std::set rbegin() method

since C++11

// Non const version
iterator end() noexcept;

// Const version
iterator end() const noexcept;

// Const version
const_iterator cend() const noexcept;

until C++11

// Non const version
iterator end();

// Const version
const_iterator cend();

Returns a reverse iterator

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

note

This method doesn't actually reverse the container, it just returns an iterator that points to the last element of it, 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 <set>

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

crbegin

#include <set>

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

Const container

rbegin

#include <set>

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

crbegin

#include <set>

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

Example

Main.cpp

#include <iostream>
#include <set>
 
int main()
{
    std::set<unsigned> rep{1, 2, 3, 4, 1, 2, 3, 4};
 
    for (auto it = rep.crbegin(); it != rep.crend(); ++it) {
        for (auto n = *it; n > 0; --n)
            std::cout << "⏼" << ' ';
        std::cout << '\n';
    }
}

Output

⏼ ⏼ ⏼ ⏼ 
⏼ ⏼ ⏼ 
⏼ ⏼ 
⏼