std::set rend() method
- od C++11
- do C++11
// Non const version
reverse_iterator rend() noexcept;
// Const version
reverse_iterator rend() const noexcept;
// Const version
const_reverse_iterator crend() const noexcept;
// Non const version
reverse_iterator rend();
// Const version
const_reverse_iterator crend();
It corresponds to the last element of the non-reversed container.
Returns a reverse iterator
to the last element of the container. It corresponds to the element preceding the first element of the non-reversed container.It effectively returns an iterator that points past the end of the original container. Attempting to dereference a past-the-end iterator is undefined behaviour
.This method doesn't actually reverse the container, it just returns an iterator that points to the element before the first element of it,
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
- Const container
- rend
- crend
#include <set>
int main()
{
std::set<int> set = {1, 2, 3, 4, 5};
auto it = set.rend(); // Type: std::set<int>::reverse_iterator
*std::prev(it) = 5; // ✔ Ok
}
#include <set>
int main()
{
std::set<int> set = {1, 2, 3, 4, 5};
auto it = set.crend(); // Type: std::set<int>::const_reverse_iterator
*std::prev(it) = 5; // ❌ Error!
}
- rend
- crend
#include <set>
int main()
{
const std::set<int> set = {1, 2, 3, 4, 5};
auto it = set.rend(); // Type: std::set<int>::const_reverse_iterator
*std::prev(it) = 5; // ❌ Error!
}
#include <set>
int main()
{
const std::set<int> set = {1, 2, 3, 4, 5};
auto it = set.crend(); // Type: std::set<int>::const_reverse_iterator
*std::prev(it) = 5; // ❌ Error!
}
Example
#include <iostream>
#include <set>
int main()
{
std::set<unsigned> rep{1, 2, 3, 4, 1, 2, 3, 4};
for (auto it = rep.crbegin(); it != rep.crend(); ++it) {
for (auto n = *it; n > 0; --n)
std::cout << "⏼" << ' ';
std::cout << '\n';
}
}
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