From cppreference.com
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<constKey,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 (see
if this behavior is undesirable).
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
*
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
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
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>intmain(){std::unordered_map<std::string,std::string>m;// uses pair's move constructorm.emplace(std::make_pair(std::string("a"),std::string("a")));// uses pair's converting move constructorm.emplace(std::make_pair("b","abcd"));// uses pair's template constructorm.emplace("d","ddd");// emplace with duplicate key has no effectm.emplace("d","DDD");// uses pair's piecewise constructorm.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(constauto&p:m)std::cout<<p.first<<" => "<<p.second<<'\n';}Possible output:
a => a b => abcd c => cccccccccc d => ddd See also
constructs elements in-place using a hint
(public member function)
(C++17)
inserts in-place if the key does not exist, does nothing if the key exists
(public member function)
inserts elements or nodes(since C++17)
(public member function)