std::ranges::rbegin

< cpp‎ | ranges
 Defined in header  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);

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 is true, then a call to ranges::rbegin is expression-equivalent to:

1. std::forward<T>(t).rbegin(), if that expression is valid, and its return type models std::input_or_output_iterator.
2. Otherwise, rbegin(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, its return 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, if both ranges::begin(std::forward<T>(t)) and ranges::end(std::forward<T>(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.

Contents

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

 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 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.

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
}

Output:

4
15