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std::ranges::borrowed_iterator_t, std::ranges::borrowed_subrange_t

Defined in header <ranges> 
template< ranges::range R >
using borrowed_iterator_t = /* see below */;
 (1) (since C++20) 
template< ranges::range R >
using borrowed_subrange_t = /* see below */;
 (2) (since C++20)
1) std::ranges::iterator_t<R> if R models borrowed_range, std::ranges::dangling otherwise.
2) std::ranges::subrange<std::ranges::iterator_t<R>> if R models borrowed_range, std::ranges::dangling otherwise.

These two alias templates are used by some constrained algorithms to avoid returning potentially dangling iterators or views.

Possible implementation

First version 
template< std::ranges::range R >
using borrowed_iterator_t = std::conditional_t<std::ranges::borrowed_range<R>,
    std::ranges::iterator_t<R>, std::ranges::dangling>;
Second version 
template< std::ranges::range R >
using borrowed_subrange_t = std::conditional_t<std::ranges::borrowed_range<R>,
    std::ranges::subrange<std::ranges::iterator_t<R>>, std::ranges::dangling>;

See also

(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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