On Thu, Jun 5, 2025 at 4:31 PM Luc Grosheintz <luc.groshei...@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.groshei...@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.
> 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
>
>
