# std::ranges::rbegin

From cppreference.com
< cpp‎ | ranges

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

Returns an iterator to the last element of the argument.

If `T` is an array type and std::remove_all_extents_t<std::remove_reference_t<T>> is incomplete, then the call to `ranges::rbegin` is ill-formed, no diagnostic required.

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

1. `decay-copy`(t.rbegin()) (until C++23)auto(t.rbegin()) (since C++23), if that expression is valid and its type models std::input_or_output_iterator.
2. Otherwise, `decay-copy`(rbegin(t)) (until C++23)auto(rbegin(t)) (since C++23), if `T` is a class or enumeration type, that expression is valid and its type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
   - any declarations of `rbegin` found by argument-dependent lookup
   - void rbegin(auto&) = delete;
   - void rbegin(const auto&) = delete;
3. Otherwise, std::make_reverse_iterator(ranges::end(t)) if both ranges::begin(t) and ranges::end(t) are valid expressions, have the same type, and that type models std::bidirectional_iterator.

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

### Customization point objects

The name `ranges::rbegin` 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 `__rbegin_fn`.

All instances of `__rbegin_fn` are equal. The effects of invoking different instances of type `__rbegin_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::rbegin` 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::rbegin` above, `__rbegin_fn` models

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

Otherwise, no function call operator of `__rbegin_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::rbegin` is ill-formed, which also results in substitution failure.

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

The C++20 standard requires that if the underlying `rbegin` function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8 to match the implementations.

### Example

Run this code

    #include <iostream>
    #include <ranges>
    #include <span>
    #include <vector>

    int main()
    {
        std::vector<int> v = {3, 1, 4};
        auto vi = std::ranges::rbegin(v);
        std::cout << *vi << '\n';
        *vi = 42; // OK

        int a[] = {-5, 10, 15};
        auto ai = std::ranges::rbegin(a);
        std::cout << *ai << '\n';
        *ai = 42; // OK

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

        auto si = std::ranges::rbegin(std::span{a}); // OK
        static_assert(std::ranges::enable_borrowed_range<
            std::remove_cv_t<decltype(std::span{a})>>);
        *si = 42; // OK
    }

Output:

    4
    15

### See also

ranges::crbegin (C++20) | returns a reverse iterator to a read-only range (customization point object)
rbegin crbegin (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/rbegin&oldid=151333"