NAME

std::ranges::crbegin - std::ranges::crbegin

Synopsis

Defined in header <ranges>
inline namespace /*unspecified*/ {

inline constexpr /*unspecified*/ crbegin = (since C++20)
/*unspecified*/; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr std::input_or_output_iterator auto crbegin(
T&& t );

Returns an iterator to the first element of the const-qualified argument that is
treated as a reversed sequence.

range-rbegin-rend.svg

Let CT be

1. const std::remove_reference_t<T>& if the argument is a lvalue (i.e. T is an
lvalue reference type),
2. const T otherwise,

a call to ranges::crbegin is expression-equivalent to
ranges::rbegin(static_cast<CT&&>(t)).

The return type models std::input_or_output_iterator in both cases.

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::crbegin 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 __crbegin_fn.

All instances of __crbegin_fn are equal. The effects of invoking different instances
of type __crbegin_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::crbegin 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::crbegin above, __crbegin_fn models

* std::invocable<__crbegin_fn, Args...>,
* std::invocable<const __crbegin_fn, Args...>,
* std::invocable<__crbegin_fn&, Args...>, and
* std::invocable<const __crbegin_fn&, Args...>.

Otherwise, no function call operator of __crbegin_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, the call to
ranges::crbegin is ill-formed, which also results in substitution failure.

The return type models std::input_or_output_iterator in all cases.

Example

// Run this code

#include <iostream>
#include <vector>
#include <iterator>
#include <span>

int main()
{
std::vector<int> v = { 3, 1, 4 };
auto vi = std::ranges::crbegin(v);
std::cout << *vi << '\n';

int a[] = { -5, 10, 15 };
auto ai = std::ranges::crbegin(a);
std::cout << *ai << '\n';

// auto x_x = std::ranges::crbegin(std::vector<int>{6,6,6});
// ill-formed: the argument is an rvalue (see Notes ↑)

auto si = std::ranges::crbegin(std::span{a}); // OK:
static_assert(std::ranges::enable_borrowed_range<
std::remove_cv_t<decltype(std::span{a})>>);
std::cout << *si << '\n';
}

Output:

4
15
15

See also

ranges::rbegin returns a reverse iterator to a range
(C++20) (customization point object)
rbegin returns a reverse iterator to the beginning of a container or array
crbegin (function template)
(C++14)