std::multiset rbegin() method

od C++11

// Non const version
reverse_iterator rbegin() noexcept;

// Const version
reverse_iterator rbegin() const noexcept;

// Const version
const_reverse_iterator crbegin() const noexcept;

do C++11

// Non const version
reverse_iterator rbegin();

// Const version
const_reverse_iterator crbegin();

Returns a reverse iterator

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

zanotuj

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

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

crbegin

#include <map>

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

Const container

rbegin

#include <map>

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

crbegin

#include <map>

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

Example

Main.cpp

#include <iostream>
#include <set>
 
int main()
{
    std::multiset<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

⏼ ⏼ ⏼ ⏼ 
⏼ ⏼ ⏼ ⏼ 
⏼ ⏼ ⏼ 
⏼ ⏼ ⏼ 
⏼ ⏼ 
⏼ ⏼ 
⏼ 
⏼