std::multimap rbegin() method

od C++11

// Non const version
reverse_iterator rbegin() noexcept;

// Const version
reverse_iterator rbegin() const noexcept;

// Const version
const_reverse_iterator crbegin() const noexcept;

do C++11

// Non const version
reverse_iterator rbegin();

// Const version
const_reverse_iterator crbegin();

Returns a reverse iterator

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

zanotuj

This method doesn't actually reverse the vector, it just returns an iterator that points to the last element of the vector, 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 - O(1).

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 <map>

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

crbegin

#include <map>

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

Const container

rbegin

#include <map>

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

crbegin

#include <map>

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

Example

Main.cpp

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
 
int main()
{
    std::multimap<std::string, int> multimap {
        { "█", 1 },
        { "▒", 5 },
        { "░", 3 },
        { "▓", 7 },
        { "▓", 8 },
        { "░", 4 },
        { "▒", 6 },
        { "█", 2 },
    };
 
    std::cout << "Print out in reverse order using const reverse iterators:\n";
    std::for_each(multimap.crbegin(), multimap.crend(),
        [](std::pair<const std::string, int> const& e) {
            std::cout << "{ \"" << e.first << "\", " << e.second << " };\n";
        });
 
    multimap.rbegin()->second = 42; // OK: non-const value is modifiable
//  multimap.crbegin()->second = 42; // Error: can't modify the const value
}

Output

Print out in reverse order using const reverse iterators:
{ "▓", 8 };
{ "▓", 7 };
{ "▒", 6 };
{ "▒", 5 };
{ "░", 4 };
{ "░", 3 };
{ "█", 2 };
{ "█", 1 };