23 Containers library [containers]

23.7 Views [views]

23.7.3 Multidimensional access [views.multidim]

23.7.3.5 Accessor policy [mdspan.accessor]

23.7.3.5.4 Class template aligned_accessor [mdspan.accessor.aligned]

23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]

namespace std { template<class ElementType, size_t ByteAlignment> struct aligned_accessor { using offset_policy = default_accessor<ElementType>; using element_type = ElementType; using reference = ElementType&; using data_handle_type = ElementType*; static constexpr size_t byte_alignment = ByteAlignment; constexpr aligned_accessor() noexcept = default; template<class OtherElementType, size_t OtherByteAlignment> constexpr aligned_accessor( aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept; template<class OtherElementType> constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept; template<class OtherElementType> constexpr operator default_accessor<OtherElementType>() const noexcept; constexpr reference access(data_handle_type p, size_t i) const noexcept; constexpr typename offset_policy::data_handle_type offset( data_handle_type p, size_t i) const noexcept; }; }
1
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Mandates:
  • (1.1)
    byte_alignment is a power of two, and
  • (1.2)
    byte_alignment >= alignof(ElementType) is true.
2
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aligned_accessor meets the accessor policy requirements.
3
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ElementType is required to be a complete object type that is neither an abstract class type nor an array type.
4
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Each specialization of aligned_accessor is a trivially copyable type that models semiregular.
5
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[0, n) is an accessible range for an object p of type data_handle_type and an object of type aligned_accessor if and only if
  • (5.1)
    [p, p + n) is a valid range, and,
  • (5.2)
    if n is greater than zero, then is_sufficiently_aligned<byte_alignment>(p) is true.
6
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[Example 1: 
The following function compute uses is_sufficiently_aligned to check whether a given mdspan with default_accessor has a data handle with sufficient alignment to be used with aligned_accessor<float, 4 * sizeof(float)>.
If so, the function dispatches to a function compute_using_fourfold_overalignment that requires fourfold over-alignment of arrays, but can therefore use hardware-specific instructions, such as four-wide SIMD (Single Instruction Multiple Data) instructions.
Otherwise, compute dispatches to a possibly less optimized function compute_without_requiring_overalignment that has no over-alignment requirement.
void compute_using_fourfold_overalignment( std::mdspan<float, std::dims<1>, std::layout_right, std::aligned_accessor<float, 4 * alignof(float)>> x); void compute_without_requiring_overalignment( std::mdspan<float, std::dims<1>, std::layout_right> x); void compute(std::mdspan<float, std::dims<1>> x) { constexpr auto byte_alignment = 4 * sizeof(float); auto accessor = std::aligned_accessor<float, byte_alignment>{}; auto x_handle = x.data_handle(); if (std::is_sufficiently_aligned<byte_alignment>(x_handle)) { compute_using_fourfold_overalignment(std::mdspan{x_handle, x.mapping(), accessor}); } else { compute_without_requiring_overalignment(x); } } — end example]