered_multimap(size_type n, const hasher& hf, const allocator_type& a); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a); unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a); unordered_multimap(std::initializer_list<value_type> il, size_type n, const allocator_type& a); unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ~unordered_multimap(); unordered_multimap& operator=(const unordered_multimap& other); unordered_multimap& operator=(unordered_multimap&& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_move_assignable_v<Hash> && boost::is_nothrow_move_assignable_v<Pred>); unordered_multimap& operator=(std::initializer_list<value_type> il); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& obj); iterator insert(value_type&& obj); template<class P> iterator insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class P> iterator insert(const_iterator hint, P&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); void insert(std::initializer_list<value_type> il); node_type extract(const_iterator position); node_type extract(const key_type& k); template<class K> node_type extract(K&& k); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& k); iterator erase(const_iterator first, const_iterator last); void quick_erase(const_iterator position); void erase_return_void(const_iterator position); void swap(unordered_multimap& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_swappable_v<Hash> && boost::is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // map operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq); template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> const_iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; // Deduction Guides template<class InputIterator, class Hash = boost::hash<iterkeytype<InputIterator>>, class Pred = std::equal_to<iterkeytype<InputIterator>>, class Allocator = std::allocator<itertoalloctype<InputIterator>>> unordered_multimap(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_multimap<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_multimap<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Allocator) > unordered_multimap<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, Allocator) > unordered_multimap<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) > unordered_multimap<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Allocator) > unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(std::initializer_list<std::pair<Key, T>>, Allocator) > unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Hash, Allocator) > unordered_multimap<Key, T, Hash, std::equal_to<Key>, Allocator>; // Equality Comparisons template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x, const unordered_multimap<Key, T, Hash, Pred, Alloc>& y); template<class Key, class T, class Hash, class Pred, class Alloc> bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x, const unordered_multimap<Key, T, Hash, Pred, Alloc>& y); // swap template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x, unordered_multimap<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // Erasure template<class K, class T, class H, class P, class A, class Predicate> typename unordered_multimap<K, T, H, P, A>::size_type erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred); // Pmr aliases (C++17 and up) namespace unordered::pmr { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>> using unordered_multimap = boost::unordered_multimap<Key, T, Hash, Pred, std::pmr::polymorphic_allocator<std::pair<const Key, T>>>; } }
Description
Template Parameters
Key 

T 

Hash 
A unary function object type that acts a hash function for a 
Pred 
A binary function object that implements an equivalence relation on values of type 
Allocator 
An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported. 
The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.
The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.
Configuration macros
BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0
Globally define this macro to support loading of unordered_multimap
s saved to
a Boost.Serialization archive with a version of Boost prior to Boost 1.84.
Typedefs
typedef implementationdefined iterator;
An iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Convertible to const_iterator
.
typedef implementationdefined const_iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
typedef implementationdefined local_iterator;
An iterator with the same value type, difference type and pointer and reference type as iterator.
A local_iterator
object can be used to iterate through a single bucket.
typedef implementationdefined const_local_iterator;
A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.
A const_local_iterator object can be used to iterate through a single bucket.
typedef implementationdefined node_type;
See node_handle_map for details.
Constructors
Default Constructor
unordered_multimap();
Constructs an empty container using hasher()
as the hash function,
key_equal()
as the key equality predicate, allocator_type()
as the allocator
and a maximum load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Bucket Count Constructor
explicit unordered_multimap(size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash
function, eql
as the key equality predicate, a
as the allocator and a maximum
load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Iterator Range Constructor
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 
If the defaults are used, 
Copy Constructor
unordered_multimap(const unordered_multimap& other);
The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.
If Allocator::select_on_container_copy_construction
exists and has the right signature, the allocator will be constructed from its result.
Requires: 

Move Constructor
unordered_multimap(unordered_multimap&& other);
The move constructor.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Iterator Range Constructor with Allocator
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);
Constructs an empty container using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Allocator Constructor
explicit unordered_multimap(const Allocator& a);
Constructs an empty container, using allocator a
.
Copy Constructor with Allocator
unordered_multimap(const unordered_multimap& other, const Allocator& a);
Constructs an container, copying other
's contained elements, hash function, predicate, maximum load factor, but using allocator a
.
Move Constructor with Allocator
unordered_multimap(unordered_multimap&& other, const Allocator& a);
Construct a container moving other
's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a
.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Initializer List Constructor
unordered_multimap(std::initializer_list<value_type> il,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from il
into it.
Requires: 
If the defaults are used, 
Bucket Count Constructor with Allocator
unordered_multimap(size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default hash function and key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Bucket Count Constructor with Hasher and Allocator
unordered_multimap(size_type n, const hasher& hf, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator, with the default key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

initializer_list Constructor with Allocator
unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a);
Constructs an empty container using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Allocator
unordered_multimap(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

Destructor
~unordered_multimap();
Note: 
The destructor is applied to every element, and all memory is deallocated 
Assignment
Copy Assignment
unordered_multimap& operator=(const unordered_multimap& other);
The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.
If Alloc::propagate_on_container_copy_assignment
exists and Alloc::propagate_on_container_copy_assignment::value
is true
, the allocator is overwritten, if not the copied elements are created using the existing allocator.
Requires: 

Move Assignment
unordered_multimap& operator=(unordered_multimap&& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_move_assignable_v<Hash> &&
boost::is_nothrow_move_assignable_v<Pred>);
The move assignment operator.
If Alloc::propagate_on_container_move_assignment
exists and Alloc::propagate_on_container_move_assignment::value
is true
, the allocator is overwritten, if not the moved elements are created using the existing allocator.
Requires: 

Initializer List Assignment
unordered_multimap& operator=(std::initializer_list<value_type> il);
Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.
Requires: 

Iterators
begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns: 
An iterator referring to the first element of the container, or if the container is empty the pasttheend value for the container. 
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns: 
An iterator which refers to the pasttheend value for the container. 
cbegin
const_iterator cbegin() const noexcept;
Returns: 
A 
cend
const_iterator cend() const noexcept;
Returns: 
A 
Modifiers
emplace
template<class... Args> iterator emplace(Args&&... args);
Inserts an object, constructed with the arguments args
, in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Inserts an object, constructed with the arguments args, in the container.
position
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Copy Insert
iterator insert(const value_type& obj);
Inserts obj
in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert
iterator insert(value_type&& obj);
Inserts obj
in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Emplace Insert
template<class P> iterator insert(P&& obj);
Inserts an element into the container by performing emplace(std::forward<P>(value))
.
Only participates in overload resolution if std::is_constructible<value_type, P&&>::value
is true
.
Returns: 
An iterator pointing to the inserted element. 
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
Inserts obj
in the container.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
Inserts obj
in the container.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Emplace Insert with Hint
template<class P> iterator insert(const_iterator hint, P&& obj);
Inserts an element into the container by performing emplace_hint(hint, std::forward<P>(value))
.
Only participates in overload resolution if std::is_constructible<value_type, P&&>::value
is true
.
hint
is a suggestion to where the element should be inserted.
Returns: 
An iterator pointing to the inserted element. 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);
Inserts a range of elements into the container.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Initializer List
void insert(std::initializer_list<value_type> il);
Inserts a range of elements into the container.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Extract by Iterator
node_type extract(const_iterator position);
Removes the element pointed to by position
.
Returns: 
A 
Notes: 
A node extracted using this method can be inserted into a compatible 
Extract by Key
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);
Removes an element with key equivalent to k
.
Returns: 
A 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
A node extracted using this method can be inserted into a compatible The 
Insert with node_handle
iterator insert(node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
.
Requires: 

Returns: 
If Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to hasher the function has no effect. 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);
Erase the element pointed to by position
.
Returns: 
The iterator following 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated. 
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);
Erase all elements with key equivalent to k
.
Returns: 
The number of elements erased. 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
The 
Erase Range
iterator erase(const_iterator first, const_iterator last);
Erases the elements in the range from first
to last
.
Returns: 
The iterator following the erased elements  i.e. 
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
quick_erase
void quick_erase(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
erase_return_void
void erase_return_void(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
swap
void swap(unordered_multimap& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_swappable_v<Hash> &&
boost::is_nothrow_swappable_v<Pred>);
Swaps the contents of the container with the parameter.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
clear
void clear() noexcept;
Erases all elements in the container.
Postconditions: 

Throws: 
Never throws an exception. 
merge
template<class H2, class P2>
void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
template<class H2, class P2>
void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
Attempt to "merge" two containers by iterating source
and extracting all nodes in source
and inserting them into *this
.
Because source
can have a different hash function and key equality predicate, the key of each node in
source
is rehashed using this>hash_function()
and then, if required, compared using this>key_eq()
.
The behavior of this function is undefined if this>get_allocator() != source.get_allocator()
.
This function does not copy or move any elements and instead simply relocates the nodes from source
into *this
.
Notes: 

