[PATCH v1] libstdc++: Implement default_accessor from mdspan.

[Tomasz Kaminski]

On Fri, Jun 6, 2025 at 5:41 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

>
>
> On 6/6/25 14:12, Tomasz Kaminski wrote:
> > On Thu, Jun 5, 2025 at 4:31 PM Luc Grosheintz <luc.grosheintz@gmail.com>
> > wrote:
> >
> >> libstdc++-v3/ChangeLog:
> >>
> >>          * include/std/mdspan (default_accessor): New class.
> >>          * src/c++23/std.cc.in: Register default_accessor.
> >>          * testsuite/23_containers/mdspan/default_accessor.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/default_accessor.cc  | 59 +++++++++++++++++++
> >>
> > LGTM. I would only change a bit layout of test directories, to have
> > 23_containers/mdspan/accessors/default.cc
> > as we already have over aligned one, and few more were proposed.
>
> Makes sense, I'll move the file and send v2 after layouts are
> merged, that way I can be sure it'll apply cleanly.
>
Layouts are now merged.

>
> >
> >>   3 files changed, 87 insertions(+), 1 deletion(-)
> >>   create mode 100644
> >> libstdc++-v3/testsuite/23_containers/mdspan/default_accessor.cc
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index 4a3e863bed5..8c6b7f49155 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
> >> +    {
> >> +      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 9a52a7e7728..51111627d7d 100644
> >> --- a/libstdc++-v3/src/c++23/std.cc.in
> >> +++ b/libstdc++-v3/src/c++23/std.cc.in
> >> @@ -1847,7 +1847,8 @@ 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/default_accessor.cc
> >> b/libstdc++-v3/testsuite/23_containers/mdspan/default_accessor.cc
> >> new file mode 100644
> >> index 00000000000..303833d4857
> >> --- /dev/null
> >> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/default_accessor.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;
> >> +}
> >> --
> >> 2.49.0
> >>
> >>
> >
>
>
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