std::vector rend() method

since C++11

// Non const version
reverse_iterator rend() noexcept;

// Const version
reverse_iterator rend() const noexcept;

// Const version
const_reverse_iterator crend() const noexcept;

until C++11

// Non const version
reverse_iterator rend();

// Const version
const_reverse_iterator crend();

It corresponds to the last element of the non-reversed vector.

Returns a reverse iterator

to the last element of the reversed vector. It corresponds to the element preceding the first element of the non-reversed vector.

It effectively returns an iterator that points past the end of the original vector. Attempting to dereference a past-the-end iterator is undefined behaviour

.

note

This method doesn't actually reverse the vector, it just returns an iterator that points to the element before the first element of the array, and whose +, -, --, ++ operators have slightly changed implementations.

For example it++ decrements the internal pointer and it-- increments it (so that traversing the container in a reverse order actually works).

If the container is empty, the returned iterator will be equal to rbegin().

Parameters

(none)

Return value

Reverse iterator to the first element.

Complexity

Constant.

Difference between rend and crend

For a const container c, rend and crend are the same - c.rend() == c.crend()

For non-const container of type c they return different iterators:

Non const container

rend

#include <map> 

int main()
{
    std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.rend(); // Type: std::map<int, float>::reverse_iterator
    *std::prev(it) = 5; // ✔ Ok
}

crend

#include <map>

int main()
{
    std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.crend(); // Type: std::map<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

Const container

rend

#include <map>

int main()
{
    const std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.rend(); // Type: std::map<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

crend

#include <map>

int main()
{
    const std::map<int, float> map = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.crend(); // Type: std::map<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

Example

Main.cpp

#include <algorithm>
#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.
    std::for_each(nums.rbegin(), nums.rend(), [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';
 
    // Sums all integers in the vector nums (if any), printing only the result.
    std::cout << "Sum of nums: "
              << std::accumulate(nums.rbegin(), nums.rend(), 0) << '\n';
 
    // Prints the first fruit in the vector fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "First fruit: " << *fruits.rbegin() << '\n';
 
    if (empty.rbegin() == empty.rend())
        std::cout << "vector 'empty' is indeed empty.\n";
}

Output

16 8 4 2 1
Sum of nums: 31
First fruit: raspberry
vector 'empty' is indeed empty.