std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::insert_or_assign
From cppreference.com
< cpp | container | unordered map
| template< class M > std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj ); |
(1) | (since C++17) |
| template< class M > std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj ); |
(2) | (since C++17) |
template< class K, class M > std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj ); |
(3) | (since C++26) |
| template< class M > iterator insert_or_assign( const_iterator hint, const Key& k, M&& obj ); |
(4) | (since C++17) |
| template< class M > iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj ); |
(5) | (since C++17) |
template< class K, class M > iterator insert_or_assign( const_iterator hint, K&& k, M&& obj ); |
(6) | (since C++26) |
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 and KeyEqual are both transparent. 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.
Contents |
[edit] 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 |
[edit] 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.
[edit] Complexity
1-3) Same as for
emplace.4-6) Same as for
emplace_hint.[edit] 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
|
__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). |
[edit] 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
[edit] See also
| access or insert specified element (public member function) | |
| access specified element with bounds checking (public member function) | |
| inserts elements or nodes(since C++17) (public member function) | |
| constructs element in-place (public member function) |
