std::vector 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.

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

crbegin

#include <map>

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

Const container

rbegin

#include <map>

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

crbegin

#include <map>

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

Example

Main.cpp

#include <iomanip>
#include <iostream>
#include <map>
#include <string_view>
 
int main() 
{
    const std::map<int, std::string_view> coins {
        {10, "dime"},
        {100, "dollar"},
        {50, "half dollar"},
        {5, "nickel"},
        {1, "penny"},
        {25, "quarter"}
        }; // initializer entries in name alphabetical order
    std::cout << "US coins in circulation, largest to smallest denomination:\n";
    for (auto it = coins.crbegin(); it != coins.crend(); ++it) {
        std::cout << std::setw(11) << it->second << " = ¢" << it->first << '\n';
    }
}