std::unordered_map operator=

od C++17

// (1) Non const version only
unordered_map& operator=( const unordered_map& other );

// (2) Non const version only
unordered_map& operator=( unordered_map&& other ) noexcept(/* see below */);

// (3) Non const version only
unordered_map& operator=( std::initializer_list<value_type> ilist );

do C++17

// (1) Non const version only
unordered_map& operator=( const unordered_map& other );

// (2) Non const version only
unordered_map& operator=( unordered_map&& other );

// (3) Non const version only
unordered_map& operator=( std::initializer_list<value_type> ilist );

Replaces the contents of the container with the contents of another.

• (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).

important

other is in a valid but unspecified state afterwards.

If std::allocator_traits<Alloc>::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.

Parameters

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

Return value

*this

Complexity

• (1) Linear in the size of *this and other - O(size() + other.size())
• (2)
  Linear in the size of *this - O(size()).
  If the allocators do not compare equal and do not propagate, linear in the size of *this and other - O(size() + other.size()).
• (3) Linear in the size of *this and ilist - O(size() + ilist.size()).

Exceptions

od 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)

do C++17

May throw implementation-defined exceptions.

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.

Example

Main.cpp

#include <unordered_map>
#include <iterator>
#include <iostream>
#include <utility>
#include <initializer_list>
 
void print(auto const comment, auto const& container)
{
    auto size = std::size(container);
    std::cout << comment << "{ ";
    for (auto const& [key, value]: container)
        std::cout << '{' << key << ',' << value << (--size ? "}, " : "} ");
    std::cout << "}\n";
}
 
int main()
{
    std::unordered_map<int, int> x { {1,1}, {2,2}, {3,3} }, y, z;
    const auto w = { std::pair<const int, int>{4,4}, {5,5}, {6,6}, {7,7} };
 
    std::cout << "Initially:\n";
    print("x = ", x);
    print("y = ", y);
    print("z = ", z);
 
    std::cout << "Copy assignment copies data from x to y:\n";
    y = x;
    print("x = ", x);
    print("y = ", y);
 
    std::cout << "Move assignment moves data from x to z, modifying both x and z:\n";
    z = std::move(x);
    print("x = ", x);
    print("z = ", z);
 
    std::cout << "Assignment of initializer_list w to z:\n";
    z = w;
    print("w = ", w);
    print("z = ", z);
}

Possible output

Initially:
x = { {3,3}, {2,2}, {1,1} }
y = { }
z = { }
Copy assignment copies data from x to y:
x = { {3,3}, {2,2}, {1,1} }
y = { {3,3}, {2,2}, {1,1} }
Move assignment moves data from x to z, modifying both x and z:
x = { }
z = { {3,3}, {2,2}, {1,1} }
Assignment of initializer_list w to z:
w = { {4,4}, {5,5}, {6,6}, {7,7} }
z = { {7,7}, {6,6}, {5,5}, {4,4} }