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

From cppreference.com

reverse_iterator rend();
(1) (noexcept since C++11)
(constexpr since C++20)		 
const_reverse_iterator rend() const;
(2) (noexcept since C++11)
(constexpr since C++20)		 
const_reverse_iterator crend() const noexcept;
(3) (since C++11)
(constexpr since C++20)

Returns a reverse iterator to the element following the last element of the reversed vector. It corresponds to the element preceding the first element of the non-reversed vector. This element acts as a placeholder, attempting to access it results in undefined behavior.

Return value

Reverse iterator to the element following the last element.

Complexity

Constant.

Notes

libc++ backports crend() to C++98 mode.

Example

#include <algorithm>
#include <cassert>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>

int main()
{
    std::vector<int> nums{1, 2, 4, 8, 16};
    std::vector<std::string> fruits{"orange", "apple", "raspberry"};
    std::vector<char> empty;

    // Print vector nums.
    std::for_each(nums.crbegin(), nums.crend(),
                  [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';

    // Sum all integers in the vector nums, printing only the result.
    std::cout << "Sum of nums: "
              << std::accumulate(nums.crbegin(), nums.crend(), 0) << '\n';

    // Print the last fruit in the vector fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "Last fruit: " << *fruits.crbegin() << '\n';

    if (empty.crbegin() == empty.crend())
        std::cout << "vector ‘empty’ is indeed empty.\n";

    // Modify the last element in nums.
    *nums.rbegin() = 32;
    assert(*nums.crbegin() == 32);
}

Output: 

16 8 4 2 1
Sum of nums: 31
Last fruit: raspberry
vector ‘empty’ is indeed empty.

See also

returns a reverse iterator to the beginning
(public member function) [edit]
returns a reverse end iterator for a container or array
(function template) [edit]