Defined in header <ranges> | ||
|---|---|---|
template<class R>
concept borrowed_range =
ranges::range<R> &&
(std::is_lvalue_reference_v<R> ||
ranges::enable_borrowed_range<std::remove_cvref_t<R>>);
| (1) | (since C++20) |
template<class R> inline constexpr bool enable_borrowed_range = false; | (2) | (since C++20) |
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.enable_borrowed_range variable template is used to indicate whether a range is a borrowed_range. The primary template is defined as false.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.
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 | (since C++23) |
| Specialization for | (since C++23) |
Users 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 expressions and have type const bool.
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>
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
|
(C++20) | a placeholder type indicating that an iterator or a subrange should not be returned since it would be dangling (class) |
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https://en.cppreference.com/w/cpp/ranges/borrowed_range