NAME

std::unordered_map::operator= - std::unordered_map::operator=

Synopsis

unordered_map& operator=( const unordered_map& other (1) (since C++11)
);
unordered_map& operator=( unordered_map&& other ); (since C++11)
(until C++17)
unordered_map& operator=( unordered_map&& other ) (2) (since C++17)
noexcept(/* see below */);
unordered_map& operator=( (3) (since C++11)
std::initializer_list<value_type> ilist );

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.

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.
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.

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 (since C++17)

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

&& std::is_nothrow_move_assignable<Pred>::value)

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

The following code uses operator= to assign one std::unordered_map to another:

// Run this code

#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} }