NAME¶

std::unordered_map::insert_or_assign - std::unordered_map::insert_or_assign

Synopsis¶

template< class M >
std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj (1) (since C++17)
);
template< class M > (2) (since C++17)
std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj );
template< class K, class M > (3) (since C++26)
std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj );
template< class M >
iterator insert_or_assign( const_iterator hint, const Key& k, M&& (4) (since C++17)
obj );
template< class M >
iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj (5) (since C++17)
);
template< class K, class M > (6) (since C++26)
iterator insert_or_assign( const_iterator hint, K&& k, M&& obj );

1,4) If a key equivalent to k already exists in the container, assigns
std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does
not exist, inserts the new value as if by insert, constructing it from value_type(k,
std::forward<M>(obj)).
2,5) Same as (1,4), except the mapped value is constructed from
value_type(std::move(k), std::forward<M>(obj)).
3,6) If a key equivalent to k already exists in the container, assigns
std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does
not exist, constructs an object u of value_type with std::forward<K>(k),
std::forward<M>(obj)), then inserts u into *this. If hash_function()(u.first) !=
hash_function()(k) || contains(u.first) is true, the behavior is undefined. The
value_type must be EmplaceConstructible into unordered_map from std::forward<K>(k),
std::forward<M>(obj). This overload participates in overload resolution only if
Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type.
This assumes that such Hash is callable with both K and Key type, and that the
KeyEqual is transparent, which, together, allows calling this function without
constructing an instance of Key.

The behavior is undefined
(until C++20)
The program is ill-formed
(since C++20) if std::is_assignable_v<mapped_type&, M&&> is false.

If after the operation the new number of elements is greater than old
max_load_factor() * bucket_count() 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
obj - the value to insert or assign

Return value¶

1-3) The bool component is true if the insertion took place and false if the
assignment took place. The iterator component is pointing at the element that was
inserted or updated.
4-6) Iterator pointing at the element that was inserted or updated.

Complexity¶

1-3) Same as for emplace.
4-6) Same as for emplace_hint.

Notes¶

insert_or_assign returns more information than operator[] and does not require
default-constructibility of the mapped type.

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
Heterogeneous overloads for the __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) remaining member functions in
ordered and unordered associative
containers. Overloads (3) and (6).

Example¶

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>

void print_node(const auto& node)
{
std::cout << '[' << node.first << "] = " << node.second << '\n';
}

void print_result(auto const& pair)
{
std::cout << (pair.second ? "inserted: " : "assigned: ");
print_node(*pair.first);
}

int main()
{
std::unordered_map<std::string, std::string> myMap;

print_result(myMap.insert_or_assign("a", "apple"));
print_result(myMap.insert_or_assign("b", "banana"));
print_result(myMap.insert_or_assign("c", "cherry"));
print_result(myMap.insert_or_assign("c", "clementine"));

for (const auto& node : myMap)
print_node(node);
}

Possible output:¶

inserted: [a] = apple
inserted: [b] = banana
inserted: [c] = cherry
assigned: [c] = clementine
[c] = clementine
[a] = apple
[b] = banana

See also¶

operator[] access or insert specified element
(public member function)
at access specified element with bounds checking
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)
emplace constructs element in-place
(public member function)