std::multimap rend() method

od C++11

// Non const version
reverse_iterator rend() noexcept;

// Const version
reverse_iterator rend() const noexcept;

// Const version
const_reverse_iterator crend() const noexcept;

do C++11

// Non const version
reverse_iterator rend();

// Const version
const_reverse_iterator crend();

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

Returns a reverse iterator

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

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

.

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 <map> 

int main()
{
    std::multimap<int, float> multimap = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.rend(); // Type: std::multimap<int, float>::reverse_iterator
    *std::prev(it) = 5; // ✔ Ok
}

crend

#include <map>

int main()
{
    std::multimap<int, float> multimap = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.crend(); // Type: std::multimap<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

Const container

rend

#include <map>

int main()
{
    const std::multimap<int, float> multimap = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.rend(); // Type: std::multimap<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

crend

#include <map>

int main()
{
    const std::multimap<int, float> multimap = { {1, 1.f}, {2, 3.f}, {5, 8.f} };
    auto it = arr.crend(); // Type: std::multimap<int, float>::reverse_const_iterator
    *std::prev(it) = 5; // ❌ Error! 
}

Example

Main.cpp

#include <iostream>
#include <iomanip>
#include <chrono>
#include <map>
#include <string_view>
using namespace std::chrono;
 
// until C++20 chrono operator<< ready
std::ostream& operator<<(std::ostream& os, const year_month_day& ymd) {
    return os << std::setfill('0') << static_cast<int>(ymd.year()) << '/'
              << std::setw(2) << static_cast<unsigned>(ymd.month()) << '/'
              << std::setw(2) << static_cast<unsigned>(ymd.day());
}
 
int main() 
{
    const std::multimap<year_month_day, int> messages {
        { February/17/2023 , 10 },
        { February/17/2023 , 20 },
        { February/16/2022 , 30 },
        { October/22/2022  , 40 },
        { June/14/2022     , 50 },
        { November/23/2021 , 60 },
        { December/10/2022 , 55 },
        { December/12/2021 , 45 },
        { April/1/2020     , 42 },
        { April/1/2020     , 24 },
    };
    std::cout << "Messages received, reverse date order:\n";
    for (auto it = messages.crbegin(); it != messages.crend(); ++it) {
        std::cout << it->first << " : " << it->second << '\n';
    }
}

Output

Messages received, reverse date order:
2023/02/17 : 20
2023/02/17 : 10
2022/12/10 : 55
2022/10/22 : 40
2022/06/14 : 50
2022/02/16 : 30
2021/12/12 : 45
2021/11/23 : 60
2020/04/01 : 24
2020/04/01 : 42