std::ranges::rbegin - cppreference.com (original) (raw)

| Defined in header | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------------------------------------ | | 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 ); | | (since C++20) |

Returns an iterator to the last element of the argument.

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.

Customization point objects

The name ranges::rbegin denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

[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 #include #include #include   int main() { std::vector 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:

[edit] Defect reports

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

[edit] See also

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