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Comparison with Associative Containers

Table 32.4. Interface differences.

Associative Containers

Unordered Associative Containers

Parameterized by an ordering relation Compare

Parameterized by a function object Hash and an equivalence relation Pred

Keys can be compared using key_compare which is accessed by member function key_comp(), values can be compared using value_compare which is accessed by member function value_comp().

Keys can be hashed using hasher which is accessed by member function hash_function(), and checked for equality using key_equal which is accessed by member function key_eq(). There is no function object for compared or hashing values.

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 k1, k2 are considered equivalent if !Compare(k1, k2) && !Compare(k2, k1)

Keys k1, k2 are considered equivalent if Pred(k1, k2)

Member function lower_bound(k) and upper_bound(k)

No equivalent. Since the elements aren't ordered lower_bound and upper_bound would be meaningless.

equal_range(k) returns an empty range at the position that k would be inserted if k isn't present in the container.

equal_range(k) returns a range at the end of the container if k isn't present in the container. It can't return a positioned range as k could be inserted into multiple place. To find out the bucket that k would be inserted into use bucket(k). But remember that an insert can cause the container to rehash - meaning that the element can be inserted into a different bucket.

iterator, const_iterator are of the bidirectional category.

iterator, const_iterator are of at least the forward category.

Iterators, pointers and references to the container's elements are never invalidated.

Iterators can be invalidated by calls to insert or rehash. Pointers and references to the container's elements are never invalidated.

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

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 ==, !=, <, <=, >, >= operators.

Can be compared using the == and != operators.

When inserting with a hint, implementations are permitted to ignore the hint.

erase never throws an exception

The containers' hash or predicate function can throw exceptions from erase


Table 32.5. Complexity Guarantees

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 value_comp()

Average case O(N), worst case O(N2)

Insert a single element

logarithmic

Average case constant, worst case linear

Insert a single element with a hint

Amortized constant if t elements inserted right after hint, logarithmic otherwise

Average case constant, worst case linear (ie. the same as a normal insert).

Inserting a range of N elements

N log(size()+N)

Average case O(N), worst case O(N * size())

Erase by key, k

O(log(size()) + count(k))

Average case: O(count(k)), Worst case: O(size())

Erase a single element by iterator

Amortized constant

Average case: O(1), Worst case: O(size())

Erase a range of N elements

O(log(size()) + N)

Average case: O(N), Worst case: O(size())

Clearing the container

O(size())

O(size())

Find

logarithmic

Average case: O(1), Worst case: O(size())

Count

O(log(size()) + count(k))

Average case: O(1), Worst case: O(size())

equal_range(k)

logarithmic

Average case: O(count(k)), Worst case: O(size())

lower_bound,upper_bound

logarithmic

n/a



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