std::ranges::borrowed_range,std::ranges::enable_borrowed_range(3)

NAME
std::ranges::borrowed_range,std::ranges::enable_borrowed_range - std::ranges::borrowed_range,std::ranges::enable_borrowed_range

Synopsis
   Defined in header <ranges>
   template< class R >
   concept borrowed_range =
       ranges::range<R> && (1) (since C++20)
       (std::is_lvalue_reference_v<R> ||
        ranges::enable_borrowed_range<std::remove_cvref_t<R>>);
   template< class R > (2) (since C++20)
   inline constexpr bool enable_borrowed_range = false;
   1) The concept borrowed_range defines the requirements of a range such that a
   function can take it by value and return iterators obtained from it without danger
   of dangling.
   2) The enable_borrowed_range variable template is used to indicate whether a range
   is a borrowed_range. The primary template is defined as false.

Specializations
   Specializations of enable_borrowed_range for all specializations of the following
   standard templates are defined as true:
     * std::basic_string_view
     * std::span
     * std::ranges::subrange
     * std::ranges::ref_view
     * std::ranges::empty_view
     * std::ranges::iota_view
   Specialization of enable_borrowed_range for the following standard range adaptors
   are defined as true if and only if std::ranges::enable_borrowed_range<V> is true,
   where V is the underlying view type:
     * std::ranges::owning_view
     * std::ranges::take_view
     * std::ranges::drop_view
     * std::ranges::drop_while_view
     * std::ranges::common_view
     * std::ranges::reverse_view
     * std::ranges::elements_view
     * std::ranges::adjacent_view (since C++23)
   Specialization for std::ranges::zip_view is defined as true if and
   only if (std::ranges::enable_borrowed_range<Vs> && ...) is true, where (since C++23)
   Vs... are all view types it adapts.
   A program may specialize enable_borrowed_range to true for cv-unqualified
   program-defined types which model borrowed_range, and false for types which do not.
   Such specializations shall be usable in constant expression and have type const
   bool.

Example
   Demonstrates the specializations of enable_borrowed_range for program defined types.
   Such specializations protect against potentially dangling results. // Run this code
 #include <algorithm>
 #include <array>
 #include <cstddef>
 #include <iostream>
 #include <ranges>
 #include <span>
 #include <type_traits>
 template<typename T, std::size_t N>
 struct MyRange : std::array<T, N> {};
 template<typename T, std::size_t N>
 inline constexpr bool std::ranges::enable_borrowed_range<MyRange<T, N>> = false;
 template<typename T, std::size_t N>
 struct MyBorrowedRange : std::span<T, N> {};
 template<typename T, std::size_t N>
 inline constexpr bool std::ranges::enable_borrowed_range<MyBorrowedRange<T, N>> = true;
 int main()
 {
     static_assert(std::ranges::range<MyRange<int, 8>>);
     static_assert(std::ranges::borrowed_range<MyRange<int, 8>> == false);
     static_assert(std::ranges::range<MyBorrowedRange<int, 8>>);
     static_assert(std::ranges::borrowed_range<MyBorrowedRange<int, 8>> == true);
     auto getMyRangeByValue = []{ return MyRange<int, 4>{{1, 2, 42, 3}}; };
     auto dangling_iter = std::ranges::max_element(getMyRangeByValue());
     static_assert(std::is_same_v<std::ranges::dangling, decltype(dangling_iter)>);
 // *dangling_iter; // compilation error (i.e. dangling protection works.)
     auto my = MyRange<int, 4>{{1, 2, 42, 3}};
     auto valid_iter = std::ranges::max_element(my);
     std::cout << *valid_iter << ' '; // OK: 42
     auto getMyBorrowedRangeByValue = []
     {
         static int sa[4]{1, 2, 42, 3};
         return MyBorrowedRange<int, std::size(sa)>{sa};
     };
     auto valid_iter2 = std::ranges::max_element(getMyBorrowedRangeByValue());
     std::cout << *valid_iter2 << '\n'; // OK: 42
 }

Output:
 42 42

See also
   ranges::dangling a placeholder type indicating that an iterator or a subrange should
   (C++20) not be returned since it would be dangling
                    (class)