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Re: RFC: allowing compound assignment operators with designated initializers


On Mon, 15 Oct 2018 at 20:08, Gabriel Paubert <paubert@iram.es> wrote:
>
> On Mon, Oct 15, 2018 at 08:11:42PM +0200, Florian Weimer wrote:
> > * Jonathan Wakely:
> >
> > > On Sun, 14 Oct 2018 at 20:46, Florian Weimer <fw@deneb.enyo.de> wrote:
> > >>
> > >> * Rasmus Villemoes:
> > >>
> > >> > This is something I've sometimes found myself wishing was supported. The
> > >> > idea being that one can say
> > >> >
> > >> > unsigned a[] = { [0] = 1, [1] = 3, [0] |= 4, ...}
> > >> >
> > >> > which would end up initializing a[0] to 5. As a somewhat realistic
> > >> > example, suppose one is trying to build a bitmap at compile time, but
> > >> > the bits to set are not really known in the sense that one can group
> > >> > those belonging to each index in a usual | expression. Something like
> > >> >
> > >> > #define _(e) [e / 8] |= 1 << (e % 8)
> > >> > const u8 error_bitmap[] = { _(EINVAL), _(ENAMETOOLONG), _(EBUSY), ... }
> > >>
> > >> I think it wouldn't be too hard to extend std::bitset with more
> > >> compile-time operations to support this, if that's what you need.
> > >
> > > It's already doable using C++17:
> >
> > I didn't doubt that, it's just that I'd expect to be able to use
> > std::bitset for this.
> >
> > > template<int... N>
> > > constexpr auto
> > > make_error_bitmap()
> > > {
> > >   using std::uint8_t;
> > >   using std::array;
> > >   constexpr auto max_index = std::max_element({N...}) / 8;
> > >   array<uint8_t, max_index+1> a;
> > >   [[maybe_unused]] uint8_t sink[] = { a[N/8] |= (1 << (N%8)), ... };
> > >   return a;
> > > }
> > >
> > > constexpr uint8_t error_bitmap = make_error_bitmap<EINVAL,
> > > ENAMETOOLONG, EBUSY>();
> > >
> > > (This won't compile in C++14 because std::array can't be modified in a
> > > constant expression until C++17).
> >
> > You wrote that without testing it?  I'm impressed.  It's really close.
> >
> > template<int... N>
> > constexpr auto
> > make_error_bitmap()
> > {
> >   using std::uint8_t;
> >   using std::array;
> >   constexpr auto max_index = std::max({ N... });
> >   array<uint8_t, max_index+1> a{};
> >   [[maybe_unused]] uint8_t sink[] = { a[N/8] |= (1 << (N%8)) ... };
> >   return a;
> > }
> >
>
> Hmm, isn't the array roughly 8 times too large?

Yes, it looks like I pasted the wrong version of  the code, which is
why  it had the stray comma that Florian corrected. The final version
of the code I actually wrote is:

#include <array>
#include <algorithm>
#include <cstdint>

template<int... N>
constexpr auto
make_error_bitmap()
{
  using std::uint8_t;
  using std::array;
  constexpr auto max_index = std::max({N...}) / 8;
  array<uint8_t, max_index+1> a{};
  [[maybe_unused]] uint8_t sink[] = { a[N/8] |= (1 << (N%8)) ... };
  return a;
}

constexpr auto error_bitmap = make_error_bitmap<EINVAL, ENAMETOOLONG, EBUSY>();

(Note that max_index has the division by 8)


>
> IOW, shouldn't you declare "array<uint8_t, (max_index+7)/8> a{};" ?
>
> > constexpr auto error_bitmap = make_error_bitmap<EINVAL, ENAMETOOLONG, EBUSY>();
> >
> > It seems to produce the intended bit pattern.
>
> Did you think of big-endian machines (just curious)?

The code does what the OP asked for, I didn't try to figure out if
what it did made sense.


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