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

Inserts a new element into the container constructed in-place with the given args , if there is no element with the key in the container.

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

Return value

A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.

Exceptions

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

Complexity

If insertion occurs, linear in the size of the container, otherwise logarithmic in the size of the container

Example

#include <iostream>
#include <string>
#include <utility>
#include <flat_map>
 
int main()
{
    std::flat_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");
 
    // emplace with duplicate key has no effect
    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'));
    // an alternative is: m.try_emplace("c", 10, 'c');
 
    for (const auto& p : m)
        std::cout << p.first << " => " << p.second << '\n';
}

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

See also