std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::emplace

 

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std::unordered_multimap

Member types
Member functions
unordered_multimap::unordered_multimap unordered_multimap::~unordered_multimap	unordered_multimap::operator= unordered_multimap::get_allocator
Iterators
unordered_multimap::beginunordered_multimap::cbegin	unordered_multimap::endunordered_multimap::cend
Capacity
unordered_multimap::size unordered_multimap::max_size	unordered_multimap::empty
Modifiers
unordered_multimap::clear unordered_multimap::insert unordered_multimap::insert_range (C++23) unordered_multimap::emplace unordered_multimap::emplace_hint	unordered_multimap::erase unordered_multimap::swap unordered_multimap::extract (C++17) unordered_multimap::merge (C++17)
Lookup
unordered_multimap::count unordered_multimap::find	unordered_multimap::contains (C++20) unordered_multimap::equal_range
Bucket interface
unordered_multimap::begin(size_type)unordered_multimap::cbegin(size_type) unordered_multimap::end(size_type)unordered_multimap::cend(size_type)	unordered_multimap::bucket_count unordered_multimap::max_bucket_count unordered_multimap::bucket_size unordered_multimap::bucket
Hash policy
unordered_multimap::load_factor unordered_multimap::max_load_factor	unordered_multimap::rehash unordered_multimap::reserve
Observers
unordered_multimap::hash_function	unordered_multimap::key_eq
Non-member functions
operator==operator!= (C++11)(C++11)(until C++20)	std::swap(std::unordered_multimap) (C++11) erase_if(std::unordered_multimap) (C++20)
Deduction guides (C++17)

 

template< class... Args > iterator emplace( Args&&... args );		(since C++11)

Inserts a new element into the container constructed in-place with the given args.

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)....

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

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.

Parameters

args

-

arguments to forward to the constructor of the element

Return value

An iterator to the inserted element.

Exceptions

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

Complexity

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

Example

Run this code

#include <iostream>
#include <string>
#include <utility>
#include <unordered_map>
 
int main()
{
    std::unordered_multimap<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 
    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'));
 
    for (const auto& p : m)
        std::cout << p.first << " => " << p.second << '\n';
}

Possible output:

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

See also

emplace_hint	constructs elements in-place using a hint (public member function)
try_emplace	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)