std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::try_emplace

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< cpp‎ | container‎ | unordered map

template < class... Args > std::pair <iterator, bool > try_emplace( const Key& 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 < class K, class... Args > std::pair <iterator, bool > try_emplace( K&& k, Args&&... args ) ;	(3)	(since C++26)
template < class... Args > iterator try_emplace( const_iterator hint, const Key& k, Args&&... args ) ;	(4)	(since C++17)
template < class... Args > iterator try_emplace( const_iterator hint, Key&& k, Args&&... args ) ;	(5)	(since C++17)
template < class K, class... Args > iterator try_emplace( const_iterator hint, 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

1) Behaves like emplace 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 emplace 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 emplace 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 emplace_hint 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 emplace_hint 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 emplace_hint 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 EmplaceConstructible into unordered_map

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

If hash_function( ) (u.first ) ! = hash_function( ) (k) || contains(u.first ) is true, the behavior is undefined, where u

6) This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent

If hash_function( ) (u.first ) ! = hash_function( ) (k) || contains(u.first ) is true, the behavior is undefined, where u

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.

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 emplace:
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

4-6) Same as for emplace_hint:

Complexity

1-3) Same as for emplace:
Amortized constant on average, worst case linear in the size of the container.

4-6) Same as for emplace_hint:

Notes

Unlike insert or emplace , 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 emplace, which requires the arguments to construct a value_type (that is, a std::pair

Overloads (3,6) can be called without constructing an object of type Key.

Feature-test macro	Value	Std	Feature
`__cpp_lib_unordered_map_try_emplace`	`201411L`	(C++17)	`std::unordered_map::try_emplace`, std::unordered_map::insert_or_assign
`__cpp_lib_associative_heterogeneous_insertion`	`202311L`	(C++26)	Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6)

Example

Run this code

#include <iostream>
#include <string>
#include <unordered_map>
#include <utility>
 
void print_node(const auto& node)
{
    std::cout << '[' << node.first << "] = " << node.second << '\n';
}
 
void print_result(auto const& pair)
{
    std::cout << (pair.second ? "inserted: " : "ignored:  ");
    print_node(*pair.first);
}
 
int main()
{
    using namespace std::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 (const auto& 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

emplace	constructs element in-place (public member function)
emplace_hint	constructs elements in-place using a hint (public member function)
insert	inserts elements or nodes(since C++17) (public member function)

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