std::deque rbegin() method

od C++11

// Non const version
iterator rbegin() noexcept;

// Const version
const_iterator rbegin() const noexcept;
const_iterator crbegin() const noexcept;

do C++11

// Non const version
iterator rbegin();

// Const version
const_iterator rbegin() const;

Returns a reverse iterator

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

zanotuj

This method doesn't actually reverse the deque, it just returns an iterator that points to the last 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 rend().

Parameters

(none)

Return value

Reverse iterator to the first element.

Complexity

Constant.

Difference between rbegin and crbegin

For a const container c, rbegin and crbegin are the same - c.rbegin() == c.crbegin()

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

Non const container

rbegin

#include <deque>

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

crbegin

#include <deque>

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

Const container

rbegin

#include <deque>

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

crbegin

#include <deque>

int main()
{
    const std::deque<int> arr = { 1, 2, 3};
    auto it = arr.crbegin(); // Type: std::deque<int>::reverse_const_iterator
    *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.