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

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< cpp‎ | container‎ | vector

 
C++
 
Containers library
Sequence
(C++11)
(C++11)
Associative
Unordered associative
(C++11)
(C++11)
(C++11)
(C++11)
Adaptors
Views
(C++20)
 
std::vector
Member functions
vector::operator=
Element access
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
(C++11)
Modifiers
(C++11)
(C++11)
Non-member functions
(C++20)(C++20)
(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 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.

(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 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>::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 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::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 assignment:\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 assignment:\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 assignment:
 nums1: 6, nums2: 6, nums3: 0
After move assignment:
 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

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