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std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::operator=

From cppreference.com
 
 
Containers library
Sequence
(C++11)
Associative
Unordered associative
Adaptors
Views
(C++20)
 
 
unordered_multimap& operator=( const unordered_multimap& other );
(1) (since C++11)
(2)
unordered_multimap& operator=( unordered_multimap&& other );
(since C++11)
(until C++17)
unordered_multimap& operator=( unordered_multimap&& other ) noexcept(/* see below */);
(since C++17)
unordered_multimap& operator=( std::initializer_list<value_type> ilist );
(3) (since C++11)

Replaces the contents of the container.

1) Copy assignment operator. Replaces the contents with a copy of the contents of other.
If std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true, the allocator of *this is replaced by a copy of that of other. If the allocator of *this after assignment would compare unequal to its old value, the old allocator is used to deallocate the memory, then the new allocator is used to allocate it before copying the elements. Otherwise, the memory owned by *this may be reused when possible. In any case, the elements originally belong to *this may be either destroyed or replaced by element-wise copy-assignment.
2) Move assignment operator. Replaces the contents with those of other using move semantics (i.e. the data in other is moved from other into this container). other is in a valid but unspecified state afterwards.
If std::allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true, the allocator of *this is replaced by a copy of that of other. If it is false and the allocators of *this and other do not compare equal, *this cannot take ownership of the memory owned by other and must move-assign each element individually, allocating additional memory using its own allocator as needed. In any case, all elements originally belong to *this are either destroyed or replaced by element-wise move-assignment.
3) Replaces the contents with those identified by initializer list ilist.

Contents

[edit] Parameters

other - another container to use as data source
ilist - initializer list to use as data source

[edit] Return value

*this

[edit] Complexity

1) Linear in the size of *this and other.
2) Linear in the size of *this unless the allocators do not compare equal and do not propagate, in which case linear in the size of *this and other.
3) Linear in the size of *this and ilist.

[edit] Exceptions

May throw implementation-defined exceptions.

(until C++17)
1,3) May throw implementation-defined exceptions.
2)
noexcept specification:  
noexcept(std::allocator_traits<Allocator>::is_always_equal::value

&& std::is_nothrow_move_assignable<Hash>::value

&& std::is_nothrow_move_assignable<Pred>::value)
(since C++17)

[edit] Notes

After container move assignment (overload (2)), unless element-wise move assignment is forced by incompatible allocators, references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

[edit] Example

The following code uses operator= to assign std::unordered_multimap or std::initializer_list to another std::unordered_multimap.

#include <iostream>
#include <string_view>
#include <unordered_map>
 
void display_sizes(const std::unordered_multimap<int, int> &map1,
                   const std::unordered_multimap<int, int> &map2,
                   const std::unordered_multimap<int, int> &map3,
                   const std::string_view comment = "")
{
    std::cout << "map1: " << map1.size() << ", "
              << "map2: " << map2.size() << ", "
              << "map3: " << map3.size() << " ("
              << comment << ")\n";
}
 
int main()
{
    std::unordered_multimap<int, int> map1 {{3, 1}, {4, 1}, {5, 9},
                                            {6, 1}, {7, 1}, {8, 9}};
    std::unordered_multimap<int, int> map2;
    std::unordered_multimap<int, int> map3;
 
    display_sizes(map1, map2, map3, "initial sizes");
 
    // copy assignment copies data from map1 to map2
    map2 = map1;
 
    display_sizes(map1, map2, map3, "sizes after assignment");
 
    // move assignment moves data from map1 to map3,
    // modifying both map1 and map3
    map3 = std::move(map1);
 
    display_sizes(map1, map2, map3, "sizes after move assignment");
 
    // assignment of an initializer_list, see overload (3)
    map2 = { {1, 1}, {2, 2}, {3, 3} };
 
    display_sizes(map1, map2, map3, "sizes after initializer_list assignment");
}

Output:

map1: 6, map2: 0, map3: 0 (initial sizes)
map1: 6, map2: 6, map3: 0 (sizes after assignment)
map1: 0, map2: 6, map3: 6 (sizes after move assignment)
map1: 0, map2: 3, map3: 6 (sizes after initializer_list assignment)

[edit] See also

constructs the unordered_multimap
(public member function) [edit]