Lookup
find
iterator find(const key_type& k);
const_iterator find(const key_type& k) const;
template<class K>
iterator find(const K& k);
template<class K>
const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq) const;
Returns: 
An iterator pointing to an element with key equivalent to 
Notes: 
The templated overloads containing The 
count
size_type count(const key_type& k) const;
template<class K>
size_type count(const K& k) const;
Returns: 
The number of elements with key equivalent to 
Notes: 
The 
contains
bool contains(const key_type& k) const;
template<class K>
bool contains(const K& k) const;
Returns: 
A boolean indicating whether or not there is an element with key equal to 
Notes: 
The 
equal_range
std::pair<iterator, iterator> equal_range(const key_type& k);
std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
template<class K>
std::pair<iterator, iterator> equal_range(const K& k);
template<class K>
std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns: 
A range containing all elements with key equivalent to 
Notes: 
The 
Bucket Interface
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns: 
An upper bound on the number of buckets. 
bucket_size
size_type bucket_size(size_type n) const;
Requires: 

Returns: 
The number of elements in bucket 
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns: 
The index of the bucket which would contain an element with key 
Postconditions: 
The return value is less than 
Notes: 
The 
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the first element in the bucket with index 
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the 'one past the end' element in the bucket with index 
cbegin
const_local_iterator cbegin(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the first element in the bucket with index 
cend
const_local_iterator cend(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the 'one past the end' element in the bucket with index 
Hash Policy
max_load_factor
float max_load_factor() const noexcept;
Returns: 
Returns the current maximum load factor. 
Set max_load_factor
void max_load_factor(float z);
Effects: 
Changes the container’s maximum load factor, using 
rehash
void rehash(size_type n);
Changes the number of buckets so that there are at least n
buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count()
associated with the container.
When size() == 0
, rehash(0)
will deallocate the underlying buckets array.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
reserve
void reserve(size_type n);
Equivalent to a.rehash(ceil(n / a.max_load_factor()))
, or a.rehash(1)
if n > 0
and a.max_load_factor() == std::numeric_limits<float>::infinity()
.
Similar to rehash
, this function can be used to grow or shrink the number of buckets in the container.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
Deduction Guides
A deduction guide will not participate in overload resolution if any of the following are true:

It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter. 
It has an
Allocator
template parameter and a type that does not qualify as an allocator is deduced for that parameter. 
It has a
Hash
template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. 
It has a
Pred
template parameter and a type that qualifies as an allocator is deduced for that parameter.
A size_type
parameter type in a deduction guide refers to the size_type
member type of the
container type deduced by the deduction guide. Its default value coincides with the default value
of the constructor selected.
itervaluetype
template<class InputIterator> using itervaluetype = typename std::iterator_traits<InputIterator>::value_type; // exposition only
iterkeytype
template<class InputIterator> using iterkeytype = std::remove_const_t< std::tuple_element_t<0, itervaluetype<InputIterator>>>; // exposition only
itermappedtype
template<class InputIterator> using itermappedtype = std::tuple_element_t<1, itervaluetype<InputIterator>>; // exposition only
itertoalloctype
template<class InputIterator> using itertoalloctype = std::pair< std::add_const_t<std::tuple_element_t<0, itervaluetype<InputIterator>>>, std::tuple_element_t<1, itervaluetype<InputIterator>>>; // exposition only
Equality Comparisons
operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
Return true
if x.size() == y.size()
and for every equivalent key group in x
, there is a group in y
for the same key, which is a permutation (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);
Return false
if x.size() == y.size()
and for every equivalent key group in x
, there is a group in y
for the same key, which is a permutation (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
Swap
template<class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Swaps the contents of x
and y
.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Effects: 

Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
erase_if
template<class K, class T, class H, class P, class A, class Predicate>
typename unordered_multimap<K, T, H, P, A>::size_type
erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);
Traverses the container c
and removes all elements for which the supplied predicate returns true
.
Returns: 
The number of erased elements. 
Notes: 
Equivalent to:

Serialization
unordered_multimap
s can be archived/retrieved by means of
Boost.Serialization using the API provided
by this library. Both regular and XML archives are supported.
Saving an unordered_multimap to an archive
Saves all the elements of an unordered_multimap
x
to an archive (XML archive) ar
.
Requires: 

Loading an unordered_multimap from an archive
Deletes all preexisting elements of an unordered_multimap
x
and inserts
from an archive (XML archive) ar
restored copies of the elements of the
original unordered_multimap
other
saved to the storage read by ar
.
Requires: 

Note: 
If the archive was saved using a release of Boost prior to Boost 1.84,
the configuration macro 
Saving an iterator/const_iterator to an archive
Saves the positional information of an iterator
(const_iterator
) it
to an archive (XML archive) ar
. it
can be and end()
iterator.
Requires: 
The 
Loading an iterator/const_iterator from an archive
Makes an iterator
(const_iterator
) it
point to the restored position of
the original iterator
(const_iterator
) saved to the storage read by
an archive (XML archive) ar
.
Requires: 
If 
Class Template unordered_set
boost::unordered_set
— An unordered associative container that stores unique values.
Synopsis
// #include <boost/unordered/unordered_set.hpp> namespace boost { template<class Key, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key>> class unordered_set { public: // types using key_type = Key; using value_type = Key; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename std::allocator_traits<Allocator>::pointer; using const_pointer = typename std::allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = std::size_t; using difference_type = std::ptrdiff_t; using iterator = implementationdefined; using const_iterator = implementationdefined; using local_iterator = implementationdefined; using const_local_iterator = implementationdefined; using node_type = implementationdefined; using insert_return_type = implementationdefined; // construct/copy/destroy unordered_set(); explicit unordered_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(const unordered_set& other); unordered_set(unordered_set&& other); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, const allocator_type& a); explicit unordered_set(const Allocator& a); unordered_set(const unordered_set& other, const Allocator& a); unordered_set(unordered_set&& other, const Allocator& a); unordered_set(std::initializer_list<value_type> il, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(size_type n, const allocator_type& a); unordered_set(size_type n, const hasher& hf, const allocator_type& a); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a); unordered_set(std::initializer_list<value_type> il, const allocator_type& a); unordered_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a); unordered_set(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ~unordered_set(); unordered_set& operator=(const unordered_set& other); unordered_set& operator=(unordered_set&& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_move_assignable_v<Hash> && boost::is_nothrow_move_assignable_v<Pred>); unordered_set& operator=(std::initializer_list<value_type> il); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> std::pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); std::pair<iterator, bool> insert(const value_type& obj); std::pair<iterator, bool> insert(value_type&& obj); template<class K> std::pair<iterator, bool> insert(K&& k); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class K> iterator insert(const_iterator hint, K&& k); template<class InputIterator> void insert(InputIterator first, InputIterator last); void insert(std::initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& k); template<class K> node_type extract(K&& k); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& k); iterator erase(const_iterator first, const_iterator last); void quick_erase(const_iterator position); void erase_return_void(const_iterator position); void swap(unordered_set& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_swappable_v<Hash> && boost::is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // set operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq); template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> const_iterator find(CompatibleKey const& k, CompatibleHash const& hash, CompatiblePredicate const& eq) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; // Deduction Guides template<class InputIterator, class Hash = boost::hash<itervaluetype<InputIterator>>, class Pred = std::equal_to<itervaluetype<InputIterator>>, class Allocator = std::allocator<itervaluetype<InputIterator>>> unordered_set(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_set<itervaluetype<InputIterator>, Hash, Pred, Allocator>; template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>, class Allocator = std::allocator<T>> unordered_set(std::initializer_list<T>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_set<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_set(InputIterator, InputIterator, typename see below::size_type, Allocator) > unordered_set<itervaluetype<InputIterator>, boost::hash<itervaluetype<InputIterator>>, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_set(InputIterator, InputIterator, Allocator) > unordered_set<itervaluetype<InputIterator>, boost::hash<itervaluetype<InputIterator>>, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_set(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) > unordered_set<itervaluetype<InputIterator>, Hash, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class T, class Allocator> unordered_set(std::initializer_list<T>, typename see below::size_type, Allocator) > unordered_set<T, boost::hash<T>, std::equal_to<T>, Allocator>; template<class T, class Allocator> unordered_set(std::initializer_list<T>, Allocator) > unordered_set<T, boost::hash<T>, std::equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_set(std::initializer_list<T>, typename see below::size_type, Hash, Allocator) > unordered_set<T, Hash, std::equal_to<T>, Allocator>; // Equality Comparisons template<class Key, class Hash, class Pred, class Alloc> bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& x, const unordered_set<Key, Hash, Pred, Alloc>& y); template<class Key, class Hash, class Pred, class Alloc> bool operator!=(const unordered_set<Key, Hash, Pred, Alloc>& x, const unordered_set<Key, Hash, Pred, Alloc>& y); // swap template<class Key, class Hash, class Pred, class Alloc> void swap(unordered_set<Key, Hash, Pred, Alloc>& x, unordered_set<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // Erasure template<class K, class H, class P, class A, class Predicate> typename unordered_set<K, H, P, A>::size_type erase_if(unordered_set<K, H, P, A>& c, Predicate pred); // Pmr aliases (C++17 and up) namespace unordered::pmr { template<class Key, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>> using unordered_set = boost::unordered_set<Key, Hash, Pred, std::pmr::polymorphic_allocator<Key>>; } }
Description
Template Parameters
Key 

