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

 

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Ranges library (C++20)

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Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted as_input_viewviews::as_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

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 > 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

See More

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

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.

Unconditionally borrowed ranges in the standard library

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

Conditionally borrowed ranges in the standard library

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::adjacent_view std::ranges::as_const_view std::ranges::as_rvalue_view std::ranges::chunk_view

(since C++23)

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

std::ranges::enumerate_view

(since C++23)

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

std::ranges::slide_view std::ranges::stride_view

(since C++23)

• std::ranges::take_view

std::ranges::as_input_view

(since C++26)

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

(since C++23)

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>
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:

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)