220 41267 article Path: news.gmane.org!.POSTED!not-for-mail From: Richard Hodges Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: constexpr also in switch() and even in for(). 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[209.85.220.41]) by mx.google.com with SMTPS id u17sor9358121itb.18.2018.12.14.02.32.34 for (Google Transport Security); Fri, 14 Dec 2018 02:32:34 -0800 (PST) Received-SPF: pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a24:64e:: with SMTP id 75mr2779780itv.15.1544783553914; Fri, 14 Dec 2018 02:32:33 -0800 (PST) In-Reply-To: <4fe7ab83-be8f-4f62-a1a4-72b39fb540d3@isocpp.org> X-Original-Sender: hodges.r@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=XkCoTg3p; spf=pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hodges.r@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:41267 Archived-At: --000000000000fc6c9e057cf8f215 Content-Type: text/plain; charset="UTF-8" On Fri, 14 Dec 2018 at 16:20, Alberto Barbati wrote: > Hi Frank, > > I believe you are missing the point of "if constexpr". The real usefulness > of "if constexpr" is that it makes the branch that is not selected a > "discarded statement" so that it doesn't trigger compilation errors if it > is ill-formed after template parameters substitution. In the case of the > for statement, that usefulness simply isn't there, because there's only one > statement to consider, so either it is ill-formed for all iterations or > none of them. The idea of having the iteration variable constexpr is > intriguing, but I'm not totally sure that it has a real usefulness, > especially after the introduction of consteval functions. The switch > statement is a bit more complicated, as its structure is too "flexible", > with jumps at labels that may not respect scopes. Because of that a switch > is *not* equivalent to a cascade of if-statements at least in the general > case. > > Anyway, your proposal indicates a possibile solution but doesn't describe > a real problem that the proposal is trying to solve. I am afraid but "It > would be nice to have this..." is not a satisfactory description of real > problem. > I think the use case and problem domain are quite well understood. consider runtime-to-compile-time state dispatcher: auto disp = dispatcher { foo, bar, baz, [] { foo(); bar(); }, [] { foo(); bar(); baz(); }, }; disp(4); // execute foo(), bar(), baz() (see the full working horror show here: https://coliru.stacked-crooked.com/a/1cab88f58a218b02) It would be very convenient (and much more readable) to be able to write: for constexpr (std::size_t = 0 ; i < std::tuple_size_v(decltype(disp)) ; ++i) { if (i == which) // if there is a match... { std::get(disp)(); // ... dispatch the function. i is constexpr break; // ... and exit the search early } } else { // none of the iterations issued a break; throw std::invalid_argument("index too large: "s + std::to_string(which); } The advantages become more pronounced with the new hypothetical approach when we consider that the dispatch is no longer limited to one argument or return type. It can be easy selected on the basis of the (constexpr) value of i. > Alberto > > > -- > You received this message because you are subscribed to the Google Groups > "ISO C++ Standard - Future Proposals" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to std-proposals+unsubscribe@isocpp.org. > To post to this group, send email to std-proposals@isocpp.org. > To view this discussion on the web visit > https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/4fe7ab83-be8f-4f62-a1a4-72b39fb540d3%40isocpp.org > > . > -- Richard Hodges hodges.r@gmail.com office: +442032898513 home: +376841522 mobile: +376380212 (this will be *expensive* outside Andorra!) skype: madmongo facebook: hodges.r -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CALvx3hbM2dPOgHh%3D2NvjhoC2kuv9doA9xWdVKJ2cT_upipRKfw%40mail.gmail.com. --000000000000fc6c9e057cf8f215 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Fri, 14 Dec 2018 at 16:20, Alberto Barbati &= lt;albertobarbati@gmail.com= > wrote:
Hi Frank,

I believe you are miss= ing the point of "if constexpr". The real usefulness of "if = constexpr" is that it makes the branch that is not selected a "di= scarded statement" so that it doesn't trigger compilation errors i= f it is ill-formed after template parameters substitution. In the case of t= he for statement, that usefulness simply isn't there, because there'= ;s only one statement to consider, so either it is ill-formed for all itera= tions or none of them. The idea of having the iteration variable constexpr = is intriguing, but I'm not totally sure that it has a real usefulness, = especially after the introduction of consteval functions. The switch statem= ent is a bit more complicated, as its structure is too "flexible"= , with jumps at labels that may not respect scopes. Because of that a switc= h is *not* equivalent to a cascade of if-statements at least in the general= case.

Anyway, your proposal indicates a possibile= solution but doesn't describe a real problem that the proposal is tryi= ng to solve. I am afraid but "It would be nice to have this..." is not a satisfactory descripti= on of real problem.

= I think the use case and problem domain are quite well understood. consider= runtime-to-compile-time state dispatcher:

=C2=A0 =C2=A0 auto disp =3D dispatcher
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 fo= o,
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 bar,
=C2=A0= =C2=A0 =C2=A0 =C2=A0 baz,
=C2=A0 =C2=A0 =C2=A0 =C2=A0 [] { foo(); bar(); },
=C2=A0 =C2=A0 =C2=A0 =C2=A0 [] { foo(); ba= r(); baz(); },
=C2=A0 = =C2=A0 };

<= /div>
=C2=A0 =C2=A0 dis= p(4);=C2=A0 // execute foo(), bar(), baz()

<= /div>
(see the full working horror show here:=C2=A0https://coliru.stacked-crooke= d.com/a/1cab88f58a218b02)

It would be very con= venient (and much more readable) to be able to write:

<= div>for constexpr (std::siz= e_t =3D 0 ; i < std::tuple_size_v(decltype(disp)) ;=C2=A0++i)
=
{
=C2=A0 if (i =3D=3D which)=C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 // if there is a match...
=C2=A0 {
=C2=A0 =C2=A0 std::get<i>(disp)();=C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0// ... dispatch the function. i is constexpr
=C2=A0 =C2=A0 break;=C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0// ... and exit the search early
=C2=A0 }
}
else
{
=C2=A0 // none of the iterations issued a break;
=C2=A0 throw s= td::invalid_argument("index too large: "s=C2=A0+ std::to_string(w= hich);
}
=C2=A0
The advantages become more pronounced with the new= hypothetical approach when we consider that the dispatch is no longer limi= ted to one argument or return type. It can be easy selected on the basis of= the (constexpr) value of i.


Albe= rto


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To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit = https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/4fe7ab83-be8f-= 4f62-a1a4-72b39fb540d3%40isocpp.org.


--
Richard Hodgesoffice: +442032898513
home: +376841522
mobile:=C2=A0+= 376380212 (this will be *expensive* outside Andorra!)
skype: madm= ongo
facebook: hodges.r

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