Hash 
A unary function object type that acts a hash function for a 
Pred 
A binary function object that implements an equivalence relation on values of type 
Allocator 
An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported. 
The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.
The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.
Configuration macros
BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0
Globally define this macro to support loading of unordered_set
s saved to
a Boost.Serialization archive with a version of Boost prior to Boost 1.84.
Typedefs
typedef implementationdefined iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Convertible to const_iterator
.
typedef implementationdefined const_iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
typedef implementationdefined local_iterator;
An iterator with the same value type, difference type and pointer and reference type as iterator.
A local_iterator
object can be used to iterate through a single bucket.
typedef implementationdefined const_local_iterator;
A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.
A const_local_iterator object can be used to iterate through a single bucket.
typedef implementationdefined node_type;
A class for holding extracted container elements, modelling NodeHandle.
typedef implementationdefined insert_return_type;
A specialization of an internal class template:
template<class Iterator, class NodeType>
struct insert_return_type // name is exposition only
{
Iterator position;
bool inserted;
NodeType node;
};
with Iterator
= iterator
and NodeType
= node_type
.
Constructors
Default Constructor
unordered_set();
Constructs an empty container using hasher()
as the hash function,
key_equal()
as the key equality predicate, allocator_type()
as the allocator
and a maximum load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Bucket Count Constructor
explicit unordered_set(size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash
function, eql
as the key equality predicate, a
as the allocator and a maximum
load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Iterator Range Constructor
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 
If the defaults are used, 
Copy Constructor
unordered_set(const unordered_set& other);
The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.
If Allocator::select_on_container_copy_construction
exists and has the right signature, the allocator will be constructed from its result.
Requires: 

Move Constructor
unordered_set(unordered_set&& other);
The move constructor.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Iterator Range Constructor with Allocator
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, const allocator_type& a);
Constructs an empty container using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Allocator Constructor
explicit unordered_set(const Allocator& a);
Constructs an empty container, using allocator a
.
Copy Constructor with Allocator
unordered_set(const unordered_set& other, const Allocator& a);
Constructs an container, copying other
's contained elements, hash function, predicate, maximum load factor, but using allocator a
.
Move Constructor with Allocator
unordered_set(unordered_set&& other, const Allocator& a);
Construct a container moving other
's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a
.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Initializer List Constructor
unordered_set(std::initializer_list<value_type> il,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from il
into it.
Requires: 
If the defaults are used, 
Bucket Count Constructor with Allocator
unordered_set(size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default hash function and key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Bucket Count Constructor with Hasher and Allocator
unordered_set(size_type n, const hasher& hf, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator, with the default key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

initializer_list Constructor with Allocator
unordered_set(std::initializer_list<value_type> il, const allocator_type& a);
Constructs an empty container using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Allocator
unordered_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_set(std::initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

Destructor
~unordered_set();
Note: 
The destructor is applied to every element, and all memory is deallocated 
Assignment
Copy Assignment
unordered_set& operator=(const unordered_set& other);
The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.
If Alloc::propagate_on_container_copy_assignment
exists and Alloc::propagate_on_container_copy_assignment::value
is true
, the allocator is overwritten, if not the copied elements are created using the existing allocator.
Requires: 

Move Assignment
unordered_set& operator=(unordered_set&& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_move_assignable_v<Hash> &&
boost::is_nothrow_move_assignable_v<Pred>);
The move assignment operator.
If Alloc::propagate_on_container_move_assignment
exists and Alloc::propagate_on_container_move_assignment::value
is true
, the allocator is overwritten, if not the moved elements are created using the existing allocator.
Requires: 

Initializer List Assignment
unordered_set& operator=(std::initializer_list<value_type> il);
Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.
Requires: 

Iterators
begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns: 
An iterator referring to the first element of the container, or if the container is empty the pasttheend value for the container. 
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns: 
An iterator which refers to the pasttheend value for the container. 
cbegin
const_iterator cbegin() const noexcept;
Returns: 
A 
cend
const_iterator cend() const noexcept;
Returns: 
A 
Modifiers
emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent value.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent value. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent value.
position
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Transparent Insert
template<class K> std::pair<iterator, bool> insert(K&& k);
Inserts an element constructed from std::forward<K>(k)
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The bool component of the return type is true if an insert took place. If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This overload only participates in overload resolution if 
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Transparent Insert with Hint
template<class K> iterator insert(const_iterator hint, K&& k);
Inserts an element constructed from std::forward<K>(k)
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This overload only participates in overload resolution if 
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Initializer List
void insert(std::initializer_list<value_type>);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Extract by Iterator
node_type extract(const_iterator position);
Removes the element pointed to by position
.
Returns: 
A 
Notes: 
In C++17 a node extracted using this method can be inserted into a compatible 
Extract by Value
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);
Removes an element with key equivalent to k
.
Returns: 
A 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
In C++17 a node extracted using this method can be inserted into a compatible The 
Insert with node_handle
insert_return_type insert(node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
If Otherwise if there was already an element with an equivalent key, returns an Otherwise if the insertion succeeded, returns an 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. In C++17 this can be used to insert a node extracted from a compatible 
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
if and only if there is no element in the container with an equivalent key.
If there is already an element in the container with an equivalent key has no effect on nh
(i.e. nh
still contains the node.)
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If If there was already an element in the container with an equivalent key returns an iterator pointing to that. Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);
Erase the element pointed to by position
.
Returns: 
The iterator following 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated. 
Erase by Value
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);
Erase all elements with key equivalent to k
.
Returns: 
The number of elements erased. 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
The 
Erase Range
iterator erase(const_iterator first, const_iterator last);
Erases the elements in the range from first
to last
.
Returns: 
The iterator following the erased elements  i.e. 
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
quick_erase
void quick_erase(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
erase_return_void
void erase_return_void(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
swap
void swap(unordered_set& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_swappable_v<Hash> &&
boost::is_nothrow_swappable_v<Pred>);
Swaps the contents of the container with the parameter.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
clear
void clear() noexcept;
Erases all elements in the container.
Postconditions: 

Throws: 
Never throws an exception. 
merge
template<class H2, class P2>
void merge(unordered_set<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_set<Key, H2, P2, Allocator>&& source);
template<class H2, class P2>
void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
Attempt to "merge" two containers by iterating source
and extracting any node in source
that is not contained
in *this
and then inserting it into *this
.
Because source
can have a different hash function and key equality predicate, the key of each node in
source
is rehashed using this>hash_function()
and then, if required, compared using this>key_eq()
.
The behavior of this function is undefined if this>get_allocator() != source.get_allocator()
.
This function does not copy or move any elements and instead simply relocates the nodes from source
into *this
.
Notes: 

Lookup
find
iterator find(const key_type& k);
const_iterator find(const key_type& k) const;
template<class K>
iterator find(const K& k);
template<class K>
const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
CompatiblePredicate const& eq) const;
Returns: 
An iterator pointing to an element with key equivalent to 
Notes: 
The templated overloads containing The 
count
size_type count(const key_type& k) const;
template<class K>
size_type count(const K& k) const;
Returns: 
The number of elements with key equivalent to 
Notes: 
The 
contains
bool contains(const key_type& k) const;
template<class K>
bool contains(const K& k) const;
Returns: 
A boolean indicating whether or not there is an element with key equal to 
Notes: 
The 
equal_range
std::pair<iterator, iterator> equal_range(const key_type& k);
std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
template<class K>
std::pair<iterator, iterator> equal_range(const K& k);
template<class K>
std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns: 
A range containing all elements with key equivalent to 
Notes: 
The 
Bucket Interface
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns: 
An upper bound on the number of buckets. 
bucket_size
size_type bucket_size(size_type n) const;
Requires: 

Returns: 
The number of elements in bucket 
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns: 
The index of the bucket which would contain an element with key 
Postconditions: 
The return value is less than 
Notes: 
The 
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the first element in the bucket with index 
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the 'one past the end' element in the bucket with index 
cbegin
const_local_iterator cbegin(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the first element in the bucket with index 
cend
const_local_iterator cend(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the 'one past the end' element in the bucket with index 
Hash Policy
max_load_factor
float max_load_factor() const noexcept;
Returns: 
Returns the current maximum load factor. 
Set max_load_factor
void max_load_factor(float z);
Effects: 
Changes the container’s maximum load factor, using 
rehash
void rehash(size_type n);
Changes the number of buckets so that there are at least n
buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count()
associated with the container.
When size() == 0
, rehash(0)
will deallocate the underlying buckets array.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
reserve
void reserve(size_type n);
Equivalent to a.rehash(ceil(n / a.max_load_factor()))
, or a.rehash(1)
if n > 0
and a.max_load_factor() == std::numeric_limits<float>::infinity()
.
Similar to rehash
, this function can be used to grow or shrink the number of buckets in the container.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
Deduction Guides
A deduction guide will not participate in overload resolution if any of the following are true:

