NAME¶

std::unordered_multimap::emplace - std::unordered_multimap::emplace

Synopsis¶

template< class... Args > (since C++11)
iterator emplace( Args&&... args );

Inserts a new element into the container constructed in-place with the given args.

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)....

Careful use of emplace allows the new element to be constructed while avoiding
unnecessary copy or move operations.

If after the operation the new number of elements is greater than old
max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise
(no rehashing), iterators are not invalidated.

Parameters¶

args - arguments to forward to the constructor of the element

Return value¶

An iterator to the inserted element.

Exceptions¶

If an exception is thrown for any reason, this function has no effect (strong
exception safety guarantee).

Complexity¶

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

Example¶

// Run this code

#include <iostream>
#include <string>
#include <utility>
#include <unordered_map>

int main()
{
std::unordered_multimap<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");

// emplace with duplicate key
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
d => DDD

See also¶

emplace_hint constructs elements in-place using a hint
(public member function)
inserts in-place if the key does not exist, does nothing if the key
try_emplace exists
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)