std::vector<T,Allocator>::operator=

 

C++

Language

Standard Library Headers

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Named requirements

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Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

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Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

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Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

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

span (C++20)

 

std::vector

Member functions

vector::vector

vector::~vector

vector::operator=

vector::assign

vector::get_allocator

Element access

vector::at

vector::operator[]

vector::front

vector::back

vector::data (C++11)

Iterators

vector::beginvector::cbegin (C++11)

vector::endvector::cend (C++11)

vector::rbeginvector::crbegin (C++11)

vector::rendvector::crend (C++11)

Capacity

vector::empty

vector::size

vector::max_size

vector::reserve

vector::capacity

vector::shrink_to_fit (C++11)

Modifiers

vector::clear

vector::insert

vector::emplace (C++11)

vector::erase

vector::push_back

vector::emplace_back (C++11)

vector::pop_back

vector::resize

vector::swap

Non-member functions

std::swap

eraseerase_if (C++20)(C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

Deduction guides(C++17)

 

	(1)
vector& operator=( const vector& other );			(until C++20)
constexpr vector& operator=( const vector& other );			(since C++20)
		(2)
vector& operator=( vector&& other );				(since C++11) (until C++17)
vector& operator=( vector&& other ) noexcept(/* see below */);				(since C++17) (until C++20)
constexpr vector& operator=( vector&& other ) noexcept(/* see below */);				(since C++20)
			(3)
vector& operator=( std::initializer_list<T> ilist );					(since C++11) (until C++20)
constexpr vector& operator=( std::initializer_list<T> ilist );					(since C++20)

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 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::value 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 elements originally present in *this are either destroyed or replaced by elementwise 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 2) noexcept specification:   noexcept(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value || std::allocator_traits<Allocator>::is_always_equal::value)

(since C++17)

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

Example

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

Run this code

#include <vector>
#include <iostream>
 
void display_sizes(char const* comment,
                   const std::vector<int>& nums1,
                   const std::vector<int>& nums2,
                   const std::vector<int>& nums3)
{
    std::cout << comment
              << " nums1: " << nums1.size() << ','
              << " nums2: " << nums2.size() << ','
              << " nums3: " << nums3.size() << '\n';
}
 
void display(char const* comment, const std::vector<int>& v)
{
    std::cout << comment << "{ ";
    for (int e : v) {
        std::cout << e << ' ';
    }
    std::cout << "}\n";
}
 
int main()
{
    std::vector<int> nums1 {3, 1, 4, 6, 5, 9};
    std::vector<int> nums2;
    std::vector<int> nums3;
 
    display_sizes("Initially:\n", nums1, nums2, nums3);
 
    // copy assignment copies data from nums1 to nums2
    nums2 = nums1;
 
    display_sizes("After assigment:\n", nums1, nums2, nums3);
 
    // move assignment moves data from nums1 to nums3,
    // modifying both nums1 and nums3
    nums3 = std::move(nums1);
 
    display_sizes("After move assigment:\n", nums1, nums2, nums3);
 
    display("Now nums3 = ", nums3);
 
    // copy assignment of an initializer_list copies data to nums3
    nums3 = {1, 2, 3};
 
    display("After assignment of initializer_list \n nums3 = ", 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
Now nums3 = { 3 1 4 6 5 9 }
After assignment of initializer_list
 nums3 = { 1 2 3 }

See also

(constructor)	constructs the vector (public member function)
assign	assigns values to the container (public member function)