public member function
<map>

std::map::emplace

template <class... Args>
  pair<iterator,bool> emplace (Args&&... args);
Construct and insert element
Inserts a new element in the map if its key is unique. This new element is constructed in place using args as the arguments for the construction of a value_type (which is an object of a pair type).

The insertion only takes place if no other element in the container has a key equivalent to the one being emplaced (keys in a map container are unique).

If inserted, this effectively increases the container size by one.

Internally, map containers keep all their elements sorted by their key following the criterion specified by its comparison object. The element is always inserted in its respective position following this ordering.

The element is constructed in-place by calling allocator_traits::construct with args forwarded.

A similar member function exists, insert, which either copies or moves existing objects into the container.

Parameters

args
Arguments used to construct a new object of the mapped type for the inserted element.
Arguments forwarded to construct the new element (of type pair<const key_type, mapped_type>).
This can be one of:
- Two arguments: one for the key, the other for the mapped value.
- A single argument of a pair type with a value for the key as first member, and a value for the mapped value as second.
- piecewise_construct as first argument, and two additional arguments with tuples to be forwarded as arguments for the key value and for the mapped value respectivelly.
See pair::pair for more info.

Return value

If the function successfully inserts the element (because no equivalent element existed already in the map), the function returns a pair of an iterator to the newly inserted element and a value of true.

Otherwise, it returns an iterator to the equivalent element within the container and a value of false.

Member type iterator is a bidirectional iterator type that points to an element.
pair is a class template declared in <utility> (see pair).

Example

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// map::emplace
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> mymap;

  mymap.emplace('x',100);
  mymap.emplace('y',200);
  mymap.emplace('z',100);

  std::cout << "mymap contains:";
  for (auto& x: mymap)
    std::cout << " [" << x.first << ':' << x.second << ']';
  std::cout << '\n';

  return 0;
}

Output:
mymap contains: [x:100] [y:200] [z:100]

Complexity

Logarithmic in the container size.

Iterator validity

No changes.

Data races

The container is modified.
Concurrently accessing existing elements is safe, although iterating ranges in the container is not.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the container.
If allocator_traits::construct is not supported with the appropriate arguments, it causes undefined behavior.

See also

map::emplace_hint
Construct and insert element with hint (public member function )
map::insert
Insert elements (public member function )
map::erase
Erase elements (public member function )

C++