std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::emplace (3) - Linux Manuals

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

NAME

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

Synopsis

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

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

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 an iterator to the inserted element.

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_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");

      // 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