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

 

C++

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std::map

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::size map::max_size map::empty Observers map::key_comp map::value_comp Modifiers map::clear map::insert map::erase map::swap map::extract (C++17) map::merge (C++17) 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) Lookup map::count map::find map::contains (C++20) map::equal_range map::lower_bound map::upper_bound

Non-member functions

operator==operator<=> (C++20) std::swap(std::map) erase_if(std::map) (C++20)   operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)

Deduction guides (C++17)

 

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.

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.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.

No iterators or references are invalidated.

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

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");
 
    // 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 (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)