NAME¶

std::flat_map::insert_or_assign - std::flat_map::insert_or_assign

Synopsis¶

template< class M >
std::pair<iterator, bool> insert_or_assign( const key_type& k, M&& (1) (since C++23)
obj );
template< class M >
std::pair<iterator, bool> insert_or_assign( key_type&& k, M&& obj (2) (since C++23)
);
template< class K, class M > (3) (since C++23)
std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj );
template< class M >
iterator insert_or_assign( const_iterator hint, const key_type& k, (4) (since C++23)
M&& obj );
template< class M >
iterator insert_or_assign( const_iterator hint, key_type&& k, M&& (5) (since C++23)
obj );
template< class K, class M > (6) (since C++23)
iterator insert_or_assign( const_iterator hint, K&& k, M&& obj );

1,2) 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
* (1,2) try_emplace(std::forward<decltype(k)>(k), std::forward<M>(obj)),
* (4,5) try_emplace(hint, std::forward<decltype(k)>(k), std::forward<M>(obj)).
The program is ill-formed if either std::is_assignable_v<mapped_type&, M> or
std::is_constructible_v<mapped_type, M> is false.
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. Otherwise,
equivalent to
* (3) try_emplace(std::forward<K>(k), std::forward<M>(obj)),
* (6) try_emplace(hint, std::forward<K>(k), std::forward<M>(obj)).
The conversion from k into key_type must construct an object u, for which find(k) ==
find(u) is true. Otherwise, the behavior is undefined.
These overloads participate in overload resolution only if :
* The qualified-id Compare::is_transparent is valid and denotes a type.
* std::is_constructible_v<key_type, K> is true.
* std::is_assignable_v<mapped_type&, M> is true.
* std::is_constructible_v<mapped_type, M> is true.

Information on iterator invalidation is copied from here

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.

Example¶

// Run this code

#include <flat_map>
#include <iostream>
#include <string>

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::flat_map<std::string, std::string> map;

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

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

Output:¶

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

See also¶

operator[] access or insert specified element
(public member function)
at access specified element with bounds checking
(public member function)
insert inserts elements
(public member function)
emplace constructs element in-place
(public member function)
inserts in-place if the key does not exist, does nothing if the key
try_emplace exists
(public member function)