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 M>
iterator insert_or_assign( const_iterator hint, const Key& k, M&& (3) (since C++17)
obj );
template <class M>
iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj (4) (since C++17)
);

1,3) 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))
2,4) Same as (1,3), except the mapped value is constructed from
value_type(std::move(k), std::forward<M>(obj))

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 an insertion occurs and results in a rehashing of the container, 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
obj - the value to insert or assign

Return value¶

1,2) 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
3,4) Iterator pointing at the element that was inserted or updated

Complexity¶

1,2) Same as for emplace
3,4) 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: __cpp_lib_unordered_map_try_emplace

Example¶

// Run this code

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

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

auto 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
(C++11) (public member function)
at access specified element with bounds checking
(C++11) (public member function)
inserts elements
insert or nodes
(C++11) (since C++17)
(public member function)
emplace constructs element in-place
(C++11) (public member function)