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cppdraft/std/iterator/tags.md

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+[std.iterator.tags]
+
+# 24 Iterators library [[iterators]](./#iterators)
+
+## 24.4 Iterator primitives [[iterator.primitives]](iterator.primitives#std.iterator.tags)
+
+### 24.4.2 Standard iterator tags [std.iterator.tags]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2744)
+
+It is often desirable for a
+function template specialization
+to find out what is the most specific category of its iterator
+argument, so that the function can select the most efficient algorithm at compile time[.](#1.sentence-1)
+
+To facilitate this, the
+library introduces[*category tag*](#def:category_tag "24.4.2 Standard iterator tags [std.iterator.tags]") classes which are used as compile time tags for algorithm selection[.](#1.sentence-2)
+
+They are:output_iterator_tag,input_iterator_tag,forward_iterator_tag,bidirectional_iterator_tag,random_access_iterator_tag,
+andcontiguous_iterator_tag[.](#1.sentence-3)
+
+For every iterator of typeI,iterator_traits<I>​::​iterator_category shall be defined to be a category tag that describes the
+iterator's behavior[.](#1.sentence-4)
+
+Additionally,iterator_traits<I>​::​iterator_concept may be used to indicate conformance to
+the iterator concepts ([[iterator.concepts]](iterator.concepts "24.3.4 Iterator concepts"))[.](#1.sentence-5)
+
+namespace std {struct output_iterator_tag { }; struct input_iterator_tag { }; struct forward_iterator_tag: public input_iterator_tag { }; struct bidirectional_iterator_tag: public forward_iterator_tag { }; struct random_access_iterator_tag: public bidirectional_iterator_tag { }; struct contiguous_iterator_tag: public random_access_iterator_tag { };}
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2788)
+
+[*Example [1](#example-1)*:
+
+A program-defined iterator BinaryTreeIterator can be included into the bidirectional iterator category by
+specializing the iterator_traits template:template<class T> struct iterator_traits<BinaryTreeIterator<T>> {using iterator_category = bidirectional_iterator_tag; using difference_type = ptrdiff_t; using value_type = T; using pointer = T*; using reference = T&;};
+
+— *end example*]
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L2804)
+
+[*Example [2](#example-2)*:
+
+Ifevolve() is well-defined for bidirectional iterators, but can be implemented more
+efficiently for random access iterators, then the implementation is as follows:template<class BidirectionalIterator>inline void evolve(BidirectionalIterator first, BidirectionalIterator last) { evolve(first, last, typename iterator_traits<BidirectionalIterator>::iterator_category());}template<class BidirectionalIterator>void evolve(BidirectionalIterator first, BidirectionalIterator last,
+ bidirectional_iterator_tag) {// more generic, but less efficient algorithm}template<class RandomAccessIterator>void evolve(RandomAccessIterator first, RandomAccessIterator last,
+ random_access_iterator_tag) {// more efficient, but less generic algorithm}
+
+— *end example*]