std::set rend() method

since C++11

// Non const version
reverse_iterator rend() noexcept;

// Const version
reverse_iterator rend() const noexcept;

// Const version
const_reverse_iterator crend() const noexcept;

until C++11

// Non const version
reverse_iterator rend();

// Const version
const_reverse_iterator crend();

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

Returns a reverse iterator

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

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

.

note

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

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

crend

#include <set>

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

Const container

rend

#include <set>

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

crend

#include <set>

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

⏼ ⏼ ⏼ ⏼ 
⏼ ⏼ ⏼ 
⏼ ⏼ 
⏼