NAME¶

std::ranges::rbegin - std::ranges::rbegin

Synopsis¶

Defined in header <ranges>
inline namespace /*unspecified*/ {

inline constexpr /*unspecified*/ rbegin = (since C++20)
/*unspecified*/; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr std::input_or_output_iterator auto rbegin(
T&& t );

Returns an iterator to the last element of the argument.

range-rbegin-rend.svg

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¶

// Run this code

#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

See also¶

ranges::crbegin returns a reverse iterator to a read-only range
(C++20) (customization point object)
rbegin returns a reverse iterator to the beginning of a container or array
crbegin (function template)
(C++14)