Init

cppdraft/readable/traits.md

@@ -0,0 +1,91 @@
+[readable.traits]
+
+# 24 Iterators library [[iterators]](./#iterators)
+
+## 24.3 Iterator requirements [[iterator.requirements]](iterator.requirements#readable.traits)
+
+### 24.3.2 Associated types [[iterator.assoc.types]](iterator.assoc.types#readable.traits)
+
+#### 24.3.2.2 Indirectly readable traits [readable.traits]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L808)
+
+To implement algorithms only in terms of indirectly readable types,
+it is often necessary
+to determine the value type that corresponds to
+a particular indirectly readable type[.](#1.sentence-1)
+
+Accordingly, it is required that if R is the name of a type that
+models the [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") concept ([[iterator.concept.readable]](iterator.concept.readable "24.3.4.2 Concept indirectly_­readable")),
+the typeiter_value_t<R> be defined as the indirectly readable type's value type[.](#1.sentence-2)
+
+[🔗](#lib:indirectly_readable_traits)
+
+template<class> struct *cond-value-type* { }; // *exposition only*template<class T>requires is_object_v<T>struct *cond-value-type*<T> {using value_type = remove_cv_t<T>;};
+
+template<class T>concept [*has-member-value-type*](#concept:has-member-value-type "24.3.2.2 Indirectly readable traits [readable.traits]") = requires { typename T::value_type; }; // *exposition only*template<class T>concept [*has-member-element-type*](#concept:has-member-element-type "24.3.2.2 Indirectly readable traits [readable.traits]") = requires { typename T::element_type; }; // *exposition only*template<class> struct indirectly_readable_traits { };
+
+template<class T>struct indirectly_readable_traits<T*>: *cond-value-type*<T> { };
+
+template<class I>requires is_array_v<I>struct indirectly_readable_traits<I> {using value_type = remove_cv_t<remove_extent_t<I>>;};
+
+template<class I>struct indirectly_readable_traits<const I>: indirectly_readable_traits<I> { };
+
+template<[*has-member-value-type*](#concept:has-member-value-type "24.3.2.2 Indirectly readable traits [readable.traits]") T>struct indirectly_readable_traits<T>: *cond-value-type*<typename T::value_type> { };
+
+template<[*has-member-element-type*](#concept:has-member-element-type "24.3.2.2 Indirectly readable traits [readable.traits]") T>struct indirectly_readable_traits<T>: *cond-value-type*<typename T::element_type> { };
+
+template<[*has-member-value-type*](#concept:has-member-value-type "24.3.2.2 Indirectly readable traits [readable.traits]") T>requires [*has-member-element-type*](#concept:has-member-element-type "24.3.2.2 Indirectly readable traits [readable.traits]")<T>struct indirectly_readable_traits<T> { };
+
+template<[*has-member-value-type*](#concept:has-member-value-type "24.3.2.2 Indirectly readable traits [readable.traits]") T>requires [*has-member-element-type*](#concept:has-member-element-type "24.3.2.2 Indirectly readable traits [readable.traits]")<T> &&[same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>struct indirectly_readable_traits<T>: *cond-value-type*<typename T::value_type> { };
+
+template<class T> using iter_value_t = *see below*;
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L874)
+
+Let RI be remove_cvref_t<I>[.](#2.sentence-1)
+
+The type iter_value_t<I> denotes
+
+- [(2.1)](#2.1)
+
+indirectly_readable_traits<RI>​::​value_type if iterator_traits<RI> names a specialization
+generated from the primary template, and
+
+- [(2.2)](#2.2)
+
+iterator_traits<RI>​::​value_type otherwise[.](#2.sentence-2)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L887)
+
+Class template indirectly_readable_traits may be specialized
+on program-defined types[.](#3.sentence-1)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L891)
+
+[*Note [1](#note-1)*:
+
+Some legacy output iterators define a nested type named value_type that is an alias for void[.](#4.sentence-1)
+
+These types are not [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") and have no associated value types[.](#4.sentence-2)
+
+— *end note*]
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L899)
+
+[*Note [2](#note-2)*:
+
+Smart pointers like shared_ptr<int> are [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") and
+have an associated value type, but a smart pointer like shared_ptr<void> is not [indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") and has no associated value type[.](#5.sentence-1)
+
+— *end note*]