< 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);

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. std::forward<T>(t).rbegin() converted to its decayed type,, if that expression with conversion is valid, and its conveted type models std::input_or_output_iterator.
  2. Otherwise, rbegin(std::forward<T>(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(std::forward<T>(t))) 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.


[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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:
    *si = 42; // OK



[edit] See also

returns a reverse iterator to a read-only range
(customization point object) [edit]
returns a reverse iterator to a container or array
(function template) [edit]