std::ranges::rbegin

inline namespace /*unspecified*/ {
    inline constexpr /*unspecified*/ rbegin = /*unspecified*/;
}

template< class T >
    requires /* see below */
constexpr std::input_or_output_iterator auto rbegin( T&& t );

Returns an iterator to the last element of the argument.

Let t be an object of type T. 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. t.rbegin() converted to its decayed type, if that expression with conversion is valid, and its converted type models std::input_or_output_iterator.
2. Otherwise, rbegin(t) converted to its decayed type, if T is a class or enumeration type, the aforementioned unqualified call with conversion is valid, its converted type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
   • void rbegin(auto&) = delete;
   • void rbegin(const auto&) = delete;
   • any declarations of rbegin found by argument-dependent lookup.
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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if.

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

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

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

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See also