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

[Luc Grosheintz]

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.

> 
>>   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
>>
>>
>