On Thu, Jun 26, 2025 at 3:40 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> 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.groshei...@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
Yes, I think that makes sense. Thanks.
>
>
> > + 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;
> > +}
>
>
