On Tue, Jul 8, 2025 at 12:08 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> On Tue, 8 Jul 2025 at 10:54, Luc Grosheintz <luc.groshei...@gmail.com>
> wrote:
> >
> > This implements P3029R1. In P3029R1, the CTAD for span is refined to
> > permit deducing the extent of the span from an integral constant, e.g.
> >
> > span((T*) ptr, integral_constant<size_t, 5>{});
> >
> > is deduced as span<T, 5>. Similarly, in
> >
> > auto exts = extents(integral_constant<int, 2>);
> > auto md = mdspan((T*) ptr, integral_constant<int, 2>);
> >
> > exts and md have types extents<size_t, 2> and mdspan<double,
> > extents<size_t, 2>>, respectively.
> >
> > PR libstdc++/120914
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/span (span): Update CTAD to enable
> > integral constants [P3029R1].
> > * include/std/mdspan (extents): ditto.
> > (mdspan): ditto.
> > * testsuite/23_containers/span/deduction.cc: Test deduction
> > guide.
> > * testsuite/23_containers/mdspan/extents/misc.cc: ditto.
> > * testsuite/23_containers/mdspan/mdspan.cc: ditto.
> >
> > Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> > ---
> > libstdc++-v3/include/std/mdspan | 8 ++----
> > libstdc++-v3/include/std/span | 20 ++++++++++++++-
> > .../23_containers/mdspan/extents/misc.cc | 20 +++++++++++++++
> > .../testsuite/23_containers/mdspan/mdspan.cc | 25 +++++++++++++++++++
> > .../testsuite/23_containers/span/deduction.cc | 3 +++
> > 5 files changed, 69 insertions(+), 7 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> > index 0fd78570b3e..152dcb3e92a 100644
> > --- a/libstdc++-v3/include/std/mdspan
> > +++ b/libstdc++-v3/include/std/mdspan
> > @@ -406,10 +406,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > template<typename _IndexType, size_t... _Counts>
> > auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
> > -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
> > -
> > - template<typename _Tp>
> > - consteval size_t
> > - __dynamic_extent() { return dynamic_extent; }
> > }
> >
> > template<typename _IndexType, size_t _Rank>
> > @@ -419,7 +415,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > template<typename... _Integrals>
> > requires (is_convertible_v<_Integrals, size_t> && ...)
> > explicit extents(_Integrals...) ->
> > - extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>;
> > + extents<size_t, __detail::__maybe_static_ext<_Integrals>...>;
> >
> > struct layout_left
> > {
> > @@ -1316,7 +1312,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > && (sizeof...(_Integrals) > 0)
> > explicit mdspan(_ElementType*, _Integrals...)
> > -> mdspan<_ElementType,
> > - extents<size_t,
> __mdspan::__dynamic_extent<_Integrals>()...>>;
> > + extents<size_t,
> __detail::__maybe_static_ext<_Integrals>...>>;
> >
> > template<typename _ElementType, typename _OIndexType, size_t _Nm>
> > mdspan(_ElementType*, span<_OIndexType, _Nm>)
> > diff --git a/libstdc++-v3/include/std/span
> b/libstdc++-v3/include/std/span
> > index 49ab9109d83..5629a71b9bd 100644
> > --- a/libstdc++-v3/include/std/span
> > +++ b/libstdc++-v3/include/std/span
> > @@ -476,6 +476,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > };
> >
> > // deduction guides
> > + namespace __detail
> > + {
> > + template<typename _Tp>
> > + concept __integral_constant_like =
> is_integral_v<decltype(_Tp::value)>
> > + && !is_same_v<bool, remove_const_t<decltype(_Tp::value)>>
> > + && convertible_to<_Tp, decltype(_Tp::value)>
> > + && equality_comparable_with<_Tp, decltype(_Tp::value)>
> > + && bool_constant<_Tp() == _Tp::value>::value
> > + && bool_constant<static_cast<decltype(_Tp::value)>(_Tp()) ==
> _Tp::value>
> > + ::value;
> > +
> > + template<typename _Tp>
> > + constexpr size_t __maybe_static_ext = dynamic_extent;
> > +
> > + template<__integral_constant_like _Tp>
> > + constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
>
> Are the braces here to detect narrowing conversions?
