std::ranges::rbegin
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 ); |
(since C++20) |
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:
-
t.rbegin()
converted to its decayed type, if that expression with conversion is valid, and its converted type modelsstd::input_or_output_iterator
. - Otherwise,
rbegin(t)
converted to its decayed type, ifT
is a class or enumeration type, the aforementioned unqualified call with conversion is valid, its converted type modelsstd::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.
-
- Otherwise,
std::make_reverse_iterator(ranges::end(t))
if bothranges::begin(t)
andranges::end(t)
are valid expressions, have the same type, and that type modelsstd::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
andf
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
#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
(C++20)
|
returns a reverse iterator to a read-only range (customization point object) |
(C++14)
|
returns a reverse iterator to the beginning of a container or array (function template) |
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