Re: [PATCH] libstdc++: Make __max_size_type and __max_diff_type structural

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 16 Aug 2023 at 17:07, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Mon, Apr 24, 2023 at 12:23 PM Patrick Palka <ppalka@redhat.com> wrote:
> >
> > This patch makes these integer-class type structural types by changing
> > their private data members into public ones, which allows them to be
> > used as NTTP types.  I'm not sure if this is required by the standard
> > but it seems handy.
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> Ping


I'm not sure about this one. I am pretty sure it's not required, and
I'm not sure it's needed.

Do we have a use for it ourselves? Users shouldn't be using this type
directly, or relying on properties that the standard doesn't specify,
so I don't think they should be using it as a structural type.


>
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/bits/max_size_type.h (__max_size_type::_M_val): Make
> >         public instead of private.
> >         (__max_size_type::_M_msb): Likewise.
> >         (__max_diff_type::_M_rep): Likewise.
> >         * testsuite/std/ranges/iota/max_size_type.cc: Verify
> >         __max_diff_type and __max_size_type are structural.
> > ---
> >  libstdc++-v3/include/bits/max_size_type.h               | 4 ++--
> >  libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc | 7 +++++++
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h
> > index 4796135d073..d6705bbe4c8 100644
> > --- a/libstdc++-v3/include/bits/max_size_type.h
> > +++ b/libstdc++-v3/include/bits/max_size_type.h
> > @@ -423,10 +423,11 @@ namespace ranges
> >        using __rep = unsigned long long;
> >  #endif
> >        static constexpr size_t _S_rep_bits = sizeof(__rep) * __CHAR_BIT__;
> > -    private:
> > +
> >        __rep _M_val = 0;
> >        unsigned _M_msb:1 = 0;
> >
> > +    private:
> >        constexpr explicit
> >        __max_size_type(__rep __val, int __msb) noexcept
> >         : _M_val(__val), _M_msb(__msb)
> > @@ -750,7 +751,6 @@ namespace ranges
> >        { return !(__l < __r); }
> >  #endif
> >
> > -    private:
> >        __max_size_type _M_rep = 0;
> >
> >        friend class __max_size_type;
> > diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> > index 985acd5a803..9afd05d5acf 100644
> > --- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> > +++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
> > @@ -400,6 +400,13 @@ static_assert(max_diff_t(max_size_t(1)
> >                          << (numeric_limits<max_size_t>::digits-1))
> >               == numeric_limits<max_diff_t>::min());
> >
> > +// Verify that the types are structural types and can therefore be used
> > +// as NTTP types.
> > +template<max_size_t V> struct Su { static_assert(V*V == V+132); };
> > +template<max_diff_t V> struct Ss { static_assert(V*V == V+132); };
> > +template struct Su<12>;
> > +template struct Ss<12>;
> > +
> >  int
> >  main()
> >  {
> > --
> > 2.40.0.374.g7580f92ffa
> >
>