NAME

std::unordered_map::emplace - std::unordered_map::emplace

Synopsis

template< class... Args > (since C++11)
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.

Careful use of emplace allows the new element to be constructed while avoiding
unnecessary copy or move operations. 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).... The element may be constructed
even if there already is an element with the key in the container, in which case the
newly constructed element will be destroyed immediately.

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise
iterators are not affected. References are not invalidated. Rehashing occurs only if
the new number of elements is greater than max_load_factor()*bucket_count().

Parameters

args - arguments to forward to the constructor of the element

Return value

Returns a pair consisting of an iterator to the inserted element, or the
already-existing element if no insertion happened, and a bool denoting whether the
insertion took place (true if insertion happened, false if it did not).

Exceptions

If an exception is thrown by any operation, this function has no effect (strong
exception guarantee).

Complexity

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

Example

// Run this code

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

int main()
{
std::unordered_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");

// uses pair's piecewise constructor
m.emplace(std::piecewise_construct,
std::forward_as_tuple("c"),
std::forward_as_tuple(10, 'c'));
// as of C++17, m.try_emplace("c", 10, 'c'); can be used

for (const auto &p : m) {
std::cout << p.first << " => " << p.second << '\n';
}
}

Possible output:

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