It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter. 
It has an
Allocator
template parameter and a type that does not qualify as an allocator is deduced for that parameter. 
It has a
Hash
template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. 
It has a
Pred
template parameter and a type that qualifies as an allocator is deduced for that parameter.
A size_type
parameter type in a deduction guide refers to the size_type
member type of the
container type deduced by the deduction guide. Its default value coincides with the default value
of the constructor selected.
itervaluetype
template<class InputIterator> using itervaluetype = typename std::iterator_traits<InputIterator>::value_type; // exposition only
Equality Comparisons
operator==
template<class Key, class Hash, class Pred, class Alloc>
bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& x,
const unordered_set<Key, Hash, Pred, Alloc>& y);
Return true
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
operator!=
template<class Key, class Hash, class Pred, class Alloc>
bool operator!=(const unordered_set<Key, Hash, Pred, Alloc>& x,
const unordered_set<Key, Hash, Pred, Alloc>& y);
Return false
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
Swap
template<class Key, class Hash, class Pred, class Alloc>
void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
unordered_set<Key, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Swaps the contents of x
and y
.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Effects: 

Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
erase_if
template<class K, class H, class P, class A, class Predicate>
typename unordered_set<K, H, P, A>::size_type
erase_if(unordered_set<K, H, P, A>& c, Predicate pred);
Traverses the container c
and removes all elements for which the supplied predicate returns true
.
Returns: 
The number of erased elements. 
Notes: 
Equivalent to:

Serialization
unordered_set
s can be archived/retrieved by means of
Boost.Serialization using the API provided
by this library. Both regular and XML archives are supported.
Saving an unordered_set to an archive
Saves all the elements of an unordered_set
x
to an archive (XML archive) ar
.
Requires: 

Loading an unordered_set from an archive
Deletes all preexisting elements of an unordered_set
x
and inserts
from an archive (XML archive) ar
restored copies of the elements of the
original unordered_set
other
saved to the storage read by ar
.
Requires: 

Note: 
If the archive was saved using a release of Boost prior to Boost 1.84,
the configuration macro 
Saving an iterator/const_iterator to an archive
Saves the positional information of an iterator
(const_iterator
) it
to an archive (XML archive) ar
. it
can be and end()
iterator.
Requires: 
The 
Loading an iterator/const_iterator from an archive
Makes an iterator
(const_iterator
) it
point to the restored position of
the original iterator
(const_iterator
) saved to the storage read by
an archive (XML archive) ar
.
Requires: 
If 
Class Template unordered_multiset
boost::unordered_multiset
— An unordered associative container that stores values. The same key can be stored multiple times.
Synopsis
// #include <boost/unordered/unordered_set.hpp> namespace boost { template<class Key, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key>> class unordered_multiset { public: // types using key_type = Key; using value_type = Key; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename std::allocator_traits<Allocator>::pointer; using const_pointer = typename std::allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = std::size_t; using difference_type = std::ptrdiff_t; using iterator = implementationdefined; using const_iterator = implementationdefined; using local_iterator = implementationdefined; using const_local_iterator = implementationdefined; using node_type = implementationdefined; // construct/copy/destroy unordered_multiset(); explicit unordered_multiset(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(const unordered_multiset& other); unordered_multiset(unordered_multiset&& other); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a); explicit unordered_multiset(const Allocator& a); unordered_multiset(const unordered_multiset& other, const Allocator& a); unordered_multiset(unordered_multiset&& other, const Allocator& a); unordered_multiset(std::initializer_list<value_type> il, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(size_type n, const allocator_type& a); unordered_multiset(size_type n, const hasher& hf, const allocator_type& a); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a); unordered_multiset(std::initializer_list<value_type> il, const allocator_type& a); unordered_multiset(std::initializer_list<value_type> il, size_type n, const allocator_type& a) unordered_multiset(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ~unordered_multiset(); unordered_multiset& operator=(const unordered_multiset& other); unordered_multiset& operator=(unordered_multiset&& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_move_assignable_v<Hash> && boost::is_nothrow_move_assignable_v<Pred>); unordered_multiset& operator=(std::initializer_list<value_type> il); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& obj); iterator insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); void insert(std::initializer_list<value_type> il); node_type extract(const_iterator position); node_type extract(const key_type& k); template<class K> node_type extract(K&& k); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void quick_erase(const_iterator position); void erase_return_void(const_iterator position); void swap(unordered_multiset&) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value && boost::is_nothrow_swappable_v<Hash> && boost::is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // set operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> iterator find(CompatibleKey const&, CompatibleHash const&, CompatiblePredicate const&); template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate> const_iterator find(CompatibleKey const&, CompatibleHash const&, CompatiblePredicate const&) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; // Deduction Guides template<class InputIterator, class Hash = boost::hash<itervaluetype<InputIterator>>, class Pred = std::equal_to<itervaluetype<InputIterator>>, class Allocator = std::allocator<itervaluetype<InputIterator>>> unordered_multiset(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_multiset<itervaluetype<InputIterator>, Hash, Pred, Allocator>; template<class T, class Hash = boost::hash<T>, class Pred = std::equal_to<T>, class Allocator = std::allocator<T>> unordered_multiset(std::initializer_list<T>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_multiset<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Allocator) > unordered_multiset<itervaluetype<InputIterator>, boost::hash<itervaluetype<InputIterator>>, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_multiset(InputIterator, InputIterator, Allocator) > unordered_multiset<itervaluetype<InputIterator>, boost::hash<itervaluetype<InputIterator>>, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) > unordered_multiset<itervaluetype<InputIterator>, Hash, std::equal_to<itervaluetype<InputIterator>>, Allocator>; template<class T, class Allocator> unordered_multiset(std::initializer_list<T>, typename see below::size_type, Allocator) > unordered_multiset<T, boost::hash<T>, std::equal_to<T>, Allocator>; template<class T, class Allocator> unordered_multiset(std::initializer_list<T>, Allocator) > unordered_multiset<T, boost::hash<T>, std::equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_multiset(std::initializer_list<T>, typename see below::size_type, Hash, Allocator) > unordered_multiset<T, Hash, std::equal_to<T>, Allocator>; // Equality Comparisons template<class Key, class Hash, class Pred, class Alloc> bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& x, const unordered_multiset<Key, Hash, Pred, Alloc>& y); template<class Key, class Hash, class Pred, class Alloc> bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& x, const unordered_multiset<Key, Hash, Pred, Alloc>& y); // swap template<class Key, class Hash, class Pred, class Alloc> void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x, unordered_multiset<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // Erasure template<class K, class H, class P, class A, class Predicate> typename unordered_multiset<K, H, P, A>::size_type erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred); // Pmr aliases (C++17 and up) namespace unordered::pmr { template<class Key, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>> using unordered_multiset = boost::unordered_multiset<Key, Hash, Pred, std::pmr::polymorphic_allocator<Key>>; } }
Description
Template Parameters
Key 

Hash 
A unary function object type that acts a hash function for a 
Pred 
A binary function object that implements an equivalence relation on values of type 
Allocator 
An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported. 
The elements are organized into buckets. Keys with the same hash code are stored in the same bucket and elements with equivalent keys are stored next to each other.
The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.
Configuration macros
BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0
Globally define this macro to support loading of unordered_multiset
s saved to
a Boost.Serialization archive with a version of Boost prior to Boost 1.84.
Typedefs
typedef implementationdefined iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Convertible to const_iterator
.
typedef implementationdefined const_iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
typedef implementationdefined local_iterator;
An iterator with the same value type, difference type and pointer and reference type as iterator.
A local_iterator
object can be used to iterate through a single bucket.
typedef implementationdefined const_local_iterator;
A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.
A const_local_iterator object can be used to iterate through a single bucket.
typedef implementationdefined node_type;
See node_handle_set for details.
Constructors
Default Constructor
unordered_multiset();
Constructs an empty container using hasher()
as the hash function,
key_equal()
as the key equality predicate, allocator_type()
as the allocator
and a maximum load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Bucket Count Constructor
explicit unordered_multiset(size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash
function, eql
as the key equality predicate, a
as the allocator and a maximum
load factor of 1.0
.
Postconditions: 

Requires: 
If the defaults are used, 
Iterator Range Constructor
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 
If the defaults are used, 
Copy Constructor
unordered_multiset(const unordered_multiset& other);
The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.
If Allocator::select_on_container_copy_construction
exists and has the right signature, the allocator will be constructed from its result.
Requires: 

Move Constructor
unordered_multiset(unordered_multiset&& other);
The move constructor.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Iterator Range Constructor with Allocator
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, const allocator_type& a);
Constructs an empty container using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Allocator Constructor
explicit unordered_multiset(const Allocator& a);
Constructs an empty container, using allocator a
.
Copy Constructor with Allocator
unordered_multiset(const unordered_multiset& other, const Allocator& a);
Constructs an container, copying other
's contained elements, hash function, predicate, maximum load factor, but using allocator a
.
Move Constructor with Allocator
unordered_multiset(unordered_multiset&& other, const Allocator& a);
Construct a container moving other
's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a
.
Notes: 
This is implemented using Boost.Move. 
Requires: 

