pon., 5 gru 2022 o 00:30 Jason McKesson via Std-Proposals
<std-proposals_at_[hidden]> napisał(a):
>
> On Sun, Dec 4, 2022 at 6:23 PM Lénárd Szolnoki via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
> >
> > Hi,
> >
> > On 4 December 2022 22:40:25 GMT, Frederick Virchanza Gotham via Std-Proposals <std-proposals_at_[hidden]> wrote:
> > >I wanted to write the following function containing a fold expression:
> > >
> > >template<typename... Types>
> > >auto Func(Types... a)
> > >{
> > > using std::get;
> > > decltype(pointers) &p = pointers;
> > >
> > > (if constexpr(requires{ get<Types>(p)->Method(a...); }) return
> > >get<Types>(p)->Method(a...), ...);
> > >}
> > >
> > >but since we can't have 'if constexpr' inside a fold expression, I
> > >instead had to write:
> > >
> > >template<typename... Types>
> > >auto Func(Types... a)
> > >{
> > >using std::get;
> > >
> > >decltype(pointers) &p = pointers;
> > >
> > > if constexpr(requires{get< 0>(p)->Method(a...); })return get<
> > >0>(p)->Method(a...);
> > >else if constexpr(requires{get< 1>(p)->Method(a...); })return get<
> > >1>(p)->Method(a...);
> > >else if constexpr(requires{get< 2>(p)->Method(a...); })return get<
> > >2>(p)->Method(a...);
> > >else if constexpr(requires{get< 3>(p)->Method(a...); })return get<
> > >3>(p)->Method(a...);
> > >else if constexpr(requires{get< 4>(p)->Method(a...); })return get<
> > >4>(p)->Method(a...);
> > >else if constexpr(requires{get< 5>(p)->Method(a...); })return get<
> > >5>(p)->Method(a...);
> > >else if constexpr(requires{get< 6>(p)->Method(a...); })return get<
> > >6>(p)->Method(a...);
> > >else if constexpr(requires{get< 7>(p)->Method(a...); })return get<
> > >7>(p)->Method(a...);
> > >else if constexpr(requires{get< 8>(p)->Method(a...); })return get<
> > >8>(p)->Method(a...);
> > >else if constexpr(requires{get< 9>(p)->Method(a...); })return get<
> > >9>(p)->Method(a...);
> > >else if constexpr(requires{get<10>(p)->Method(a...); })return
> > >get<10>(p)->Method(a...);
> > >else if constexpr(requires{get<11>(p)->Method(a...); })return
> > >get<11>(p)->Method(a...);
> > >else if constexpr(requires{get<12>(p)->Method(a...); })return
> > >get<12>(p)->Method(a...);
> > >else if constexpr(requires{get<13>(p)->Method(a...); })return
> > >get<13>(p)->Method(a...);
> > >else if constexpr(requires{get<14>(p)->Method(a...); })return
> > >get<14>(p)->Method(a...);
> > >else if constexpr(requires{get<15>(p)->Method(a...); })return
> > >get<15>(p)->Method(a...);
> > >}
> > >
> > >Is there some how some way that we can allow 'if constexpr' inside a
> > >fold expression? I know that we can have a fold expression containing
> > >a lambda (and that the lambda can contain 'if constexpr'), but in this
> > >example we need 'if constexpr' by itself.
> >
> > Note that apart from an if of if constexpr statement, you can't have a return statement in a fold expression either. There is absolutely no way to short-circuit a fold expression, unless it's fold over && or ||, and even then, it needs to be the built-in operator. This limits fold expressions somewhat.
> >
> > There were some proposals to introduce for loops over packs, I don't know the state of those. That would solve this issue quite elegantly.
>
> The proposal is P1306: expansion statements. I don't really know what
> happened to it. According to P2632, it has finished design work, but
> it hasn't moved forward past EWG. It seems to be stalled out as "needs
> updates". I guess the people proposing it lost interest or something.

It’s a shame that it is stuck in limbo.
What exactly was needed to push it forward?


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