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std::ranges::crend

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< cpp‎ | ranges
 
 
Ranges library
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(C++23)
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(until C++23)(C++23)


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

    inline constexpr /* unspecified */ crend = /* unspecified */;

}
(since C++20)
(customization point object)
Call signature
template< class T >

    requires /* see below */

constexpr /* see below */ auto crend( T&& t );
(since C++20)

Returns a sentinel for the constant iterator (since C++23) indicating the end of a const-qualified (until C++23) range that 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),
  • const T otherwise.

A call to ranges::crend is expression-equivalent to ranges::rend(static_cast<CT&&>(t)).

(until C++23)

If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to ranges::crend is expression-equivalent to:

  • std::const_sentinel<decltype(U)>(U) for some expression U equivalent to ranges::rend(/*possibly-const-range*/(t)).

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

(since C++23)

If ranges::crend(e) is valid for an expression e, where decltype((e)) is T, then CT models std::ranges::range, and (until C++23) std::sentinel_for<S, I> is true in all cases, where S is decltype(ranges::crend(e)), and I is decltype(ranges::crbegin(e)). Additionally, S models constant-iterator if it models input_iterator. (since C++23)

Customization point objects

The name ranges::crend 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 __crend_fn.

All instances of __crend_fn are equal. The effects of invoking different instances of type __crend_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::crend 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::crend above, __crend_fn models

Otherwise, no function call operator of __crend_fn participates in overload resolution.

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    int a[]{4, 6, -3, 9, 10};
    std::cout << "Array backwards: ";
    namespace ranges = std::ranges;
    ranges::copy(ranges::rbegin(a), ranges::rend(a),
                 std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::cout << "Vector backwards: ";
    std::vector v{4, 6, -3, 9, 10};
    ranges::copy(ranges::rbegin(v), ranges::rend(v),
                 std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

Output:

Array backwards: 10 9 -3 6 4
Vector backwards: 10 9 -3 6 4

[edit] See also

returns a reverse end iterator to a range
(customization point object) [edit]
(C++14)
returns a reverse end iterator for a container or array
(function template) [edit]