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.


[edit] Parameters

args - arguments to forward to the constructor of the element

[edit] Return value

An iterator to the inserted element.

[edit] Exceptions

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

[edit] Complexity

Linear in the size of the container

[edit] 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
              std::forward_as_tuple(10, 'c'));
    for (const auto& p : m)
        std::cout << p.first << " => " << p.second << '\n';


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

[edit] 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]