NAME¶

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

Synopsis¶

Defined in header <ranges>
Defined in header <iterator>
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

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 meaning of rbegin is
established as if by performing argument-dependent lookup only.
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.

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2602R2 C++20 there's machinery to prohibit certain removed such machinery
non-member rbegin found by ADL

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)