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cppdraft/incrementable/traits.md

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+[incrementable.traits]
+
+# 24 Iterators library [[iterators]](./#iterators)
+
+## 24.3 Iterator requirements [[iterator.requirements]](iterator.requirements#incrementable.traits)
+
+### 24.3.2 Associated types [[iterator.assoc.types]](iterator.assoc.types#incrementable.traits)
+
+#### 24.3.2.1 Incrementable traits [incrementable.traits]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L744)
+
+To implement algorithms only in terms of incrementable types,
+it is often necessary to determine the difference type that
+corresponds to a particular incrementable type[.](#1.sentence-1)
+
+Accordingly,
+it is required that if WI is the name of a type that models the[weakly_incrementable](iterator.concept.winc#concept:weakly_incrementable "24.3.4.4 Concept weakly_­incrementable [iterator.concept.winc]") concept ([[iterator.concept.winc]](iterator.concept.winc "24.3.4.4 Concept weakly_­incrementable")),
+the typeiter_difference_t<WI> be defined as the incrementable type's difference type[.](#1.sentence-2)
+
+[🔗](#lib:incrementable_traits)
+
+namespace std {template<class> struct incrementable_traits { }; template<class T>requires is_object_v<T>struct incrementable_traits<T*> {using difference_type = ptrdiff_t; }; template<class I>struct incrementable_traits<const I>: incrementable_traits<I> { }; template<class T>requires requires { typename T::difference_type; }struct incrementable_traits<T> {using difference_type = typename T::difference_type; }; template<class T>requires (!requires { typename T::difference_type; } &&requires(const T& a, const T& b) { { a - b } -> [integral](concepts.arithmetic#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]"); })struct incrementable_traits<T> {using difference_type = make_signed_t<decltype(declval<T>() - declval<T>())>; }; template<class T>using iter_difference_t = *see below*;}
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L790)
+
+Let RI be remove_cvref_t<I>[.](#2.sentence-1)
+
+The type iter_difference_t<I> denotes
+
+- [(2.1)](#2.1)
+
+incrementable_traits<RI>​::​difference_type if iterator_traits<RI> names a specialization
+generated from the primary template, and
+
+- [(2.2)](#2.2)
+
+iterator_traits<RI>​::​difference_type otherwise[.](#2.sentence-2)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L803)
+
+Users may specialize incrementable_traits on program-defined types[.](#3.sentence-1)