std::vector<T,Allocator>::size - cppreference.com
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std::vector<T,Allocator>::size

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size_type size() const;
(noexcept since C++11)
(constexpr since C++20)

Returns the number of elements in the container, i.e., std::distance(begin(), end()).

Return value

The number of elements in the container.

Complexity

Constant.

Example

The following code uses size to display the number of elements in a std::vector<int>:

#include <cassert>
#include <vector>
 
int main()
{
    std::vector<int> nums;
    assert(nums.size() == 0);
    nums = {1, 2, 3, 4};
    assert(nums.size() == 4);
}

See also