Difference between revisions of "cpp/ranges/view"
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@2@ The {{tt|enable_view}} variable template is used to indicate whether a {{lconcept|range}} is a {{tt|view}}. {{c|/*is-derived-from-view-interface*/<T>}} is {{c|true}} if and only if {{tt|T}} has exactly one public base class {{c|ranges::view_interface<U>}} for some type {{tt|U}}, and {{tt|T}} has no base classes of type {{c|ranges::view_interface<V>}} for any other type {{tt|V}}. | @2@ The {{tt|enable_view}} variable template is used to indicate whether a {{lconcept|range}} is a {{tt|view}}. {{c|/*is-derived-from-view-interface*/<T>}} is {{c|true}} if and only if {{tt|T}} has exactly one public base class {{c|ranges::view_interface<U>}} for some type {{tt|U}}, and {{tt|T}} has no base classes of type {{c|ranges::view_interface<V>}} for any other type {{tt|V}}. | ||
| − | <br>Users may specialize {{tt|enable_view}} to {{c|true}} for cv-unqualified program-defined types which model {{tt|view}}, and {{c|false}} for types which do not. Such specializations must be [[cpp/language/constant expression#Usable in constant expressions|usable in constant expressions]] and have type {{c|const bool}}. | + | <br>Users may specialize {{tt|enable_view}} to {{c|true}} for cv-unqualified program-defined types which model {{tt|view}}, and {{c|false}} for types which do not. Such specializations must be [[cpp/language/constant expression#Usable in constant expressions|usable in constant expressions]] and have type {{c/core|const bool}}. |
@3@ Deriving from {{tt|view_base}} enables {{lconcept|range}} types to model {{tt|view}}. | @3@ Deriving from {{tt|view_base}} enables {{lconcept|range}} types to model {{tt|view}}. | ||
Revision as of 19:04, 15 June 2024
| Defined in header <ranges>
|
||
| template<class T> concept view = ranges::range<T> && std::movable<T> && ranges::enable_view<T>; |
(1) | (since C++20) |
| template<class T> constexpr bool enable_view = |
(2) | (since C++20) |
| struct view_base { }; |
(3) | (since C++20) |
1) The
view concept specifies the requirements of a range type that has suitable semantic properties for use in constructing range adaptor pipelines.2) The
Users may specialize
enable_view variable template is used to indicate whether a range is a view. /*is-derived-from-view-interface*/<T> is true if and only if T has exactly one public base class ranges::view_interface<U> for some type U, and T has no base classes of type ranges::view_interface<V> for any other type V.
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 must be usable in constant expressions and have type const bool.Contents |
Semantic requirements
1)
T models view only if:
- move construction of
Thas constant time complexity, and - if N copies and/or moves are made from a
Tobject holding M elements, then these N objects have 𝓞(N+M) destruction (which implies that a moved-fromviewobject has 𝓞(1) destruction), and - either std::copy_constructible<T> is false, or copy construction of
Thas constant time complexity, and - either std::copyable<T> is false, or copy assignment of
Thas no more time complexity than destruction followed by copy construction.
Notes
Examples of view types are:
- A
rangetype that wraps a pair of iterators, e.g., std::ranges::subrange<I>. - A
rangetype that holds its elements by std::shared_ptr and shares ownership with all its copies. - A
rangetype that generates its elements on demand, e.g., std::ranges::iota_view.
A copyable container such as std::vector<std::string> generally does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.
While views were originally described as cheaply copyable and non-owning ranges, a type is not required to be copyable or non-owning for it to model view. However, it must still be cheap to copy (if it is copyable), move, assign, and destroy, so that range adaptors will not have unexpected complexity.
By default, a type modeling movable and range is considered a view if it is publicly and unambiguously derived from view_base, or exactly one specialization of std::ranges::view_interface.
Example
A minimum view.
#include <ranges> struct ArchetypalView : std::ranges::view_interface<ArchetypalView> { int* begin(); int* end(); }; static_assert(std::ranges::view<ArchetypalView>);
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2325R3 | C++20 | view required default_initializable
|
does not require |
| LWG 3549 | C++20 | enable_view did not detect inheritance from view_interface
|
detects |
| P2415R2 | C++20 | the restriction on the time complexity of destruction was too strict | relaxed |
