Re: RFC: allowing compound assignment operators with designated initializers

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

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:

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).

Of course the response will be "but I don't want to use C++" ...