std::unordered_map::operator=

 

C++

Language

Standard library headers

Concepts

Utilities library

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Containers library

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Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

 

std::unordered_map

Member functions

unordered_map::unordered_map

unordered_map::~unordered_map

unordered_map::operator=

unordered_map::get_allocator

Iterators

unordered_map::beginunordered_map::cbegin

unordered_map::endunordered_map::cend

Capacity

unordered_map::erase

unordered_map::size

unordered_map::max_size

Modifiers

unordered_map::clear

unordered_map::insert

unordered_map::emplace

unordered_map::emplace_hint

unordered_map::erase

unordered_map::swap

Lookup

unordered_map::count

unordered_map::find

unordered_map::equal_range

Bucket interface

unordered_map::begin2

unordered_map::end2

unordered_map::bucket_count

unordered_map::max_bucket_count

unordered_map::bucket_size

unordered_map::bucket

Hash policy

unordered_map::load_factor

unordered_map::max_load_factor

unordered_map::rehash

unordered_map::reserve

Observers

unordered_map::hash_function

unordered_map::key_eq

 

unordered_map& operator=( const unordered_map& other );	(1)	(since C++11)
unordered_map& operator=( unordered_map&& other );	(2)	(since C++11)
unordered_map& 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() is true, the target allocator is replaced by a copy of the source allocator. If the target and the source allocators do not compare equal, the target (*this) allocator is used to deallocate the memory, then other's allocator is used to allocate it before copying the elements. (since C++11)

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() is true, the target allocator is replaced by a copy of the source allocator. If it is false and the source and the target allocators do not compare equal, the target cannot take ownership of the source memory and must move-assign each element individually, allocating additional memory using its own allocator as needed.

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 the other.

2) Constant unless std::allocator_traits<allocator_type>::propagate_on_container_move_assignment() is false and the allocators do not compare equal (in which case linear).

3) Linear in the size of ilist.

Example

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

Run this code

#include <unordered_map>
#include <iostream>
 
void display_sizes(const std::unordered_map<int, int> &nums1,
                   const std::unordered_map<int, int> &nums2,
                   const std::unordered_map<int, int> &nums3)
{
    std::cout << "nums1: " << nums1.size() 
              << " nums2: " << nums2.size()
              << " nums3: " << nums3.size() << '\n';
}
 
int main()
{
    std::unordered_map<int, int> nums1 {{3, 1}, {4, 1}, {5, 9}, 
                                        {6, 1}, {7, 1}, {8, 9}};
    std::unordered_map<int, int> nums2; 
    std::unordered_map<int, int> nums3;
 
    std::cout << "Initially:\n";
    display_sizes(nums1, nums2, nums3);
 
    // copy assignment copies data from nums1 to nums2
    nums2 = nums1;
 
    std::cout << "After assigment:\n"; 
    display_sizes(nums1, nums2, nums3);
 
    // move assignment moves data from nums1 to nums3,
    // modifying both nums1 and nums3
    nums3 = std::move(nums1);
 
    std::cout << "After move assigment:\n"; 
    display_sizes(nums1, nums2, nums3);
}

Output:

Initially:
nums1: 6 nums2: 0 nums3: 0
After assigment:
nums1: 6 nums2: 6 nums3: 0
After move assigment:
nums1: 0 nums2: 6 nums3: 6

See also

(constructor)

constructs the unordered_map (public member function)