template<class...Args>std::pair<iterator,bool>try_emplace(constKey&k,Args&&...args); (1) (since C++17)template<class...Args>std::pair<iterator,bool>try_emplace(Key&&k,Args&&...args); (2) (since C++17)template<classK,class...Args>std::pair<iterator,bool>try_emplace(K&&k,Args&&...args); (3) (since C++26)template<class...Args>iteratortry_emplace(const_iteratorhint,constKey&k,Args&&...args); (4) (since C++17)template<class...Args>iteratortry_emplace(const_iteratorhint,Key&&k,Args&&...args); (5) (since C++17)template<classK,class...Args>iteratortry_emplace(const_iteratorhint,K&&k,Args&&...args); (6)(since C++26)If a key equivalent to k already exists in the container, does nothing. Otherwise, inserts a new element into the container with key k and value constructed with args. In such case:
1) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(k),std::forward_as_tuple(std::forward<Args>(args)...))2) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(std::move(k)),std::forward_as_tuple(std::forward<Args>(args)...))3) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(std::forward<K>(k)),std::forward_as_tuple(std::forward<Args>(args)...))4) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(k),std::forward_as_tuple(std::forward<Args>(args)...))5) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(std::move(k)),std::forward_as_tuple(std::forward<Args>(args)...))6) Behaves like
except that the element is constructed as
value_type(std::piecewise_construct,std::forward_as_tuple(std::forward<K>(k)),std::forward_as_tuple(std::forward<Args>(args)...))1-6) If value_type is not
into unordered_map from the corresponding expression, the behavior is undefined.
3) This overload participates in overload resolution only if all following conditions are satisfied:
std::is_convertible_v<K&&,const_iterator> and std::is_convertible_v<K&&,iterator> are both false.
Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type.
If hash_function()(u.first)!=hash_function()(k)||contains(u.first) is true, the behavior is undefined, where u is the new element to be inserted.
6) This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are both valid and each denotes a type.
If hash_function()(u.first)!=hash_function()(k)||contains(u.first) is true, the behavior is undefined, where u is the new element to be inserted.
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
k - the key used both to look up and to insert if not found hint - iterator to the position before which the new element will be inserted args - arguments to forward to the constructor of the element Return value
1-3) Same as for
:
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.
4-6) Same as for
:
An iterator to the inserted element, or to the element that prevented the insertion.
Complexity
1-3) Same as for
:
Amortized constant on average, worst case linear in the size of the container.
4-6) Same as for
:
Amortized constant on average, worst case linear in the size of the container.
Notes
Unlike
or
, these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types, such as std::unordered_map<std::string,std::unique_ptr<foo>>. In addition, try_emplace treats the key and the arguments to the mapped_type separately, unlike
, which requires the arguments to construct a value_type (that is, a
).
Overloads (3,6) can be called without constructing an object of type Key.
macro ValueStdFeature
__cpp_lib_unordered_map_try_emplace
(C++17)std::unordered_map::try_emplace,
std::unordered_map::insert_or_assign
__cpp_lib_associative_heterogeneous_insertion
(C++26)Heterogeneous overloads for the remaining member functions in
and
associative
. Overloads (
) and (
). Example
Run this code
#include<iostream>#include<string>#include<unordered_map>#include<utility>voidprint_node(constauto&node){std::cout<<'['<<node.first<<"] = "<<node.second<<'\n';}voidprint_result(autoconst&pair){std::cout<<(pair.second?"inserted: ":"ignored: ");print_node(*pair.first);}intmain(){usingnamespacestd::literals;std::unordered_map<std::string,std::string>m;print_result(m.try_emplace("a","a"s));print_result(m.try_emplace("b","abcd"));print_result(m.try_emplace("c",10,'c'));print_result(m.try_emplace("c","Won't be inserted"));for(constauto&p:m)print_node(p);}Possible output:
inserted: [a] = a inserted: [b] = abcd inserted: [c] = cccccccccc ignored: [c] = cccccccccc [a] = a [b] = abcd [c] = cccccccccc See also
constructs element in-place
(public member function)
constructs elements in-place using a hint
(public member function)
inserts elements or nodes(since C++17)
(public member function)