Przejdź do głównej zawartości

std::vector rend() method

// 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;

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:

#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
}

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.
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.

std::vector rend() method

// 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;

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:

#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
}

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.
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.