24 Containers library [containers]

24.7 Views [views]

24.7.3 Multidimensional access [views.multidim]

24.7.3.4 Layout mapping [mdspan.layout]

24.7.3.4.2 Requirements [mdspan.layout.reqmts]

A type M meets the layout mapping requirements if
  • M models copyable and equality_comparable,
  • is_nothrow_move_constructible_v<M> is true,
  • is_nothrow_move_assignable_v<M> is true,
  • is_nothrow_swappable_v<M> is true, and
  • 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.
typename M::index_type
Result: typename M​::​extents_type​::​index_type.
typename M::rank_type
Result: typename M​::​extents_type​::​rank_type.
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.
m.extents()
Result: const typename M​::​extents_type&
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().
m(i...) == m(static_cast<typename M::index_type>(i)...)
Result: bool
Returns: true
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.
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]
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]
m.is_strided()
Result: bool
Returns: true only if for every rank index r of m.extents() there exists an integer such that, for all i where is a multidimensional index in m.extents() ([mdspan.overview]), m((i + )...) - m(i...) equals .
[Note 3: 
This implies that for a strided layout .
— 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]
m.stride(r)
Preconditions: m.is_strided() is true.
Result: typename M​::​index_type
Returns: as defined in m.is_strided() above.
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]
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]
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]