std::array rbegin()/crbegin() method

od C++17

// Non-const version
constexpr iterator rbegin() noexcept;

// Const version
constexpr const_iterator rbegin() const noexcept;
constexpr const_iterator crbegin() const noexcept;

do C++17

// Non-const version
iterator rbegin() noexcept;

// Const version
const_iterator rbegin() const noexcept;
const_iterator crbegin() const noexcept;

Returns a reverse iterator

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

zanotuj

This method doesn't actually reverse the array, it just returns an iterator that points to the last 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 rend().

Parameters

(none)

Return value

Reverse iterator to the first element.

Complexity

Constant.

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 <array>

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

crbegin

#include <array>

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

Const container

rbegin

#include <array>

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

crbegin

#include <array>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <string>
#include <string_view>
#include <array>
 
int main()
{
	constexpr std::array<std::string_view, 8> data = {"▁","▂","▃","▄","▅","▆","▇","█"};
 
	std::array<std::string, std::size(data)> arr;
 
	std::copy(data.cbegin(), data.cend(), arr.begin());
	//             ^              ^           ^
 
	auto print = [](const std::string_view s) { std::cout << s << ' '; };
 
	print("Print 'arr' in direct order using [cbegin, cend):\t");
	std::for_each(arr.cbegin(), arr.cend(), print);
	//                ^             ^
	print("\n\nPrint 'arr' in reverse order using [crbegin, crend):\t");
	std::for_each(arr.crbegin(), arr.crend(), print);
	//                ^^             ^^
	print("\n");
}

Possible output

Print 'arr' in direct order using [cbegin, cend):        ▁ ▂ ▃ ▄ ▅ ▆ ▇ █ 
 
Print 'arr' in reverse order using [crbegin, crend):     █ ▇ ▆ ▅ ▄ ▃ ▂ ▁