std::multimap rbegin() method
- since C++11
- until C++11
// Non const version
reverse_iterator rbegin() noexcept;
// Const version
reverse_iterator rbegin() const noexcept;
// Const version
const_reverse_iterator crbegin() const noexcept;
// 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.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
- Const container
- rbegin
- crbegin
#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
}
#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!
}
- rbegin
- crbegin
#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!
}
#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
#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
}
Print out in reverse order using const reverse iterators:
{ "▓", 8 };
{ "▓", 7 };
{ "▒", 6 };
{ "▒", 5 };
{ "░", 4 };
{ "░", 3 };
{ "█", 2 };
{ "█", 1 };
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