std::deque rend() method

since C++11

// Non const version
iterator rend() noexcept;

// Const version
const_iterator rend() const noexcept;
const_iterator crend() const noexcept;

until C++11

// Non const version
iterator rend();

// Const version
const_iterator rend() const;

Returns a reverse iterator

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

It effectively returns an iterator that points before the begin of the original deque.

danger

Attempting to dereference a before-the-begin iterator is undefined behaviour

.

note

This member function doesn't actually reverse the deque, it just returns an iterator that points to the element before the first element of the deque, 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 element following the last 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 <deque>

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

crend

#include <deque>

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

Const container

rend

#include <deque>

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

crend

#include <deque>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <deque>

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

    // Print deque.
    std::for_each(nums.rbegin(), nums.rend(), [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';

    // Sums all integers in the deque 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 deque fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "First fruit: " << *fruits.rbegin() << '\n';

    if (empty.rbegin() == empty.rend())
        std::cout << "deque 'empty' is indeed empty.\n";
}

Output

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