Init

cppdraft/range/view.md

@@ -0,0 +1,120 @@
+[range.view]
+
+# 25 Ranges library [[ranges]](./#ranges)
+
+## 25.4 Range requirements [[range.req]](range.req#range.view)
+
+### 25.4.5 Views [range.view]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1526)
+
+The [view](#concept:view "25.4.5 Views [range.view]") concept specifies the requirements of a [range](range.range#concept:range "25.4.2 Ranges [range.range]") type
+that has the semantic properties below,
+which make it suitable for use in
+constructing range adaptor pipelines ([[range.adaptors]](range.adaptors "25.7 Range adaptors"))[.](#1.sentence-1)
+
+[🔗](#concept:view)
+
+`template<class T>
+  concept [view](#concept:view "25.4.5 Views [range.view]") =
+    [range](range.range#concept:range "25.4.2 Ranges [range.range]")<T> && [movable](concepts.object#concept:movable "18.6 Object concepts [concepts.object]")<T> && enable_view<T>;
+`
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1539)
+
+T models [view](#concept:view "25.4.5 Views [range.view]") only if
+
+- [(2.1)](#2.1)
+
+T has O(1) move construction; and
+
+- [(2.2)](#2.2)
+
+move assignment of an object of type T is no more complex than destruction followed by move construction; and
+
+- [(2.3)](#2.3)
+
+if N copies and/or moves are made from an object of type T that contained M elements,
+then those N objects have O(N+M) destruction; and
+
+- [(2.4)](#2.4)
+
+[copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<T> is false, orT has O(1) copy construction; and
+
+- [(2.5)](#2.5)
+
+[copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<T> is false, or
+copy assignment of an object of type T is no more complex than destruction followed by copy construction[.](#2.sentence-1)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1564)
+
+[*Note [1](#note-1)*:
+
+The constraints on copying and moving imply that
+a moved-from object of type T has O(1) destruction[.](#3.sentence-1)
+
+— *end note*]
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1570)
+
+[*Example [1](#example-1)*:
+
+Examples of views are:
+
+- [(4.1)](#4.1)
+
+  A [range](range.range#concept:range "25.4.2 Ranges [range.range]") type that wraps a pair of iterators[.](#4.1.sentence-1)
+
+- [(4.2)](#4.2)
+
+  A [range](range.range#concept:range "25.4.2 Ranges [range.range]") type that holds its elements by shared_ptr and shares ownership with all its copies[.](#4.2.sentence-1)
+
+- [(4.3)](#4.3)
+
+  A [range](range.range#concept:range "25.4.2 Ranges [range.range]") type that generates its elements on demand[.](#4.3.sentence-1)
+
+A container such as vector<string> does not meet the semantic requirements of [view](#concept:view "25.4.5 Views [range.view]") since copying the container copies all of the elements,
+which cannot be done in constant time[.](#4.sentence-2)
+
+— *end example*]
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1589)
+
+Since the difference between [range](range.range#concept:range "25.4.2 Ranges [range.range]") and [view](#concept:view "25.4.5 Views [range.view]") is largely
+semantic, the two are differentiated with the help of enable_view[.](#5.sentence-1)
+
+[🔗](#lib:enable_view)
+
+`template<class T>
+  constexpr bool is-derived-from-view-interface = see below;            // exposition only
+template<class T>
+  constexpr bool enable_view =
+    [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<T, view_base> || is-derived-from-view-interface<T>;
+`
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1603)
+
+For a type T,*is-derived-from-view-interface*<T> is true if and only ifT has exactly one public base class view_interface<U> for some type U andT has no base classes of type view_interface<V> for any other type V[.](#6.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1612)
+
+*Remarks*: Pursuant to [[namespace.std]](namespace.std "16.4.5.2.1 Namespace std"), users may specialize enable_view to true for cv-unqualified program-defined types that model [view](#concept:view "25.4.5 Views [range.view]"),
+and false for types that do not[.](#7.sentence-1)
+
+Such specializations shall
+be usable in constant expressions ([[expr.const]](expr.const "7.7 Constant expressions")) and
+have type const bool[.](#7.sentence-2)