Initializer List Constructor
unordered_multiset(std::initializer_list<value_type> il,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate, a
as the allocator and a maximum load factor of 1.0
and inserts the elements from il
into it.
Requires: 
If the defaults are used, 
Bucket Count Constructor with Allocator
unordered_multiset(size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default hash function and key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Bucket Count Constructor with Hasher and Allocator
unordered_multiset(size_type n, const hasher& hf, const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default key equality predicate, a
as the allocator and a maximum load factor of 1.0
.
Postconditions: 

Requires: 

Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator, with the default key equality predicate and a maximum load factor of 1.0
and inserts the elements from [f, l)
into it.
Requires: 

initializer_list Constructor with Allocator
unordered_multiset(std::initializer_list<value_type> il, const allocator_type& a);
Constructs an empty container using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Allocator
unordered_multiset(std::initializer_list<value_type> il, size_type n, const allocator_type& a)
Constructs an empty container with at least n
buckets, using a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_multiset(std::initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator and a maximum load factor of 1.0 and inserts the elements from il
into it.
Requires: 

Destructor
~unordered_multiset();
Note: 
The destructor is applied to every element, and all memory is deallocated 
Assignment
Copy Assignment
unordered_multiset& operator=(const unordered_multiset& other);
The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.
If Alloc::propagate_on_container_copy_assignment
exists and Alloc::propagate_on_container_copy_assignment::value
is true
, the allocator is overwritten, if not the copied elements are created using the existing allocator.
Requires: 

Move Assignment
unordered_multiset& operator=(unordered_multiset&& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_move_assignable_v<Hash> &&
boost::is_nothrow_move_assignable_v<Pred>);
The move assignment operator.
If Alloc::propagate_on_container_move_assignment
exists and Alloc::propagate_on_container_move_assignment::value
is true
, the allocator is overwritten, if not the moved elements are created using the existing allocator.
Requires: 

Initializer List Assignment
unordered_multiset& operator=(std::initializer_list<value_type> il);
Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.
Requires: 

Iterators
begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns: 
An iterator referring to the first element of the container, or if the container is empty the pasttheend value for the container. 
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns: 
An iterator which refers to the pasttheend value for the container. 
cbegin
const_iterator cbegin() const noexcept;
Returns: 
A 
cend
const_iterator cend() const noexcept;
Returns: 
A 
Modifiers
emplace
template<class... Args> iterator emplace(Args&&... args);
Inserts an object, constructed with the arguments args, in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Inserts an object, constructed with the arguments args, in the container.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Copy Insert
iterator insert(const value_type& obj);
Inserts obj
in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert
iterator insert(value_type&& obj);
Inserts obj
in the container.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
Inserts obj
in the container.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
Inserts obj
in the container.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
An iterator pointing to the inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);
Inserts a range of elements into the container.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Insert Initializer List
void insert(std::initializer_list<value_type> il);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. 
Extract by Iterator
node_type extract(const_iterator position);
Removes the element pointed to by position
.
Returns: 
A 
Notes: 
A node extracted using this method can be inserted into a compatible 
Extract by Value
node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);
Removes an element with key equivalent to k
.
Returns: 
A 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
A node extracted using this method can be inserted into a compatible The 
Insert with node_handle
iterator insert(node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
.
Requires: 

Returns: 
If Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Insert with Hint and node_handle
iterator insert(const_iterator hint, node_type&& nh);
If nh
is empty, has no effect.
Otherwise inserts the element owned by nh
.
hint
is a suggestion to where the element should be inserted.
Requires: 

Returns: 
If Otherwise returns an iterator pointing to the newly inserted element. 
Throws: 
If an exception is thrown by an operation other than a call to hasher the function has no effect. 
Notes: 
The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. Pointers and references to elements are never invalidated. This can be used to insert a node extracted from a compatible 
Erase by Position
iterator erase(iterator position);
iterator erase(const_iterator position);
Erase the element pointed to by position
.
Returns: 
The iterator following 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated. 
Erase by Value
size_type erase(const key_type& k);
template<class K> size_type erase(K&& x);
Erase all elements with key equivalent to k
.
Returns: 
The number of elements erased. 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
The 
Erase Range
iterator erase(const_iterator first, const_iterator last);
Erases the elements in the range from first
to last
.
Returns: 
The iterator following the erased elements  i.e. 
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
quick_erase
void quick_erase(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
erase_return_void
void erase_return_void(const_iterator position);
Erase the element pointed to by position
.
Throws: 
Only throws an exception if it is thrown by In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations. 
Notes: 
This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated. 
swap
void swap(unordered_multiset&)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
boost::is_nothrow_swappable_v<Hash> &&
boost::is_nothrow_swappable_v<Pred>);
Swaps the contents of the container with the parameter.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
clear
void clear() noexcept;
Erases all elements in the container.
Postconditions: 

Throws: 
Never throws an exception. 
merge
template<class H2, class P2>
void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
template<class H2, class P2>
void merge(unordered_set<Key, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_set<Key, H2, P2, Allocator>&& source);
Attempt to "merge" two containers by iterating source
and extracting all nodes in source
and inserting them into *this
.
Because source
can have a different hash function and key equality predicate, the key of each node in
source
is rehashed using this>hash_function()
and then, if required, compared using this>key_eq()
.
The behavior of this function is undefined if this>get_allocator() != source.get_allocator()
.
This function does not copy or move any elements and instead simply relocates the nodes from source
into *this
.
Notes: 

Lookup
find
iterator find(const key_type& k);
const_iterator find(const key_type& k) const;
template<class K>
iterator find(const K& k);
template<class K>
const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
iterator find(CompatibleKey const&, CompatibleHash const&,
CompatiblePredicate const&);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
const_iterator find(CompatibleKey const&, CompatibleHash const&,
CompatiblePredicate const&) const;
Returns: 
An iterator pointing to an element with key equivalent to 
Notes: 
The templated overloads containing The 
count
size_type count(const key_type& k) const;
template<class K>
size_type count(const K& k) const;
Returns: 
The number of elements with key equivalent to 
Notes: 
The 
contains
bool contains(const key_type& k) const;
template<class K>
bool contains(const K& k) const;
Returns: 
A boolean indicating whether or not there is an element with key equal to 
Notes: 
The 
equal_range
std::pair<iterator, iterator> equal_range(const key_type& k);
std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
template<class K>
std::pair<iterator, iterator> equal_range(const K& k);
template<class K>
std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns: 
A range containing all elements with key equivalent to 
Notes: 
The 
Bucket Interface
max_bucket_count
size_type max_bucket_count() const noexcept;
Returns: 
An upper bound on the number of buckets. 
bucket_size
size_type bucket_size(size_type n) const;
Requires: 

Returns: 
The number of elements in bucket 
bucket
size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns: 
The index of the bucket which would contain an element with key 
Postconditions: 
The return value is less than 
Notes: 
The 
begin
local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the first element in the bucket with index 
end
local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires: 

Returns: 
A local iterator pointing the 'one past the end' element in the bucket with index 
cbegin
const_local_iterator cbegin(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the first element in the bucket with index 
cend
const_local_iterator cend(size_type n) const;
Requires: 

Returns: 
A constant local iterator pointing the 'one past the end' element in the bucket with index 
Hash Policy
max_load_factor
float max_load_factor() const noexcept;
Returns: 
Returns the current maximum load factor. 
Set max_load_factor
void max_load_factor(float z);
Effects: 
Changes the container’s maximum load factor, using 
rehash
void rehash(size_type n);
Changes the number of buckets so that there are at least n
buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count()
associated with the container.
When size() == 0
, rehash(0)
will deallocate the underlying buckets array.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
reserve
void reserve(size_type n);
Equivalent to a.rehash(ceil(n / a.max_load_factor()))
, or a.rehash(1)
if n > 0
and a.max_load_factor() == std::numeric_limits<float>::infinity()
.
Similar to rehash
, this function can be used to grow or shrink the number of buckets in the container.
Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
Deduction Guides
A deduction guide will not participate in overload resolution if any of the following are true:

It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter. 
It has an
Allocator
template parameter and a type that does not qualify as an allocator is deduced for that parameter. 
It has a
Hash
template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. 
It has a
Pred
template parameter and a type that qualifies as an allocator is deduced for that parameter.
A size_type
parameter type in a deduction guide refers to the size_type
member type of the
container type deduced by the deduction guide. Its default value coincides with the default value
of the constructor selected.
itervaluetype
template<class InputIterator> using itervaluetype = typename std::iterator_traits<InputIterator>::value_type; // exposition only
Equality Comparisons
operator==
template<class Key, class Hash, class Pred, class Alloc>
bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
const unordered_multiset<Key, Hash, Pred, Alloc>& y);
Return true
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
operator!=
template<class Key, class Hash, class Pred, class Alloc>
bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& x,
const unordered_multiset<Key, Hash, Pred, Alloc>& y);
Return false
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
Swap
template<class Key, class Hash, class Pred, class Alloc>
void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
unordered_multiset<Key, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Swaps the contents of x
and y
.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Effects: 

Throws: 
Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of 
Notes: 
The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors. 
erase_if
template<class K, class H, class P, class A, class Predicate>
typename unordered_multiset<K, H, P, A>::size_type
erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);
Traverses the container c
and removes all elements for which the supplied predicate returns true
.
Returns: 
The number of erased elements. 
Notes: 
Equivalent to:

