23 Containers library [containers]

23.7 Views [views]

23.7.3 Multidimensional access [views.multidim]

23.7.3.4 Layout mapping [mdspan.layout]

23.7.3.4.2 Requirements [mdspan.layout.reqmts]

A type M meets the layout mapping requirements if

(1.1)
M models copyable and equality_comparable,
(1.2)
is_nothrow_move_constructible_v<M> is true,
(1.3)
is_nothrow_move_assignable_v<M> is true,
(1.4)
is_nothrow_swappable_v<M> is true, and
(1.5)
the following types and expressions are well-formed and have the specified semantics.

typename M::extents_type

Result: A type that is a specialization of extents.

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typename M::index_type

Result: typename M::extents_type::index_type.

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typename M::rank_type

Result: typename M::extents_type::rank_type.

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typename M::layout_type

Result: A type MP that meets the layout mapping policy requirements ([mdspan.layout.policy.reqmts]) and for which is-mapping-of<MP, M> is true.

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m.extents()

Result: const typename M::extents_type&

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m(i...)

Result: typename M::index_type

Returns: A nonnegative integer less than numeric_limits<typename M::index_type>::max() and less than or equal to numeric_limits<size_t>::max().

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m(i...) == m(static_cast<typename M::index_type>(i)...)

Result: bool

Returns: true

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m.required_span_size()

Result: typename M::index_type

Returns: If the size of the multidimensional index space m.extents() is 0, then 0, else 1 plus the maximum value of m(i...) for all i.

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m.is_unique()

Result: bool

Returns: true only if for every i and j where (i != j || ...) is true, m(i...) != m(j...) is true.

[Note 1:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is unique.

— end note]

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m.is_exhaustive()

Result: bool

Returns: true only if for all k in the range [0, m.required_span_size()) there exists an i such that m(i...) equals k.

[Note 2:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is exhaustive.

— end note]

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m.is_strided()

Result: bool

Returns: true only if for every rank index r of m.extents() there exists an integer

s_{r}

such that, for all i where

(i + d_{r})

is a multidimensional index in m.extents() ([mdspan.overview]), m((i +

d_{r}

)...) - m(i...) equals

s_{r}

[Note 3:

This implies that for a strided layout

m (i_{0}, \dots, i_{k}) = m (0, \dots, 0) + i_{0} \times s_{0} + \dots + i_{k} \times s_{k}

— end note]

[Note 4:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is strided.

— end note]

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m.stride(r)

Preconditions: m.is_strided() is true.

Result: typename M::index_type

Returns:

s_{r}

as defined in m.is_strided() above.

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M::is_always_unique()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_unique() is true for all possible objects m of type M.

[Note 5:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is unique.

— end note]

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M::is_always_exhaustive()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_exhaustive() is true for all possible objects m of type M.

[Note 6:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is exhaustive.

— end note]

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M::is_always_strided()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_strided() is true for all possible objects m of type M.

[Note 7:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is strided.

— end note]