...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Copyright © 2003, 2004 Jeremy B. Maitin-Shepard
Copyright © 2005-2008 Daniel James
Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
Table of Contents
For accessing data based on key lookup, the C++ standard library offers std::set
,
std::map
, std::multiset
and std::multimap
. These are generally implemented
using balanced binary trees so that lookup time has logarithmic complexity.
That is generally okay, but in many cases a hash
table can perform better, as accessing data has constant complexity,
on average. The worst case complexity is linear, but that occurs rarely and
with some care, can be avoided.
Also, the existing containers require a 'less than' comparison object to order their elements. For some data types this is impossible to implement or isn't practical. In contrast, a hash table only needs an equality function and a hash function for the key.
With this in mind, unordered associative containers were added to the C++ standard. This is an implementation of the containers described in C++11, with some deviations from the standard in order to work with non-C++11 compilers and libraries.
unordered_set
and unordered_multiset
are defined in the header
<boost/unordered_set.hpp
>
namespace boost { template < class Key, class Hash =boost::hash
<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > classunordered_set
; template< class Key, class Hash =boost::hash
<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > classunordered_multiset
; }
unordered_map
and unordered_multimap
are defined in the header
<boost/unordered_map.hpp
>
namespace boost { 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> > > classunordered_map
; 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> > > classunordered_multimap
; }
When using Boost.TR1, these classes are included from <unordered_set>
and <unordered_map>
, with the classes added to the std::tr1
namespace.
The containers are used in a similar manner to the normal associative containers:
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:
BOOST_FOREACH(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
To store 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.
There are other differences, which are listed in the Comparison with Associative Containers section.
Last revised: December 05, 2017 at 23:51:30 GMT |