std::ranges::rend(3) | C++ Standard Libary | std::ranges::rend(3) |
NAME¶
std::ranges::rend - std::ranges::rend
Synopsis¶
Defined in header <ranges>
inline namespace /*unspecified*/ {
(since C++20)
inline constexpr /*unspecified*/ rend = /*unspecified*/; (customization
point object)
}
Call signature
template< class T >
requires /* see below */
constexpr (since C++20)
std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>
auto
rend( T&& t );
Returns a sentinel indicating the end of a reversed range.
range-rbegin-rend.svg
Let t be an object of type T. If the argument is an lvalue or
ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then
a call to
ranges::rend is expression-equivalent to:
1. t.rend() converted to its decayed type, if that expression with conversion
is
valid, and its converted type models
std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>.
2. Otherwise, rend(t) converted to its decayed type, if T is a class or
enumeration
type, the aforementioned unqualified call with conversion is valid, and its
converted type models
std::sentinel_for<decltype(ranges::rbegin(std::declval<T>()))>,
where the
overload resolution is performed with the following candidates:
* void rend(auto&) = delete;
* void rend(const auto&) = delete;
* any declarations of rend found by argument-dependent lookup.
3. Otherwise, std::make_reverse_iterator(ranges::begin(t)) if both
ranges::begin(t)
and ranges::end(t) are valid expressions, have the same type, and that type
models std::bidirectional_iterator.
In all other cases, a call to ranges::rend is ill-formed, which can result in
substitution failure when ranges::rend(t) appears in the immediate context of
a
template instantiation.
Expression-equivalent
Expression e is expression-equivalent to expression f, if
* e and f have the same effects, and
* either both are constant subexpressions or else neither is a constant
subexpression, and
* either both are potentially-throwing or else neither is
potentially-throwing
(i.e. noexcept(e) == noexcept(f)).
Customization point objects
The name ranges::rend denotes a customization point object, which is a const
function object of a literal semiregular class type. For exposition purposes,
the
cv-unqualified version of its type is denoted as __rend_fn.
All instances of __rend_fn are equal. The effects of invoking different
instances of
type __rend_fn on the same arguments are equivalent, regardless of whether
the
expression denoting the instance is an lvalue or rvalue, and is
const-qualified or
not (however, a volatile-qualified instance is not required to be invocable).
Thus,
ranges::rend can be copied freely and its copies can be used
interchangeably.
Given a set of types Args..., if std::declval<Args>()... meet the
requirements for
arguments to ranges::rend above, __rend_fn models
* std::invocable<__rend_fn, Args...>,
* std::invocable<const __rend_fn, Args...>,
* std::invocable<__rend_fn&, Args...>, and
* std::invocable<const __rend_fn&, Args...>.
Otherwise, no function call operator of __rend_fn participates in overload
resolution.
Notes¶
If the argument is an rvalue (i.e. T is an object type) and
ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, or
if it is of an array
type of unknown bound, the call to ranges::rend is ill-formed, which also
results in
substitution failure.
If ranges::rend(std::forward<T>(t)) is valid, then
decltype(ranges::rend(std::forward<T>(t))) and
decltype(ranges::begin(std::forward<T>(t))) model std::sentinel_for in
all cases,
while T models std::ranges::range.
The C++20 standard requires that if the underlying rend function call returns
a
prvalue, the return value is move-constructed from the materialized temporary
object. All implementations directly return the prvalue instead. The
requirement is
corrected by the post-C++20 proposal P0849R8 to match the
implementations.
Example¶
// Run this code
#include <algorithm>
#include <iostream>
#include <ranges>
#include <vector>
int main()
{
std::vector<int> v = { 3, 1, 4 };
namespace ranges = std::ranges;
if (ranges::find(ranges::rbegin(v), ranges::rend(v), 5) != ranges::rend(v)) {
std::cout << "found a 5 in vector `v`!\n";
}
int a[] = { 5, 10, 15 };
if (ranges::find(ranges::rbegin(a), ranges::rend(a), 5) != ranges::rend(a)) {
std::cout << "found a 5 in array `a`!\n";
}
}
Output:¶
found a 5 in array `a`!
See also¶
ranges::crend returns a reverse end iterator to a read-only range
(C++20) (customization point object)
ranges::rbegin returns a reverse iterator to a range
(C++20) (customization point object)
rend returns a reverse end iterator for a container or array
crend (function template)
(C++14)
2022.07.31 | http://cppreference.com |