# std::ranges::crend

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< cpp‎ | ranges

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.

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

- std::invocable <__crend_fn, Args...>,
- std::invocable <const __crend_fn, Args...>,
- std::invocable <__crend_fn&, Args...>, and
- std::invocable <const __crend_fn&, Args...>.

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

### Example

Run this code

    #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

### See also

ranges::rend (C++20) | returns a reverse end iterator to a range (customization point object)
rendcrend (C++14) | returns a reverse end iterator for a container or array (function template)

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