std::flat_multimap<Key,T,Compare,KeyContainer,MappedContainer>::emplace

template< class... Args >
iterator emplace( Args&&... args );

(since C++23)

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

Initializes an object t of type std::pair<key_type, mapped_type> with std::forward<Args>(args)...; if the map already contains an element whose key is equivalent to t.first, *this is unchanged. Otherwise, equivalent to:

auto key_it = ranges::upper_bound(c.keys, t.first, compare);
auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
c.keys.insert(key_it, std::move(t.first));
c.values.insert(value_it, std::move(t.second));

This overload participates in overload resolution only if std::is_constructible_v<std::pair<key_type, mapped_type>, Args...> is true.

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

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

Linear in the size of the container

Example

#include <iostream>
#include <string>
#include <utility>
#include <flat_map>

int main()
{
    std::flat_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';
}

Output:

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