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std::ranges::rbegin

From cppreference.com
< 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.

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. std::forward<T>(t).rbegin(), if that expression is valid, and its return type models std::input_or_output_iterator.
  2. Otherwise, rbegin(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, its return 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 begin found by argument-dependent lookup.
  3. Otherwise, std::make_reverse_iterator(ranges::end(std::forward<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.

Contents

[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 (denoted, for exposition purposes, as rbegin_ftor). All instances of rbegin_ftor are equal. 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_ftor will satisfy std::invocable<const rbegin_ftor&, Args...>. Otherwise, no function call operator of rbegin_ftor 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.


[edit] Example

#include <iostream>
#include <vector>
#include <ranges>
 
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
}

Output:

4
15

[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]