25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.6 Indirect callable requirements [indirectcallable]

25.3.6.4 Class template projected [projected]

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Class template projected is used to constrain algorithms that accept callable objects and projections ([defns.projection]).
It combines an indirectly_readable type I and a callable object type Proj into a new indirectly_readable type whose reference type is the result of applying Proj to the iter_reference_t of I.
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namespace std { template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj> struct projected { using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>; indirect_result_t<Proj&, I> operator*() const; // not defined }; template<weakly_incrementable I, class Proj> struct incrementable_traits<projected<I, Proj>> { using difference_type = iter_difference_t<I>; }; }