>
> (The paper doesn't mention why they're used, as far as I can see)
>
The information is in revision history (a bit hidden):
Initialized the *maybe-static-ext*'s specialization with {T::value} to
prevent conversions from negative values.
>
>
> > + }
> >
> > template<typename _Type, size_t _ArrayExtent>
> > span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
> > @@ -489,7 +506,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> > template<contiguous_iterator _Iter, typename _End>
> > span(_Iter, _End)
> > - -> span<remove_reference_t<iter_reference_t<_Iter>>>;
> > + -> span<remove_reference_t<iter_reference_t<_Iter>>,
> > + __detail::__maybe_static_ext<_End>>;
> >
> > template<ranges::contiguous_range _Range>
> > span(_Range &&)
> > diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> > index e71fdc54230..bca8901685d 100644
> > --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> > @@ -97,6 +97,25 @@ test_deduction(Extents... exts)
> > VERIFY(e == expected);
> > }
> >
> > +constexpr bool
> > +test_integral_constant_deduction()
> > +{
> > + auto verify = [](auto actual, auto expected)
> > + {
> > + static_assert(std::same_as<decltype(actual), decltype(expected)>);
> > + VERIFY(actual == expected);
> > + };
> > +
> > + constexpr auto c1 = std::integral_constant<size_t, 1>{};
> > + constexpr auto c2 = std::integral_constant<int, 2>{};
> > +
> > + verify(std::extents(1), std::extents<size_t, dyn>{1});
> > + verify(std::extents(c1), std::extents<size_t, 1>{});
> > + verify(std::extents(c2), std::extents<size_t, 2>{});
> > + verify(std::extents(c1, 2), std::extents<size_t, 1, dyn>{2});
> > + return true;
> > +}
> > +
> > constexpr bool
> > test_deduction_all()
> > {
> > @@ -104,6 +123,7 @@ test_deduction_all()
> > test_deduction<1>(1);
> > test_deduction<2>(1.0, 2.0f);
> > test_deduction<3>(int(1), short(2), size_t(3));
> > + test_integral_constant_deduction();
> > return true;
> > }
> >
> > diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> > index 9252273bf66..a650fb19bdf 100644
> > --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> > @@ -245,6 +245,28 @@ test_from_pointer_and_shape()
> > return true;
> > }
> >
> > +constexpr bool
> > +test_from_pointer_and_integral_constant()
> > +{
> > + std::array<double, 6> buffer{};
> > + double * ptr = buffer.data();
> > +
> > + auto verify = [ptr](auto actual, auto exts)
> > + {
> > + auto expected = std::mdspan<double, decltype(exts)>(ptr, exts);
> > + static_assert(std::same_as<decltype(actual), decltype(expected)>);
> > + VERIFY(actual.extents() == expected.extents());
> > + };
> > +
> > + auto c3 = std::integral_constant<int, 3>{};
> > + auto c6 = std::integral_constant<int, 6>{};
> > +
> > + verify(std::mdspan(ptr, 6), std::extents(6));
> > + verify(std::mdspan(ptr, c6), std::extents(c6));
> > + verify(std::mdspan(ptr, 2, c3), std::extents(2, c3));
> > + return true;
> > +}
> > +
> > constexpr bool
> > test_from_extents()
> > {
> > @@ -616,6 +638,9 @@ main()
> > test_from_pointer_and_shape();
> > static_assert(test_from_pointer_and_shape());
> >
> > + test_from_pointer_and_integral_constant();
> > + static_assert(test_from_pointer_and_integral_constant());
> > +
> > test_from_extents();
> > static_assert(test_from_extents());
> >
> > diff --git a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> > index dce6cedf89b..c66db90222e 100644
> > --- a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> > +++ b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> > @@ -80,4 +80,7 @@ test01()
> >
> > std::span s12(const_cast<const std::span<int>&>(s5));
> > static_assert( is_dynamic_span<int>(s12) );
> > +
> > + std::span s13(a.data(), std::integral_constant<size_t, 3>{});
> > + static_assert( is_static_span<long, 3>(s13));
> > }
> > --
> > 2.49.0
> >
>
>
