std::unordered_map<Key,T,Hash,KeyEqual,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.

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 for Insertion, False for No Insertion.

Exceptions

If an exception is thrown by any operation, this function has no effect.

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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

array (C++11)
vector
deque
forward_list (C++11)
list
set
multiset
map
multimap
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
stack
queue
priority_queue
span (C++20)

Member functions
unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iterators
unordered_map::beginunordered_map::cbegin
unordered_map::endunordered_map::cend
Capacity
unordered_map::empty
unordered_map::size
unordered_map::max_size
Modifiers
unordered_map::clear
unordered_map::insert
unordered_map::insert_or_assign (C++17)
unordered_map::emplace
unordered_map::emplace_hint
unordered_map::try_emplace (C++17)
unordered_map::erase
unordered_map::swap
unordered_map::extract (C++17)
unordered_map::merge (C++17)
Lookup
unordered_map::at
unordered_map::operator[]
unordered_map::count
unordered_map::find
unordered_map::contains (C++20)
unordered_map::equal_range
Bucket interface
unordered_map::begin(size_type)unordered_map::cbegin(size_type)
unordered_map::end(size_type)unordered_map::cend(size_type)
unordered_map::bucket_count
unordered_map::max_bucket_count
unordered_map::bucket_size
unordered_map::bucket
Hash policy
unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Observers
unordered_map::hash_function
unordered_map::key_eq
Deduction guides(C++17)