NAME¶

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

Synopsis¶

Defined in header <ranges>
Defined in header <iterator>
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 /* see below */ auto crbegin( T&& t );

Returns an iterator to the first element of the const-qualified (until C++23)
argument that is treated as a reversed sequence.
Returns a constant iterator to the first element of the argument that (since C++23)
is treated as a reversed sequence.

range-rbegin-rend.svg

Let CT be

* const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is
an lvalue reference type), (until
* const T otherwise. C++23)

A call to ranges::crbegin is expression-equivalent to
ranges::rbegin(static_cast<CT&&>(t)).
If the argument is an lvalue or
ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to
ranges::crbegin is expression-equivalent to:

* std::const_iterator<decltype(U)>(U) for some expression U equivalent to (since
ranges::rbegin(/*possibly-const-range*/(t)). C++23)

In all other cases, a call to ranges::crbegin is ill-formed, which can result
in substitution failure when the call appears in the immediate context of a
template instantiation.

The return type models std::input_or_output_iterator
and constant-iterator
(since C++23) in all cases.

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.

Example¶

// Run this code

#include <cassert>
#include <iterator>
#include <span>
#include <vector>

int main()
{
std::vector<int> v{3, 1, 4};
auto vi = std::ranges::crbegin(v);
assert(*vi == 4);
++vi; // OK, iterator object is mutable
assert(*vi == 1);
// *vi = 13; // Error: underlying element is read-only

int a[]{-5, 10, 15};
auto ai = std::ranges::crbegin(a);
assert(*ai == 15);

// 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
assert(*si == 15);
static_assert
(
std::ranges::enable_borrowed_range<std::remove_cv_t<decltype(std::span{a})>>
);
}

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)