[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)

[ð](#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)

â *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)

â *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)

[ð](#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)
