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std::unordered_map::emplace

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< cpp‎ | container‎ | unordered map
Revision as of 09:54, 2 November 2012 by P12bot (Talk | contribs)

template< class... Args >
std::pair<iterator,bool> emplace( Args&&... args );
(since C++11)

Inserts a new element into the container by constructing it in-place with the given args if there is no element with the key in the container.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations. The constructor of the new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args)....

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is equal to or greater than max_load_factor()*bucket_count().

Contents

Parameters

args - arguments to forward to the constructor of the element

Return value

Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place.

Exceptions

If an exception is thrown by any operation, this function has no effect.

Complexity

Amortized constant on average, worst case linear in the size of the container.

Example

#include <iostream>
#include <utility>
#include <string>
 
#include <unordered_map>
int main()
{
    std::unordered_map<std::string, std::string> m;
 
    // uses pair's move constructor
    m.emplace(std::make_pair(std::string("a"), std::string("a")));
 
    // uses pair's converting move constructor
    m.emplace(std::make_pair("b", "abcd"));
 
    // uses pair's template constructor
    m.emplace("d", "ddd");
 
    // uses pair's piecewise constructor
    m.emplace(std::piecewise_construct,
              std::forward_as_tuple("c"),
              std::forward_as_tuple(10, 'c'));
 
    for (const auto &p : m) {
        std::cout << p.first << " => " << p.second << '\n';
    }
}

Possible output:

a => a
b => abcd
c => cccccccccc
d => ddd

See also

constructs elements in-place using a hint
(public member function) [edit]
inserts in-place if the key does not exist, does nothing if the key exists
(public member function) [edit]
inserts elements
(public member function) [edit]