View source for cpp/named req/LayoutMapping

You do not have permission to edit this page, for the following reasons:

The action you have requested is limited to users in one of the groups: Users, Administrators.

Due to recent vandalism, editing of this page is temporarily disabled for new users.If you want to suggest a change, please leave a message on the suggestions page for new users instead (don't be surprised if that page appears vandalized!).

You can view and copy the source of this page.

{{cpp/named req/title|LayoutMapping|notes={{mark since c++23}}}}
{{cpp/named req/navbar}}

{{named req|LayoutMapping}} controls the mapping of a multidimensional index to a one-dimensional offset to data handle in {{c/core|std::mdspan}}.

===Requirements===
A type {{tt|M}} satisfies {{named req/core|LayoutMapping}} if it models {{lconcept|copyable}} and {{lconcept|equality_comparable}}, and the following are {{c|true}}:
* {{c|std::is_nothrow_move_constructible_v<M>}}
* {{c|std::is_nothrow_move_assignable_v<M>}}
* {{c|std::is_nothrow_swappable_v<M>}}

And, given the following types and values, the expressions shown in the table below are valid and have the indicated semantics:

====Legend====
{{dsc begin}}
{{dsc hitem|Type|Definition}}
{{dsc|{{tt|M}}|a layout mapping class}}
{{dsc hitem|Value|Definition}}
{{dsc|{{c|m}}|a value of type (possibly const-qualified) {{tt|M}}}}
{{dsc|{{c|i}}, {{c|j}}|packs of (possibly const-qualified) integers that are multidimensional indices in {{c|m.extents()}}}}
{{dsc|{{c|r}}|a (possibly const-qualified) rank index of {{c/core|typename M::extents_type}}}}
{{dsc|{{c|d_r}}|a pack of (possibly const-qualified) integers for which {{c|sizeof...(d_r) {{==}} M::extents_type::rank()}} is {{c|true}}, the element at rank index {{c|r}} is equal to {{c|1}}, and all other elements are equal to {{c|0}}}}
{{dsc end}}

====Member types====
{|class=wikitable
!Name||Type||Requirements
|-
|{{tt|M::extents_type}}||Specialization of class template {{c/core|std::extents}}||
|-
|{{tt|M::index_type}}||{{c/core|typename M::extents_type::index_type}}||
|-
|{{tt|M::rank_type}}||{{c/core|typename M::extents_type::rank_type}}||
|-
|{{tt|M::layout_type}}||Layout mapping policy {{tt|MP}} where {{c/core|typename MP::template mapping<E>}} is {{tt|M}}<br>for some extents type {{tt|E}}||{{named req|LayoutMappingPolicy}} for which {{tt|M}} is mapping type of {{tt|MP}}
|}

