NAME¶

std::flat_map::emplace - std::flat_map::emplace

Synopsis¶

template< class... Args > (since C++23)
std::pair<iterator, bool> emplace( Args&&... args );

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.

Information on iterator invalidation is copied from here

Parameters¶

args - arguments to forward to the constructor of the element

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¶

// Run this code

#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';
}

Output:¶

a => a
b => abcd
c => cccccccccc
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)
insert inserts elements
(public member function)