std::multiset rend() method

od C++11

// Non const version
reverse_iterator rend() noexcept;

// Const version
reverse_iterator rend() const noexcept;

// Const version
const_reverse_iterator crend() const noexcept;

do C++11

// Non const version
reverse_iterator rend();

// Const version
const_reverse_iterator crend();

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

Returns a reverse iterator

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

It effectively returns an iterator that points past the end of the original vector. Attempting to dereference a past-the-end iterator is undefined behaviour

.

zanotuj

This method doesn't actually reverse the vector, it just returns an iterator that points to the element before the first element of the array, 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 first element.

Complexity

Constant - O(1).

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 <map> 

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

crend

#include <map>

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

Const container

rend

#include <map>

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

crend

#include <map>

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

Example

Main.cpp

Run this code
#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

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