std::map<Key,T,Compare,Allocator>::insert_or_assign

 

C++

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

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Non-member function table

 

std::map

Member functions

map::map map::~map               map::operator= map::get_allocator

Element access map::at map::operator[] Iterators map::beginmap::cbegin (C++11) map::endmap::cend (C++11) map::rbeginmap::crbegin (C++11)      map::rendmap::crend (C++11) Capacity map::size map::max_size map::empty Observers map::key_comp map::value_comp Modifiers map::clear map::insert map::erase map::swap map::extract (C++17) map::merge (C++17) map::insert_range (C++23) map::insert_or_assign (C++17) map::emplace (C++11) map::emplace_hint (C++11) map::try_emplace (C++17) Lookup map::count map::find map::contains (C++20) map::equal_range map::lower_bound map::upper_bound

Non-member functions

operator==operator<=> (C++20) std::swap(std::map) erase_if(std::map) (C++20)   operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)

Deduction guides (C++17)

 

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 equal_range(u.first) == equal_range(k) is false, the behavior is undefined. The value_type must be EmplaceConstructible into map from std::forward<K>(k), std::forward<M>(obj). This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It 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.

No iterators or references are 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_map_try_emplace	201411L	(C++17)	std::map::try_emplace, std::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).

Example

Run this code

#include <iostream>
#include <string>
#include <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::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);
}

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 or nodes(since C++17) (public member function)
emplace (C++11)	constructs element in-place (public member function)