std::vector rbegin() method

od C++20

// prism-push-types:iterator,const_iterator
// Non-const version
constexpr iterator        end() noexcept;

// Const version
constexpr const_iterator  end() const noexcept;
constexpr const_iterator  cend() const noexcept;

od C++11

// prism-push-types:iterator,const_iterator
// Non-const version
iterator        end() noexcept;

// Const version
const_iterator  end() const noexcept;
const_iterator  cend() const noexcept;

do C++11

// Non-const version
iterator        end();

// Const version
const_iterator  end() const;

Returns a reverse iterator

to the first element of the reversed container.

zanotuj

This method doesn't actually reverse the container, it just returns an iterator that points to the last element of the container, 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 - O(1).

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 <vector>

int main()
{
    std::vector<int> arr = {1, 2, 3, 4, 5};
    auto it = arr.rbegin(); // Type: std::vector<int>::reverse_iterator
    *it = 5; // ✔ Ok
}

crbegin

#include <vector>

int main()
{
    std::vector<int> arr = {1, 2, 3, 4, 5};
    auto it = arr.crbegin(); // Type: std::vector<int>::reverse_const_iterator
    *it = 5; // ❌ Error! 
}

Const container

rbegin

#include <vector>

int main()
{
    const std::vector<int> arr = {1, 2, 3, 4, 5};
    auto it = arr.rbegin(); // Type: std::vector<int>::reverse_const_iterator
    *it = 5; // ❌ Error! 
}

crbegin

#include <vector>

int main()
{
    const std::vector<int> arr = {1, 2, 3, 4, 5};
    auto it = arr.crbegin(); // Type: std::vector<int>::reverse_const_iterator
    *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.