std::vector rend() method

od C++20

// prism-push-types:iterator,const_iterator
// Non-const version
constexpr iterator        rend() noexcept;

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

od C++11

// prism-push-types:iterator,const_iterator
// Non-const version
iterator        rend() noexcept;

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

do C++11

// prism-push-types:iterator,const_iterator
// Non-const version
iterator        rend();

// Const version
const_iterator  rend() const;

Returns a reverse iterator

to the last element of the reversed 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.

Undefined Behaviour

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

.

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 <vector> 

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

crend

#include <vector>

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

Const container

rend

#include <vector>

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

crend

#include <vector>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
 
int main()
{
    std::vector<int> nums {1, 2, 4, 8, 16};
    std::vector<std::string> fruits {"orange", "apple", "raspberry"};
    std::vector<char> empty;
 
    // Print vector.
    std::for_each(nums.rbegin(), nums.rend(), [](const int n) { std::cout << n << ' '; });
    std::cout << '\n';
 
    // Sums all integers in the vector nums (if any), printing only the result.
    std::cout << "Sum of nums: "
              << std::accumulate(nums.rbegin(), nums.rend(), 0) << '\n';
 
    // Prints the first fruit in the vector fruits, checking if there is any.
    if (!fruits.empty())
        std::cout << "First fruit: " << *fruits.rbegin() << '\n';
 
    if (empty.rbegin() == empty.rend())
        std::cout << "vector 'empty' is indeed empty.\n";
}

Output

16 8 4 2 1
Sum of nums: 31
First fruit: raspberry
vector 'empty' is indeed empty.