[PATCH v2 1/1] libstdc++: Implement default_accessor from mdspan.

[Luc Grosheintz]

On 6/13/25 12:40, Luc Grosheintz wrote:
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/mdspan (default_accessor): New class.
> 	* src/c++23/std.cc.in: Register default_accessor.
> 	* testsuite/23_containers/mdspan/accessors/default.cc: New test.
> 
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>   libstdc++-v3/include/std/mdspan               | 26 ++++++++
>   libstdc++-v3/src/c++23/std.cc.in              |  3 +-
>   .../23_containers/mdspan/accessors/default.cc | 59 +++++++++++++++++++
>   3 files changed, 87 insertions(+), 1 deletion(-)
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
> 
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index 6dc2441f80b..2e85ba8e6cb 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1004,6 +1004,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         [[no_unique_address]] _S_strides_t _M_strides;
>       };
>   
> +  template<typename _ElementType>
> +    struct default_accessor
> +    {

It would be easy to check the two mandates: not abstract, not array
here. Would you like a v3, with the change?

https://eel.is/c++draft/views.multidim#mdspan.accessor.default.overview-2

> +      using offset_policy = default_accessor;
> +      using element_type = _ElementType;
> +      using reference = element_type&;
> +      using data_handle_type = element_type*;
> +
> +      constexpr
> +      default_accessor() noexcept = default;
> +
> +      template<typename _OElementType>
> +	requires is_convertible_v<_OElementType(*)[], element_type(*)[]>
> +	constexpr
> +	default_accessor(default_accessor<_OElementType>) noexcept
> +	{ }
> +
> +      constexpr reference
> +      access(data_handle_type __p, size_t __i) const noexcept
> +      { return __p[__i]; }
> +
> +      constexpr data_handle_type
> +      offset(data_handle_type __p, size_t __i) const noexcept
> +      { return __p + __i; }
> +    };
> +
>   _GLIBCXX_END_NAMESPACE_VERSION
>   }
>   #endif
> diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
> index 109f590f1d1..e671aff68f8 100644
> --- a/libstdc++-v3/src/c++23/std.cc.in
> +++ b/libstdc++-v3/src/c++23/std.cc.in
> @@ -1843,7 +1843,8 @@ export namespace std
>     using std::layout_left;
>     using std::layout_right;
>     using std::layout_stride;
> -  // FIXME layout_left_padded, layout_right_padded, default_accessor and mdspan
> +  using std::default_accessor;
> +  // FIXME layout_left_padded, layout_right_padded and mdspan
>   }
>   #endif
>   
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
> new file mode 100644
> index 00000000000..303833d4857
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
> @@ -0,0 +1,59 @@
> +// { dg-do run { target c++23 } }
> +#include <mdspan>
> +
> +#include <testsuite_hooks.h>
> +
> +constexpr size_t dyn = std::dynamic_extent;
> +
> +template<typename Accessor>
> +  constexpr void
> +  test_accessor_policy()
> +  {
> +    static_assert(std::copyable<Accessor>);
> +    static_assert(std::is_nothrow_move_constructible_v<Accessor>);
> +    static_assert(std::is_nothrow_move_assignable_v<Accessor>);
> +    static_assert(std::is_nothrow_swappable_v<Accessor>);
> +  }
> +
> +constexpr bool
> +test_access()
> +{
> +  std::default_accessor<double> accessor;
> +  std::array<double, 5> a{10, 11, 12, 13, 14};
> +  VERIFY(accessor.access(a.data(), 0) == 10);
> +  VERIFY(accessor.access(a.data(), 4) == 14);
> +  return true;
> +}
> +
> +constexpr bool
> +test_offset()
> +{
> +  std::default_accessor<double> accessor;
> +  std::array<double, 5> a{10, 11, 12, 13, 14};
> +  VERIFY(accessor.offset(a.data(), 0) == a.data());
> +  VERIFY(accessor.offset(a.data(), 4) == a.data() + 4);
> +  return true;
> +}
> +
> +constexpr void
> +test_ctor()
> +{
> +  static_assert(std::is_nothrow_constructible_v<std::default_accessor<double>,
> +						std::default_accessor<double>>);
> +  static_assert(std::is_convertible_v<std::default_accessor<double>,
> +				      std::default_accessor<double>>);
> +  static_assert(!std::is_constructible_v<std::default_accessor<char>,
> +					 std::default_accessor<int>>);
> +}
> +
> +int
> +main()
> +{
> +  test_accessor_policy<std::default_accessor<double>>();
> +  test_access();
> +  static_assert(test_access());
> +  test_offset();
> +  static_assert(test_offset());
> +  test_ctor();
> +  return 0;
> +}