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    std::ranges::rbegin

    From cppreference.com
     
    C++
     
    Ranges library
    Range adaptors
     
    Defined in header <ranges>
    Defined in header <iterator>
    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.

    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 non-member rbegin found by ADL removed such machinery

    See also

    (C++20)
    		 returns a reverse iterator to a read-only range
    (customization point object)[edit]
    (C++14)
    		 returns a reverse iterator to the beginning of a container or array
    (function template) [edit]