std::string rend() method

od C++20

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

// Const version
constexpr const_iterator rend() const noexcept;
constexpr const_iterator crend() const noexcept;

od C++11

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

// Const version
const_iterator rend() const noexcept;
const_iterator crend() const noexcept;

do C++11

// Non-const version
iterator rend();

// Const version
const_iterator rend() const;

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

Returns a reverse iterator

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

Attempting to dereference a past-the-end iterator is undefined behaviour

.

zanotuj

This method doesn't actually reverse the string, it just returns an iterator that points to the element before the first element of the string, and which +, -, --, ++ 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 <string> 

int main()
{
    std::string str = "Hello";
    auto it = str.rend(); // Type: std::string::reverse_iterator
    *std::prev(it) = 'J'; // ✔ Ok
}

crend

#include <string>

int main()
{
    std::string str = "Hello";
    auto it = str.crend(); // Type: std::string::reverse_const_iterator
    *std::prev(it) = 'J'; // ❌ Error! 
}

Const container

rend

#include <string>

int main()
{
    const std::string str = "Hello";
    auto it = str.rend(); // Type: std::string::reverse_const_iterator
    *std::prev(it) = 'J'; // ❌ Error! 
}

crend

#include <string>

int main()
{
    const std::string str = "Hello";
    auto it = str.crend(); // Type: std::string::reverse_const_iterator
    *std::prev(it) = 'J'; // ❌ Error! 
}

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
  std::string s("A man, a plan, a canal: Panama");
  {
    std::string c;
    std::copy(s.rbegin(), s.rend(), std::back_inserter(c));
    std::cout << c <<'\n'; // "amanaP :lanac a ,nalp a ,nam A"
  }
 
  {
    std::string c;
    std::copy(s.crbegin(), s.crend(), std::back_inserter(c));
    std::cout << c <<'\n'; // "amanaP :lanac a ,nalp a ,nam A"
  }
}

Output

amanaP :lanac a ,nalp a ,nam A
amanaP :lanac a ,nalp a ,nam A