std::map<Key,T,Compare,Allocator>::emplace

template< class... Args > std::pair<iterator, bool> emplace( Args&&... args );		(since C++11)

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.

No iterators or references are invalidated.

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

Complexity

Logarithmic in the size of the container.

Example

Run this code

#include <iostream>
#include <string>
#include <utility>
#include <map>
 
int main()
{
    std::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';
}

Output:

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
vector<bool>
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Iterator invalidation
Member function table
Non-member function table

Member functions
map::map
map::~map
map::operator=
map::get_allocator
Element access
map::at
map::operator[]
Iterators
map::beginmap::cbegin (C++11)
map::endmap::cend (C++11)
map::rbeginmap::crbegin (C++11)
map::rendmap::crend (C++11)
Capacity
map::empty
map::size
map::max_size
Modifiers
map::clear
map::insert
map::insert_range (C++23)
map::insert_or_assign (C++17)
map::emplace (C++11)
map::emplace_hint (C++11)
map::try_emplace (C++17)
map::erase
map::swap
map::extract (C++17)
map::merge (C++17)
Lookup
map::count
map::find
map::contains (C++20)
map::equal_range
map::lower_bound
map::upper_bound
Observers
map::key_comp
map::value_comp
Non-member functions
std::swap
erase_if (C++20)
operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides (C++17)

emplace_hint (C++11)	constructs elements in-place using a hint (public member function)
try_emplace (C++17)	inserts in-place if the key does not exist, does nothing if the key exists (public member function)
insert	inserts elements or nodes (since C++17) (public member function)