tmp/tmpacl2l2h2/{from.md → to.md}
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#### Indirectly readable traits <a id="readable.traits">[[readable.traits]]</a>
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To implement algorithms only in terms of indirectly readable types, it
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is often necessary to determine the value type that corresponds to a
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particular indirectly readable type. Accordingly, it is required that if
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`R` is the name of a type that models the `indirectly_readable` concept
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[[iterator.concept.readable]], the type
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``` cpp
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iter_value_t<R>
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```
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be defined as the indirectly readable type’s value type.
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``` cpp
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template<class> struct cond-value-type { }; // exposition only
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template<class T>
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requires is_object_v<T>
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struct cond-value-type<T> {
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using value_type = remove_cv_t<T>;
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};
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template<class> struct indirectly_readable_traits { };
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template<class T>
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struct indirectly_readable_traits<T*>
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: cond-value-type<T> { };
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template<class I>
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requires is_array_v<I>
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struct indirectly_readable_traits<I> {
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using value_type = remove_cv_t<remove_extent_t<I>>;
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};
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template<class I>
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struct indirectly_readable_traits<const I>
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: indirectly_readable_traits<I> { };
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template<class T>
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requires requires { typename T::value_type; }
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struct indirectly_readable_traits<T>
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: cond-value-type<typename T::value_type> { };
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template<class T>
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requires requires { typename T::element_type; }
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struct indirectly_readable_traits<T>
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: cond-value-type<typename T::element_type> { };
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template<class T> using iter_value_t = see below;
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```
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Let R_`I` be `remove_cvref_t<I>`. The type `iter_value_t<I>` denotes
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- `indirectly_readable_traits<R_I>::value_type` if
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`iterator_traits<R_I>` names a specialization generated from the
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primary template, and
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- `iterator_traits<R_I>::value_type` otherwise.
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Class template `indirectly_readable_traits` may be specialized on
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program-defined types.
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[*Note 1*: Some legacy output iterators define a nested type named
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`value_type` that is an alias for `void`. These types are not
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`indirectly_readable` and have no associated value types. — *end note*]
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[*Note 2*: Smart pointers like `shared_ptr<int>` are
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`indirectly_readable` and have an associated value type, but a smart
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pointer like `shared_ptr<void>` is not `indirectly_readable` and has no
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associated value type. — *end note*]
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