std::ranges::borrowed_range, std::ranges::enable_borrowed_range

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

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)

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.

Semantic requirements

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

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 Vs... are all view types it adapts.

(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.

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 (C++20)

a placeholder type indicating that an iterator or a subrange should not be returned since it would be dangling (class)