====Member functions and operators====
<div style="max-width: 100%; overflow-x: scroll;">
{|class=wikitable
!Expression||Return type||Semantics
|-
|{{c|m.extents()}}||{{c/core|const typename M::extents_type&}}||Returns constant reference to associated multidimensional index space
|-
|{{c|m(i...)}}||{{c/core|typename M::index_type}}||
* Returns a nonnegative integer {{c|o}} such that {{c|o < std::numeric_limits<typename M::index_type>::max()}} and {{c|o <{{=}} std::numeric_limits<std::size_t>::max()}} are both {{c|true}}.
* Such expression is equivalent to {{c|m(static_cast<typename M::index_type>(i)...)}}.
|-
|{{c|m.required_span_size()}}||{{c/core|typename M::index_type}}||
* Returns {{c|1}} plus the maximum value of {{c|m(i...)}} for all {{c|i}} if the size of the multidimensional index space {{c|m.extents()}} is not 0. 
* Otherwise, returns {{c|0}}.
|-
|{{c|m.is_unique()}}||{{c/core|bool}}||Returns {{c|true}} only if for every {{c|i}} and {{c|j}} where {{c|(i !{{=}} j {{!!}} ...)}} is {{c|true}}, {{c|m(i...) !{{=}} m(j...)}} is {{c|true}}. <ref group="note">A mapping can return {{c|false}} even if the condition is met. For certain layouts, it is possibly not feasible to determine efficiently whether the layout is unique.</ref>
|-
|{{c|m.is_exhaustive()}}||{{c/core|bool}}||Returns {{c|true}} only if for all {{c|k}} in the range {{range|0|m.required_span_size()}}, there exists an {{c|i}} such that {{c|m(i...)}} equals {{c|k}}. <ref group="note">Same as above, but in the case of exhaustive layouts.</ref>
|-
|{{c|m.is_strided()}}||{{c/core|bool}}||Returns {{c|true}} only if for every rank index {{c|r}} of {{c|m.extents()}}, there exists an integer {{c|s_r}} such that, for all {{c|i}} where {{c|(i + d_r)}} is a multidimensional index in {{c|m.extents()}}, {{c|m((i + d_r)...) - m(i...)}} equals {{c|s_r}}. <ref group="note">Same as above, but in the case of strided layouts.</ref>
|-
|{{c|m.stride(r)}}||{{c/core|typename M::index_type}}||
* The precondition is that {{c|m.is_strided()}} is {{c|true}}.
* Returns a stride {{c|s_r}} at rank index {{c|r}} as defined in {{c|m.is_strided()}} above. 
|-
|{{c|M::is_always_unique()}}||{{c/core|bool}}||
* Returns {{c|true}} only if {{c|m.is_unique()}} is {{c|true}} for all possible objects {{c|m}} of type {{tt|M}}. <ref group="note">A mapping can return {{c|false}} even if the condition is met. For certain layout mappings, it is possibly not feasible to determine whether every instance is unique.</ref>
* The return value is always a constant expression.
|-
|{{c|M::is_always_exhaustive()}}||{{c/core|bool}}||
* Returns {{c|true}} only if {{c|m.is_exhaustive()}} is {{c|true}} for all possible objects {{c|m}} of type {{tt|M}}. <ref group="note">Same as above, but in the case of exhaustive instances.</ref>
* The return value is always a constant expression.
|-
|{{c|M::is_always_strided()}}||{{c/core|bool}}||
* Returns {{c|true}} only if {{c|m.is_strided()}} is {{c|true}} for all possible objects {{c|m}} of type {{tt|M}}. <ref group="note">Same as above, but in the case of strided instances.</ref>
* The return value is always a constant expression.
|-
|}
<references group="note"/>
</div>

===Concept===
For the constraints uses under {{lc|std::layout_stride::mapping}}, the following exposition-only concept is defined.
{{ddcla|anchor=layout-mapping-alike|expos=yes|1=
template< class M >
concept /*layout-mapping-alike*/ = requires 
{
    requires /*is-extents*/<typename M::extents_type>;
    { M::is_always_strided() }    -> std::same_as<bool>;
    { M::is_always_exhaustive() } -> std::same_as<bool>;
    { M::is_always_unique() }     -> std::same_as<bool>;
    std::bool_constant<M::is_always_strided()>::value;
    std::bool_constant<M::is_always_exhaustive()>::value;
    std::bool_constant<M::is_always_unique()>::value;
};
}}

Defines the minimal usability constraints of the {{named req/core|LayoutMapping}} requirement. This concept checks that the predicate mapping trait functions above exist, are constant expressions, and have a return type of {{c/core|bool}}.

{{c/core|/*is-extents*/<E>}} is {{c|true}} if and only if {{tt|E}} is a specialization of {{lc|std::extents}}.

