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Showing content from https://en.cppreference.com/w/cpp/language/../experimental/../string/../ranges/borrowed_range.html below:

std::ranges::borrowed_range, std::ranges::enable_borrowed_range - cppreference.com

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.

The

variable template is used to indicate whether a

is a

. The primary template is defined as

.

Let U be std::remove_reference_t<T> if T is an rvalue reference type, and T otherwise. Given a variable u of type U, T models borrowed_range only if the validity of iterators obtained from u is not tied to the lifetime of that variable.

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.

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::common_view
• std::ranges::drop_view
• std::ranges::drop_while_view
• std::ranges::elements_view

• std::ranges::owning_view
• std::ranges::reverse_view

• std::ranges::take_view

1. ↑ The underlying view V must also satisfy forward_range.

Specialization of enable_borrowed_range for the following standard range adaptors are defined as true if and only if (std::ranges::enable_borrowed_range<Vs> && ...) is true, where Vs... are all view types it adapts:

• std::ranges::zip_view

Demonstrates the specializations of enable_borrowed_range for program defined types. Such specializations protect against potentially dangling results.

#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>
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>
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:

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