24 Ranges library [ranges]

24.5 Range utilities [range.utility]

24.5.5 Dangling iterator handling [range.dangling]

1
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The tag type dangling is used together with the template aliases borrowed_­iterator_­t and borrowed_­subrange_­t to indicate that an algorithm that typically returns an iterator into or subrange of a range argument does not return an iterator or subrange which could potentially reference a range whose lifetime has ended for a particular rvalue range argument which does not model borrowed_­range ([range.range]).
namespace std::ranges { struct dangling { constexpr dangling() noexcept = default; template<class... Args> constexpr dangling(Args&&...) noexcept { } }; }
2
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[Example 1: vector<int> f(); auto result1 = ranges::find(f(), 42); // #1 static_assert(same_­as<decltype(result1), ranges::dangling>); auto vec = f(); auto result2 = ranges::find(vec, 42); // #2 static_assert(same_­as<decltype(result2), vector<int>::iterator>); auto result3 = ranges::find(subrange{vec}, 42); // #3 static_assert(same_­as<decltype(result3), vector<int>::iterator>);
The call to ranges​::​find at #1 returns ranges​::​dangling since f() is an rvalue vector; the vector could potentially be destroyed before a returned iterator is dereferenced.
However, the calls at #2 and #3 both return iterators since the lvalue vec and specializations of subrange model borrowed_­range.
— end example]