Serialization
unordered_multiset
s can be archived/retrieved by means of
Boost.Serialization using the API provided
by this library. Both regular and XML archives are supported.
Saving an unordered_multiset to an archive
Saves all the elements of an unordered_multiset
x
to an archive (XML archive) ar
.
Requires: 

Loading an unordered_multiset from an archive
Deletes all preexisting elements of an unordered_multiset
x
and inserts
from an archive (XML archive) ar
restored copies of the elements of the
original unordered_multiset
other
saved to the storage read by ar
.
Requires: 

Note: 
If the archive was saved using a release of Boost prior to Boost 1.84,
the configuration macro 
Saving an iterator/const_iterator to an archive
Saves the positional information of an iterator
(const_iterator
) it
to an archive (XML archive) ar
. it
can be and end()
iterator.
Requires: 
The 
Loading an iterator/const_iterator from an archive
Makes an iterator
(const_iterator
) it
point to the restored position of
the original iterator
(const_iterator
) saved to the storage read by
an archive (XML archive) ar
.
Requires: 
If 
Hash traits
Synopsis
// #include <boost/unordered/hash_traits.hpp> namespace boost { namespace unordered { template<typename Hash> struct hash_is_avalanching; } // namespace unordered } // namespace boost
hash_is_avalanching
template<typename Hash>
struct hash_is_avalanching;
A hash function is said to have the avalanching property if small changes in the input translate to large changes in the returned hash code —ideally, flipping one bit in the representation of the input value results in each bit of the hash code flipping with probability 50%. Approaching this property is critical for the proper behavior of openaddressing hash containers.
hash_is_avalanching<Hash>::value
is:

false
ifHash::is_avalanching
is not present, 
Hash::is_avalanching::value
if this is present and convertible at compile time to abool
, 
true
ifHash::is_avalanching
isvoid
(this usage is deprecated), 
illformed otherwise.
Users can then declare a hash function Hash
as avalanching either by embedding an appropriate is_avalanching
typedef
into the definition of Hash
, or directly by specializing hash_is_avalanching<Hash>
to a class with
an embedded compiletime constant value
set to true
.
Openaddressing and concurrent containers
use the provided hash function Hash
asis if hash_is_avalanching<Hash>::value
is true
; otherwise, they
implement a bitmixing postprocessing stage to increase the quality of hashing at the expense of
extra computational cost.
Statistics
Openaddressing and concurrent containers can be configured to keep running statistics of some internal operations affected by the quality of the supplied hash function.
Synopsis
struct statssummarytype { double average; double variance; double deviation; }; struct insertionstatstype { std::size_t count; statssummarytype probe_length; }; struct lookupstatstype { std::size_t count; statssummarytype probe_length; statssummarytype num_comparisons; }; struct statstype { insertionstatstype insertion; lookupstatstype successful_lookup, unsuccessful_lookup; };
statssummarytype
Provides the average value, variance and standard deviation of a sequence of numerical values.
insertionstatstype
Provides the number of insertion operations performed by a container and statistics on the associated probe length (number of bucket groups accessed per operation).
lookupstatstype
For successful (element found) or unsuccessful (not found) lookup, provides the number of operations performed by a container and statistics on the associated probe length (number of bucket groups accessed) and number of element comparisons per operation.
statstype
Provides statistics on insertion, successful and unsuccessful lookups performed by a container. If the supplied hash function has good quality, then:

Average probe lenghts should be close to 1.0.

For successful lookups, the average number of element comparisons should be close to 1.0.

For unsuccessful lookups, the average number of element comparisons should be close to 0.0.
These statistics can be used to determine if a given hash function can be marked as avalanching.
Class Template unordered_flat_map
boost::unordered_flat_map
— An openaddressing unordered associative container that associates unique keys with another value.
The performance of boost::unordered_flat_map
is much better than that of boost::unordered_map
or other implementations of std::unordered_map
. Unlike standard unordered associative containers,
which are nodebased, the elements of a boost::unordered_flat_map
are held directly in the bucket
array, and insertions into an already occupied bucket are diverted to available buckets in the
vicinity of the original position. This type of data layout is known as open addressing.
As a result of its using open addressing, the interface of boost::unordered_flat_map
deviates in
a number of aspects from that of boost::unordered_map
/std::unordered_map
:

value_type
must be moveconstructible. 
Pointer stability is not kept under rehashing.

begin()
is not constanttime. 
There is no API for bucket handling (except
bucket_count
) or node extraction/insertion. 
The maximum load factor of the container is managed internally and can’t be set by the user.
Other than this, boost::unordered_flat_map
is mostly a dropin replacement of nodebased standard
unordered associative containers.
Synopsis
// #include <boost/unordered/unordered_flat_map.hpp> namespace boost { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> class unordered_flat_map { public: // types using key_type = Key; using mapped_type = T; using value_type = std::pair<const Key, T>; using init_type = std::pair< typename std::remove_const<Key>::type, typename std::remove_const<T>::type >; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename std::allocator_traits<Allocator>::pointer; using const_pointer = typename std::allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = std::size_t; using difference_type = std::ptrdiff_t; using iterator = implementationdefined; using const_iterator = implementationdefined; using stats = statstype; // if statistics are enabled // construct/copy/destroy unordered_flat_map(); explicit unordered_flat_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_flat_map(InputIterator f, InputIterator l, size_type n = implementationdefined, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_flat_map(const unordered_flat_map& other); unordered_flat_map(unordered_flat_map&& other); template<class InputIterator> unordered_flat_map(InputIterator f, InputIterator l, const allocator_type& a); explicit unordered_flat_map(const Allocator& a); unordered_flat_map(const unordered_flat_map& other, const Allocator& a); unordered_flat_map(unordered_flat_map&& other, const Allocator& a); unordered_flat_map(concurrent_flat_map<Key, T, Hash, Pred, Allocator>&& other); unordered_flat_map(std::initializer_list<value_type> il, size_type n = implementationdefined const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_flat_map(size_type n, const allocator_type& a); unordered_flat_map(size_type n, const hasher& hf, const allocator_type& a); template<class InputIterator> unordered_flat_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a); template<class InputIterator> unordered_flat_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a); unordered_flat_map(std::initializer_list<value_type> il, const allocator_type& a); unordered_flat_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a); unordered_flat_map(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ~unordered_flat_map(); unordered_flat_map& operator=(const unordered_flat_map& other); unordered_flat_map& operator=(unordered_flat_map&& other) noexcept( (boost::allocator_traits<Allocator>::is_always_equal::value  boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) && std::is_same<pointer, value_type*>::value); unordered_flat_map& operator=(std::initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> std::pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); std::pair<iterator, bool> insert(const value_type& obj); std::pair<iterator, bool> insert(const init_type& obj); std::pair<iterator, bool> insert(value_type&& obj); std::pair<iterator, bool> insert(init_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, const init_type& obj); iterator insert(const_iterator hint, value_type&& obj); iterator insert(const_iterator hint, init_type&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); void insert(std::initializer_list<value_type>); template<class... Args> std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> std::pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M> std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M> std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M> std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); convertibletoiterator erase(iterator position); convertibletoiterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& k); iterator erase(const_iterator first, const_iterator last); void swap(unordered_flat_map& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value  boost::allocator_traits<Allocator>::propagate_on_container_swap::value); void clear() noexcept; template<class H2, class P2> void merge(unordered_flat_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_flat_map<Key, T, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // map operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; std::pair<iterator, iterator> equal_range(const key_type& k); std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> std::pair<iterator, iterator> equal_range(const K& k); template<class K> std::pair<const_iterator, const_iterator> equal_range(const K& k) const; // element access mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k); template<class K> mapped_type& operator[](K&& k); mapped_type& at(const key_type& k); const mapped_type& at(const key_type& k) const; template<class K> mapped_type& at(const K& k); template<class K> const mapped_type& at(const K& k) const; // bucket interface size_type bucket_count() const noexcept; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); size_type max_load() const noexcept; void rehash(size_type n); void reserve(size_type n); // statistics (if enabled) stats get_stats() const; void reset_stats() noexcept; }; // Deduction Guides template<class InputIterator, class Hash = boost::hash<iterkeytype<InputIterator>>, class Pred = std::equal_to<iterkeytype<InputIterator>>, class Allocator = std::allocator<itertoalloctype<InputIterator>>> unordered_flat_map(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_flat_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T>>> unordered_flat_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) > unordered_flat_map<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_flat_map(InputIterator, InputIterator, typename see below::size_type, Allocator) > unordered_flat_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_flat_map(InputIterator, InputIterator, Allocator) > unordered_flat_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, boost::hash<iterkeytype<InputIterator>>, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_flat_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) > unordered_flat_map<iterkeytype<InputIterator>, itermappedtype<InputIterator>, Hash, std::equal_to<iterkeytype<InputIterator>>, Allocator>; template<class Key, class T, class Allocator> unordered_flat_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Allocator) > unordered_flat_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_flat_map(std::initializer_list<std::pair<Key, T>>, Allocator) > unordered_flat_map<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_flat_map(std::initializer_list<std::pair<Key, T>>, typename see below::size_type, Hash, Allocator) > unordered_flat_map<Key, T, Hash, std::equal_to<Key>, Allocator>; // Equality Comparisons template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x, const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y); template<class Key, class T, class Hash, class Pred, class Alloc> bool operator!=(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x, const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y); // swap template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_flat_map<Key, T, Hash, Pred, Alloc>& x, unordered_flat_map<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // Erasure template<class K, class T, class H, class P, class A, class Predicate> typename unordered_flat_map<K, T, H, P, A>::size_type erase_if(unordered_flat_map<K, T, H, P, A>& c, Predicate pred); // Pmr aliases (C++17 and up) namespace unordered::pmr { template<class Key, class T, class Hash = boost::hash<Key>, class Pred = std::equal_to<Key>> using unordered_flat_map = boost::unordered_flat_map<Key, T, Hash, Pred, std::pmr::polymorphic_allocator<std::pair<const Key, T>>>; } }
Description
Template Parameters
Key 

