121 lines
4.6 KiB
Markdown
121 lines
4.6 KiB
Markdown
[range.view]
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# 25 Ranges library [[ranges]](./#ranges)
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## 25.4 Range requirements [[range.req]](range.req#range.view)
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### 25.4.5 Views [range.view]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1526)
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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
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that has the semantic properties below,
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which make it suitable for use in
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constructing range adaptor pipelines ([[range.adaptors]](range.adaptors "25.7 Range adaptors"))[.](#1.sentence-1)
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[ð](#concept:view)
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`template<class T>
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concept [view](#concept:view "25.4.5 Views [range.view]") =
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[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>;
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1539)
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T models [view](#concept:view "25.4.5 Views [range.view]") only if
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- [(2.1)](#2.1)
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T has O(1) move construction; and
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- [(2.2)](#2.2)
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move assignment of an object of type T is no more complex than destruction followed by move construction; and
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- [(2.3)](#2.3)
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if N copies and/or moves are made from an object of type T that contained M elements,
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then those N objects have O(N+M) destruction; and
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- [(2.4)](#2.4)
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[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
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- [(2.5)](#2.5)
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[copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<T> is false, or
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copy assignment of an object of type T is no more complex than destruction followed by copy construction[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1564)
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[*Note [1](#note-1)*:
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The constraints on copying and moving imply that
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a moved-from object of type T has O(1) destruction[.](#3.sentence-1)
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â *end note*]
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1570)
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[*Example [1](#example-1)*:
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Examples of views are:
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- [(4.1)](#4.1)
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A [range](range.range#concept:range "25.4.2 Ranges [range.range]") type that wraps a pair of iterators[.](#4.1.sentence-1)
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- [(4.2)](#4.2)
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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)
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- [(4.3)](#4.3)
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A [range](range.range#concept:range "25.4.2 Ranges [range.range]") type that generates its elements on demand[.](#4.3.sentence-1)
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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,
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which cannot be done in constant time[.](#4.sentence-2)
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â *end example*]
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1589)
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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
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semantic, the two are differentiated with the help of enable_view[.](#5.sentence-1)
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[ð](#lib:enable_view)
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`template<class T>
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constexpr bool is-derived-from-view-interface = see below; // exposition only
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template<class T>
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constexpr bool enable_view =
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[derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_from [concept.derived]")<T, view_base> || is-derived-from-view-interface<T>;
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`
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1603)
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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)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L1612)
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*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]"),
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and false for types that do not[.](#7.sentence-1)
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Such specializations shall
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be usable in constant expressions ([[expr.const]](expr.const "7.7 Constant expressions")) and
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have type const bool[.](#7.sentence-2)
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