===Standard library===
The following standard library types satisfy {{named req/core|LayoutMapping}} requirements:
{{dsc begin}}
{{dsc inc|cpp/container/mdspan/layout_left/dsc mapping}}
{{dsc inc|cpp/container/mdspan/layout_right/dsc mapping}}
{{dsc inc|cpp/container/mdspan/layout_stride/dsc mapping}}
{{dsc inc|cpp/container/mdspan/layout_left_padded/dsc mapping}}
{{dsc inc|cpp/container/mdspan/layout_right_padded/dsc mapping}}
{{dsc end}}

===See also===
* {{named req|LayoutMappingPolicy}}

{{langlinks|es|ja|ru|zh}}

Templates used on this page:

Template:!! (view source)
Template:== (view source)
Template:anchor (view source)
Template:box/core (view source)
Template:c (view source) (protected)
Template:c/core (view source) (protected)
Template:cpp/container/mdspan/layout left/dsc mapping (view source)
Template:cpp/container/mdspan/layout left/title (view source)
Template:cpp/container/mdspan/layout left padded/dsc mapping (view source)
Template:cpp/container/mdspan/layout left padded/title (view source)
Template:cpp/container/mdspan/layout right/dsc mapping (view source)
Template:cpp/container/mdspan/layout right/title (view source)
Template:cpp/container/mdspan/layout right padded/dsc mapping (view source)
Template:cpp/container/mdspan/layout right padded/title (view source)
Template:cpp/container/mdspan/layout stride/dsc mapping (view source)
Template:cpp/container/mdspan/layout stride/title (view source)
Template:cpp/named req/navbar (view source)
Template:cpp/named req/navbar content (view source)
Template:cpp/named req/navbar heading (view source)
Template:cpp/named req/title (view source)
Template:cpp/navbar content (view source)
Template:cpp/navbar heading (view source)
Template:cpp/title (view source) (protected)
Template:ctitle (view source) (protected)
Template:dcl/std (view source) (semi-protected)
Template:dcl begin (view source) (protected)
Template:dcl end (view source) (protected)
Template:dcl sep (view source) (protected)
Template:dcla (view source)
Template:ddcla (view source)
Template:dsc (view source) (protected)
Template:dsc begin (view source) (protected)
Template:dsc editlink (view source)
Template:dsc end (view source) (protected)
Template:dsc guard (view source) (protected)
Template:dsc hitem (view source) (protected)
Template:dsc inc (view source) (protected)
Template:dsc mark mem (view source) (protected)
Template:dsc mem tclass (view source)
Template:dsc member (view source) (protected)
Template:get lang (view source) (protected)
Template:langlinks (view source) (protected)
Template:langlinks/one (view source) (protected)
Template:lc (view source) (protected)
Template:lconcept (view source)
Template:lconcept/cat (view source)
Template:lconcept/pg (view source)
Template:lconcept/title (view source)
Template:lines (view source) (protected)
Template:ls (view source)
Template:ltt/core (view source) (protected)
Template:mark (view source) (protected)
Template:mark c++11 (view source) (protected)
Template:mark c++14 (view source) (protected)
Template:mark c++17 (view source) (protected)
Template:mark c++20 (view source)
Template:mark c++23 (view source) (protected)
Template:mark c++26 (view source)
Template:mark expos (view source)
Template:mark expos/core (view source)
Template:mark mem tclass (view source)
Template:mark rev (view source) (protected)
Template:mark since c++23 (view source)
Template:mark tooltip (view source)
Template:named req (view source)
Template:named req/core (view source)
Template:navbar (view source)
Template:navbar editlink (view source) (protected)
Template:nbsp (view source) (protected)
Template:nbspt (view source)
Template:nv (view source) (protected)
Template:nv begin (view source) (protected)
Template:nv col begin (view source) (protected)
Template:nv col end (view source) (protected)
Template:nv col next (view source) (protected)
Template:nv end (view source) (protected)
Template:nv h1 (view source) (protected)
Template:nv h2 (view source) (protected)
Template:nv ln named req (view source)
Template:nv ph (view source)
Template:range (view source)
Template:range/core (view source)
Template:small (view source) (protected)
Template:title (view source) (protected)
Template:tooltip (view source)
Template:trim (view source) (protected)
Template:tt (view source) (protected)

Return to cpp/named req/LayoutMapping.

cppreference.com

Search

Namespaces

Variants

Views

Actions

View source for cpp/named req/LayoutMapping

Navigation

Tools