# std::ranges::crbegin

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
Defined in header `<iterator>` |  |
inline namespace /* unspecified */ { inline constexpr /* unspecified */ crbegin = /* unspecified */; } |  | (since C++20) (customization point object)
Call signature |  |
template< class T > requires /* see below */ constexpr /* see below */ auto crbegin( T&& t ); |  | (since C++20)

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

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::crbegin` is expression-equivalent to ranges::rbegin (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::crbegin` is expression-equivalent to:

- std::const_iterator <decltype(U)>(U) for some expression U equivalent to ranges::rbegin (`possibly-const-range`(t)).

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.

(since C++23)

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 (C++20) | returns a reverse iterator to a range (customization point object)
rbegincrbegin (C++14) | returns a reverse iterator to the beginning of a container or array (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/ranges/crbegin&oldid=179361"