26 Ranges library [ranges]

26.4 Range requirements [range.req]

26.4.4 Views [range.view]

1
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The view concept specifies the requirements of a range type that has the semantic properties below, which make it suitable for use in constructing range adaptor pipelines ([range.adaptors]).
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template<class T> concept view = range<T> && movable<T> && enable_view<T>;
2
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T models view only if:
  • (2.1)
    T has move construction; and
  • (2.2)
    move assignment of an object of type T is no more complex than destruction followed by move construction; and
  • (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 destruction; and
  • (2.4)
    copy_constructible<T> is false, or T has copy construction; and
  • (2.5)
    copyable<T> is false, or copy assignment of an object of type T is no more complex than destruction followed by copy construction.
3
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[Note 1: 
The constraints on copying and moving imply that a moved-from object of type T has destruction.
— end note]
4
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[Example 1: 
Examples of views are:
  • (4.1)
    A range type that wraps a pair of iterators.
  • (4.2)
    A range type that holds its elements by shared_ptr and shares ownership with all its copies.
  • (4.3)
    A range type that generates its elements on demand.
A container such as vector<string> does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.
— end example]
5
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Since the difference between range and view is largely semantic, the two are differentiated with the help of enable_view.
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template<class T> constexpr bool is-derived-from-view-interface = see below; // exposition only template<class T> constexpr bool enable_view = derived_from<T, view_base> || is-derived-from-view-interface<T>;
6
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For a type T, is-derived-from-view-interface<T> is true if and only if T has exactly one public base class view_interface<U> for some type U and T has no base classes of type view_interface<V> for any other type V.
7
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Remarks: Pursuant to [namespace.std], users may specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not.
Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.