T 

Hash 
A unary function object type that acts a hash function for a 
Pred 
A binary function object that induces an equivalence relation on values of type 
Allocator 
An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported. 
The elements of the container are held into an internal bucket array. An element is inserted into a bucket determined by its hash code, but if the bucket is already occupied (a collision), an available one in the vicinity of the original position is used.
The size of the bucket array can be automatically increased by a call to insert
/emplace
, or as a result of calling
rehash
/reserve
. The load factor of the container (number of elements divided by number of buckets) is never
greater than max_load_factor()
, except possibly for small sizes where the implementation may decide to
allow for higher loads.
If hash_is_avalanching<Hash>::value
is true
, the hash function
is used asis; otherwise, a bitmixing postprocessing stage is added to increase the quality of hashing
at the expense of extra computational cost.
Configuration Macros
BOOST_UNORDERED_ENABLE_STATS
Globally define this macro to enable statistics calculation for the container. Note that this option decreases the overall performance of many operations.
Typedefs
typedef implementationdefined iterator;
An iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Convertible to const_iterator
.
typedef implementationdefined const_iterator;
A constant iterator whose value type is value_type
.
The iterator category is at least a forward iterator.
Constructors
Default Constructor
unordered_flat_map();
Constructs an empty container using hasher()
as the hash function,
key_equal()
as the key equality predicate and allocator_type()
as the allocator.
Postconditions: 

Requires: 
If the defaults are used, 
Bucket Count Constructor
explicit unordered_flat_map(size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash
function, eql
as the key equality predicate, and a
as the allocator.
Postconditions: 

Requires: 
If the defaults are used, 
Iterator Range Constructor
template<class InputIterator>
unordered_flat_map(InputIterator f, InputIterator l,
size_type n = implementationdefined,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate and a
as the allocator, and inserts the elements from [f, l)
into it.
Requires: 
If the defaults are used, 
Copy Constructor
unordered_flat_map(unordered_flat_map const& other);
The copy constructor. Copies the contained elements, hash function, predicate and allocator.
If Allocator::select_on_container_copy_construction
exists and has the right signature, the allocator will be constructed from its result.
Requires: 

Move Constructor
unordered_flat_map(unordered_flat_map&& other);
The move constructor. The internal bucket array of other
is transferred directly to the new container.
The hash function, predicate and allocator are movedconstructed from other
.
If statistics are enabled,
transfers the internal statistical information from other
and calls other.reset_stats()
.
Iterator Range Constructor with Allocator
template<class InputIterator>
unordered_flat_map(InputIterator f, InputIterator l, const allocator_type& a);
Constructs an empty container using a
as the allocator, with the default hash function and key equality predicate and inserts the elements from [f, l)
into it.
Requires: 

Allocator Constructor
explicit unordered_flat_map(Allocator const& a);
Constructs an empty container, using allocator a
.
Copy Constructor with Allocator
unordered_flat_map(unordered_flat_map const& other, Allocator const& a);
Constructs a container, copying other
's contained elements, hash function, and predicate, but using allocator a
.
Move Constructor with Allocator
unordered_flat_map(unordered_flat_map&& other, Allocator const& a);
If a == other.get_allocator()
, the elements of other
are transferred directly to the new container;
otherwise, elements are movedconstructed from those of other
. The hash function and predicate are movedconstructed
from other
, and the allocator is copyconstructed from a
.
If statistics are enabled,
transfers the internal statistical information from other
iff a == other.get_allocator()
,
and always calls other.reset_stats()
.
Move Constructor from concurrent_flat_map
unordered_flat_map(concurrent_flat_map<Key, T, Hash, Pred, Allocator>&& other);
Move construction from a concurrent_flat_map
.
The internal bucket array of other
is transferred directly to the new container.
The hash function, predicate and allocator are movedconstructed from other
.
If statistics are enabled,
transfers the internal statistical information from other
and calls other.reset_stats()
.
Complexity: 
Constant time. 
Concurrency: 
Blocking on 
Initializer List Constructor
unordered_flat_map(std::initializer_list<value_type> il,
size_type n = implementationdefined
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
Constructs an empty container with at least n
buckets, using hf
as the hash function, eql
as the key equality predicate and a
, and inserts the elements from il
into it.
Requires: 
If the defaults are used, 
Bucket Count Constructor with Allocator
unordered_flat_map(size_type n, allocator_type const& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default hash function and key equality predicate and a
as the allocator.
Postconditions: 

Requires: 

Bucket Count Constructor with Hasher and Allocator
unordered_flat_map(size_type n, hasher const& hf, allocator_type const& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, the default key equality predicate and a
as the allocator.
Postconditions: 

Requires: 

Iterator Range Constructor with Bucket Count and Allocator
template<class InputIterator>
unordered_flat_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
as the allocator and default hash function and key equality predicate, and inserts the elements from [f, l)
into it.
Requires: 

Iterator Range Constructor with Bucket Count and Hasher
template<class InputIterator>
unordered_flat_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator, with the default key equality predicate, and inserts the elements from [f, l)
into it.
Requires: 

initializer_list Constructor with Allocator
unordered_flat_map(std::initializer_list<value_type> il, const allocator_type& a);
Constructs an empty container using a
and default hash function and key equality predicate, and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Allocator
unordered_flat_map(std::initializer_list<value_type> il, size_type n, const allocator_type& a);
Constructs an empty container with at least n
buckets, using a
and default hash function and key equality predicate, and inserts the elements from il
into it.
Requires: 

initializer_list Constructor with Bucket Count and Hasher and Allocator
unordered_flat_map(std::initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& a);
Constructs an empty container with at least n
buckets, using hf
as the hash function, a
as the allocator and default key equality predicate,and inserts the elements from il
into it.
Requires: 

Destructor
~unordered_flat_map();
Note: 
The destructor is applied to every element, and all memory is deallocated 
Assignment
Copy Assignment
unordered_flat_map& operator=(unordered_flat_map const& other);
The assignment operator. Destroys previously existing elements, copyassigns the hash function and predicate from other
,
copyassigns the allocator from other
if Alloc::propagate_on_container_copy_assignment
exists and Alloc::propagate_on_container_copy_assignment::value
is true
,
and finally inserts copies of the elements of other
.
Requires: 

Move Assignment
unordered_flat_map& operator=(unordered_flat_map&& other)
noexcept((boost::allocator_traits<Allocator>::is_always_equal::value 
boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) &&
std::is_same<pointer, value_type*>::value);
The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from other
,
and moveassigns the allocator from other
if Alloc::propagate_on_container_move_assignment
exists and Alloc::propagate_on_container_move_assignment::value
is true
.
If at this point the allocator is equal to other.get_allocator()
, the internal bucket array of other
is transferred directly to the new container;
otherwise, inserts moveconstructed copies of the elements of other
.
If statistics are enabled,
transfers the internal statistical information from other
iff the final allocator is equal to other.get_allocator()
,
and always calls other.reset_stats()
.
Initializer List Assignment
unordered_flat_map& operator=(std::initializer_list<value_type> il);
Assign from values in initializer list. All previously existing elements are destroyed.
Requires: 

Iterators
begin
iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns: 
An iterator referring to the first element of the container, or if the container is empty the pasttheend value for the container. 
Complexity: 
O( 
end
iterator end() noexcept;
const_iterator end() const noexcept;
Returns: 
An iterator which refers to the pasttheend value for the container. 
cbegin
const_iterator cbegin() const noexcept;
Returns: 
A 
Complexity: 
O( 
cend
const_iterator cend() const noexcept;
Returns: 
A 
Modifiers
emplace
template<class... Args> std::pair<iterator, bool> emplace(Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. If 
emplace_hint
template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
Inserts an object, constructed with the arguments args
, in the container if and only if there is no element in the container with an equivalent key.
position
is a suggestion to where the element should be inserted. This implementation ignores it.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. If 
Copy Insert
std::pair<iterator, bool> insert(const value_type& obj);
std::pair<iterator, bool> insert(const init_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. A call of the form 
Move Insert
std::pair<iterator, bool> insert(value_type&& obj);
std::pair<iterator, bool> insert(init_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. A call of the form 
Copy Insert with Hint
iterator insert(const_iterator hint, const value_type& obj);
iterator insert(const_iterator hint, const init_type& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted. This implementation ignores it.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. A call of the form 
Move Insert with Hint
iterator insert(const_iterator hint, value_type&& obj);
iterator insert(const_iterator hint, init_type&& obj);
Inserts obj
in the container if and only if there is no element in the container with an equivalent key.
hint
is a suggestion to where the element should be inserted. This implementation ignores it.
Requires: 

Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. A call of the form 
Insert Iterator Range
template<class InputIterator> void insert(InputIterator first, InputIterator last);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. 
Insert Initializer List
void insert(std::initializer_list<value_type>);
Inserts a range of elements into the container. Elements are inserted if and only if there is no element in the container with an equivalent key.
Requires: 

Throws: 
When inserting a single element, if an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. 
try_emplace
template<class... Args>
std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class K, class... Args>
std::pair<iterator, bool> try_emplace(K&& k, Args&&... args);
Inserts a new element into the container if there is no existing element with key k
contained within it.
If there is an existing element with key k
this function does nothing.
Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
This function is similiar to emplace, with the difference that no
unlike emplace, which simply forwards all arguments to Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load. The 
try_emplace with Hint
template<class... Args>
iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
template<class K, class... Args>
iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
Inserts a new element into the container if there is no existing element with key k
contained within it.
If there is an existing element with key k
this function does nothing.
hint
is a suggestion to where the element should be inserted. This implementation ignores it.
Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
This function is similiar to emplace_hint, with the difference that no
unlike emplace_hint, which simply forwards all arguments to Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load. The 
insert_or_assign
template<class M>
std::pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
std::pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class K, class M>
std::pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
Inserts a new element into the container or updates an existing one by assigning to the contained value.
If there is an element with key k
, then it is updated by assigning std::forward<M>(obj)
.
If there is no such element, it is added to the container as:
// first two overloads
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<Key>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
// third overload
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<K>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
Returns: 
The If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators pointers and references, but only if the insert causes the load to be greater than the maximum load. The 
insert_or_assign with Hint
template<class M>
iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
template<class K, class M>
iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);
Inserts a new element into the container or updates an existing one by assigning to the contained value.
If there is an element with key k
, then it is updated by assigning std::forward<M>(obj)
.
If there is no such element, it is added to the container as:
// first two overloads
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<Key>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
// third overload
value_type(std::piecewise_construct,
std::forward_as_tuple(std::forward<K>(k)),
std::forward_as_tuple(std::forward<M>(obj)))
hint
is a suggestion to where the element should be inserted. This implementation ignores it.
Returns: 
If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. The 
Erase by Position
convertibletoiterator erase(iterator position);
convertibletoiterator erase(const_iterator position);
Erase the element pointed to by position
.
Returns: 
An opaque object implicitly convertible to the 
Throws: 
Nothing. 
Notes: 
The opaque object returned must only be discarded or immediately converted to 
Erase by Key
size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);
Erase all elements with key equivalent to k
.
Returns: 
The number of elements erased. 
Throws: 
Only throws an exception if it is thrown by 
Notes: 
The 
Erase Range
iterator erase(const_iterator first, const_iterator last);
Erases the elements in the range from first
to last
.
Returns: 
The iterator following the erased elements  i.e. 
Throws: 
Nothing in this implementation (neither the 
swap
void swap(unordered_flat_map& other)
noexcept(boost::allocator_traits<Allocator>::is_always_equal::value 
boost::allocator_traits<Allocator>::propagate_on_container_swap::value);
Swaps the contents of the container with the parameter.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Throws: 
Nothing unless 
clear
void clear() noexcept;
Erases all elements in the container.
Postconditions: 

merge
template<class H2, class P2>
void merge(unordered_flat_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
void merge(unordered_flat_map<Key, T, H2, P2, Allocator>&& source);
Moveinserts all the elements from source
whose key is not already present in *this
, and erases them from source
.
Lookup
find
iterator find(const key_type& k);
const_iterator find(const key_type& k) const;
template<class K>
iterator find(const K& k);
Returns: 
An iterator pointing to an element with key equivalent to 
Notes: 
The 
count
size_type count(const key_type& k) const;
template<class K>
size_type count(const K& k) const;
Returns: 
The number of elements with key equivalent to 
Notes: 
The 
contains
bool contains(const key_type& k) const;
template<class K>
bool contains(const K& k) const;
Returns: 
A boolean indicating whether or not there is an element with key equal to 
Notes: 
The 
equal_range
std::pair<iterator, iterator> equal_range(const key_type& k);
std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;
template<class K>
std::pair<iterator, iterator> equal_range(const K& k);
template<class K>
std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns: 
A range containing all elements with key equivalent to 
Notes: 
The 
operator[]
mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
template<class K> mapped_type& operator[](K&& k);
Effects: 
If the container does not already contain an element with a key equivalent to 
Returns: 
A reference to 
Throws: 
If an exception is thrown by an operation other than a call to 
Notes: 
Can invalidate iterators, pointers and references, but only if the insert causes the load to be greater than the maximum load. The 
at
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
template<class K> mapped_type& at(const K& k);
template<class K> const mapped_type& at(const K& k) const;
Returns: 
A reference to 
Throws: 
An exception object of type 
Notes: 
The 
Hash Policy
load_factor
float load_factor() const noexcept;
Returns: 

max_load_factor
float max_load_factor() const noexcept;
Returns: 
Returns the container’s maximum load factor. 
Set max_load_factor
void max_load_factor(float z);
Effects: 
Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with 
max_load
size_type max_load() const noexcept;
Returns: 
The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased. 
Note: 
After construction, rehash or clearance, the container’s maximum load is at least 
rehash
void rehash(size_type n);
Changes if necessary the size of the bucket array so that there are at least n
buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count()
associated with the container.
When size() == 0
, rehash(0)
will deallocate the underlying buckets array. If the provided Allocator uses fancy pointers, a default allocation is subsequently performed.
Invalidates iterators, pointers and references, and changes the order of elements.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
reserve
void reserve(size_type n);
Equivalent to a.rehash(ceil(n / a.max_load_factor()))
.
Similar to rehash
, this function can be used to grow or shrink the number of buckets in the container.
Invalidates iterators, pointers and references, and changes the order of elements.
Throws: 
The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function. 
Statistics
get_stats
stats get_stats() const;
Returns: 
A statistical description of the insertion and lookup operations performed by the container so far. 
Notes: 
Only available if statistics calculation is enabled. 
reset_stats
void reset_stats() noexcept;
Effects: 
Sets to zero the internal statistics kept by the container. 
Notes: 
Only available if statistics calculation is enabled. 
Deduction Guides
A deduction guide will not participate in overload resolution if any of the following are true:

It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter. 
It has an
Allocator
template parameter and a type that does not qualify as an allocator is deduced for that parameter. 
It has a
Hash
template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. 
It has a
Pred
template parameter and a type that qualifies as an allocator is deduced for that parameter.
A size_type
parameter type in a deduction guide refers to the size_type
member type of the
container type deduced by the deduction guide. Its default value coincides with the default value
of the constructor selected.
itervaluetype
template<class InputIterator> using itervaluetype = typename std::iterator_traits<InputIterator>::value_type; // exposition only
iterkeytype
template<class InputIterator> using iterkeytype = std::remove_const_t< std::tuple_element_t<0, itervaluetype<InputIterator>>>; // exposition only
itermappedtype
template<class InputIterator> using itermappedtype = std::tuple_element_t<1, itervaluetype<InputIterator>>; // exposition only
itertoalloctype
template<class InputIterator> using itertoalloctype = std::pair< std::add_const_t<std::tuple_element_t<0, itervaluetype<InputIterator>>>, std::tuple_element_t<1, itervaluetype<InputIterator>>>; // exposition only
Equality Comparisons
operator==
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator==(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);
Return true
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
operator!=
template<class Key, class T, class Hash, class Pred, class Alloc>
bool operator!=(const unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
const unordered_flat_map<Key, T, Hash, Pred, Alloc>& y);
Return false
if x.size() == y.size()
and for every element in x
, there is an element in y
with the same key, with an equal value (using operator==
to compare the value types).
Notes: 
Behavior is undefined if the two containers don’t have equivalent equality predicates. 
Swap
template<class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_flat_map<Key, T, Hash, Pred, Alloc>& x,
unordered_flat_map<Key, T, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Swaps the contents of x
and y
.
If Allocator::propagate_on_container_swap
is declared and Allocator::propagate_on_container_swap::value
is true
then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.
Effects: 

Throws: 
Nothing unless 
erase_if
template<class K, class T, class H, class P, class A, class Predicate>
typename unordered_flat_map<K, T, H, P, A>::size_type
erase_if(unordered_flat_map<K, T, H, P, A>& c, Predicate pred);
Traverses the container c
and removes all elements for which the supplied predicate returns true
.
Returns: 
The number of erased elements. 
Notes: 
Equivalent to:
