On Mon, Jun 30, 2025 at 9:58 AM Tomasz Kaminski <tkami...@redhat.com> wrote:
>
>
> On Mon, Jun 30, 2025 at 9:25 AM 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/accessors/default.cc: New test.
>> * testsuite/23_containers/mdspan/accessors/default_neg.cc: New
>> test.
>>
>> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
>> ---
>>
>> Version of v5 with all local changes included.
>>
> LGTM, tested locally and now running a full test suite.
> Once it will get approval from Jonathan, I will handle merging it.
>
All test passed.
>
>
>>
>> libstdc++-v3/include/std/mdspan | 31 ++++++
>> libstdc++-v3/src/c++23/std.cc.in | 3 +-
>> .../23_containers/mdspan/accessors/default.cc | 99 +++++++++++++++++++
>> .../mdspan/accessors/default_neg.cc | 23 +++++
>> 4 files changed, 155 insertions(+), 1 deletion(-)
>> create mode 100644
>> libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
>> create mode 100644
>> libstdc++-v3/testsuite/23_containers/mdspan/accessors/default_neg.cc
>>
>> diff --git a/libstdc++-v3/include/std/mdspan
>> b/libstdc++-v3/include/std/mdspan
>> index 6dc2441f80b..c72a64094b7 100644
>> --- a/libstdc++-v3/include/std/mdspan
>> +++ b/libstdc++-v3/include/std/mdspan
>> @@ -1004,6 +1004,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> [[no_unique_address]] _S_strides_t _M_strides;
>> };
>>
>> + template<typename _ElementType>
>> + struct default_accessor
>> + {
>> + static_assert(!is_array_v<_ElementType>,
>> + "ElementType must not be an array type");
>> + static_assert(!is_abstract_v<_ElementType>,
>> + "ElementType must not be an abstract class type");
>> +
>> + 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 9336118f5d9..e692caaa5f9 100644
>> --- a/libstdc++-v3/src/c++23/std.cc.in
>> +++ b/libstdc++-v3/src/c++23/std.cc.in
>> @@ -1850,7 +1850,8 @@ 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, aligned_accessor 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..c036f8ad10f
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
>> @@ -0,0 +1,99 @@
>> +// { 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;
>> +}
>> +
>> +class Base
>> +{ };
>> +
>> +class Derived : public Base
>> +{ };
>> +
>> +constexpr void
>> +test_ctor()
>> +{
>> + // T -> T
>> +
>> 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>>);
>> +
>> + // T -> const T
>> + static_assert(std::is_convertible_v<std::default_accessor<double>,
>> + std::default_accessor<const
>> double>>);
>> + static_assert(std::is_convertible_v<std::default_accessor<Derived>,
>> + std::default_accessor<const
>> Derived>>);
>> +
>> + // const T -> T
>> + static_assert(!std::is_constructible_v<std::default_accessor<double>,
>> + std::default_accessor<const
>> double>>);
>> + static_assert(!std::is_constructible_v<std::default_accessor<Derived>,
>> + std::default_accessor<const
>> Derived>>);
>> +
>> + // T <-> volatile T
>> + static_assert(std::is_convertible_v<std::default_accessor<int>,
>> + std::default_accessor<volatile
>> int>>);
>> + static_assert(!std::is_constructible_v<std::default_accessor<int>,
>> + std::default_accessor<volatile
>> int>>);
>> +
>> + // size difference
>> + static_assert(!std::is_constructible_v<std::default_accessor<char>,
>> + std::default_accessor<int>>);
>> +
>> + // signedness
>> + static_assert(!std::is_constructible_v<std::default_accessor<int>,
>> + std::default_accessor<unsigned
>> int>>);
>> + static_assert(!std::is_constructible_v<std::default_accessor<unsigned
>> int>,
>> + std::default_accessor<int>>);
>> +
>> + // Derived <-> Base
>> + static_assert(!std::is_constructible_v<std::default_accessor<Base>,
>> + std::default_accessor<Derived>>);
>> + static_assert(!std::is_constructible_v<std::default_accessor<Derived>,
>> + std::default_accessor<Base>>);
>> +
>> +}
>> +
>> +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;
>> +}
>> diff --git
>> a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default_neg.cc
>> b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default_neg.cc
>> new file mode 100644
>> index 00000000000..f8da2b569ca
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/default_neg.cc
>> @@ -0,0 +1,23 @@
>> +// { dg-do compile { target c++23 } }
>> +#include<mdspan>
>> +
>> +std::default_accessor<int[3]> a; // { dg-error "required from here" }
>> +
>> +class AbstractBase
>> +{
>> + virtual void
>> + foo() const = 0;
>> +};
>> +
>> +class Derived : public AbstractBase
>> +{
>> + void
>> + foo() const override
>> + { }
>> +};
>> +
>> +std::default_accessor<Derived> b_ok;
>> +std::default_accessor<AbstractBase> b_err; // { dg-error "required from
>> here"}
>> +
>> +// { dg-prune-output "ElementType must not be an array type" }
>> +// { dg-prune-output "ElementType must not be an abstract" }
>> --
>> 2.49.0
>>
>>
