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

From cppreference.com

 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Iterators
Capacity
Modifiers
(C++23)
(C++17)
unordered_map::try_emplace
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Bucket interface
Hash policy
Observers
Non-member functions
(until C++20)
(C++20)
Deduction guides(C++17)
 
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... Args >
iterator try_emplace( const_iterator hint, const Key& k, Args&&... args );
 (3) (since C++17)
template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&... args );
 (4) (since C++17)

Inserts a new element into the container with key k and value constructed with args, if there is no element with the key in the container.

1) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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)...))
4) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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)...))

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

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

1,2) Same as for emplace
3,4) Same as for emplace_hint

Complexity

1,2) Same as for emplace
3,4) 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).

Feature-test macro Value Std Comment
__cpp_lib_unordered_map_try_emplace 201411L (C++17) std::unordered_map::try_emplace,
std::unordered_map::insert_or_assign

Example

Run this code
#include <iostream>
#include <utility>
#include <string>
 
#include <unordered_map>
 
auto print_node = [](const auto &node) {
    std::cout << "[" << node.first << "] = " << node.second << '\n';
};
 
auto 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

(C++11)
 constructs element in-place
(public member function)
(C++11)
 constructs elements in-place using a hint
(public member function)
(C++11)
 inserts elements or nodes (since C++17)
(public member function)
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