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# std::unordered_map::operator=

From cppreference.com
< cpp‎ | container‎ | unordered map

[edit template]

unordered_map& operator=( const unordered_map& other ); | (1) | (since C++11)
| (2) |
unordered_map& operator=( unordered_map&& other ); |  | (since C++11) (until C++17)
unordered_map& operator=( unordered_map&& other ) noexcept(/* see below */); |  | (since C++17)
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. In any case, all element originally present in *this are either destroyed or replaced by elementwise move-assignment.
3) Replaces the contents with those identified by initializer list `ilist`.

### [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`.

### 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 elementwise 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 §23.2.1[container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

### [edit] Example

[edit]

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

### [edit] See also

(constructor) | constructs the `unordered_map` (public member function) [edit]

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Category:

- conditionally noexcept