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Re: RFC: allowing compound assignment operators with designated initializers
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?
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)?
Gabriel
>
> > Of course the response will be "but I don't want to use C++" ...
>
> Indeed.