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std::multimap rbegin() method

// Non const version
reverse_iterator rbegin() noexcept;

// Const version
reverse_iterator rbegin() const noexcept;

// Const version
const_reverse_iterator crbegin() const noexcept;

Returns a reverse iterator

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

note

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:

#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
}

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 };
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::multimap rbegin() method

// Non const version
reverse_iterator rbegin() noexcept;

// Const version
reverse_iterator rbegin() const noexcept;

// Const version
const_reverse_iterator crbegin() const noexcept;

Returns a reverse iterator

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

note

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:

#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
}

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 };
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.