220 36404 article Path: news.gmane.org!.POSTED!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: The current attribute/keyword dividing line is unsustainable. 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Wed, 27 Dec 2017 14:37:34 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.176.1.3 with SMTP id 3ls4424683uak.9.gmail; Wed, 27 Dec 2017 14:37:32 -0800 (PST) X-Received: by 10.31.165.202 with SMTP id o193mr2743766vke.9.1514414252424; Wed, 27 Dec 2017 14:37:32 -0800 (PST) In-Reply-To: <86919454-9af9-4b08-b7b3-fd262b46cdd5@isocpp.org> X-Original-Sender: arthur.j.odwyer@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:36404 Archived-At: ------=_Part_17782_1456175466.1514414251976 Content-Type: multipart/alternative; boundary="----=_Part_17783_967905290.1514414251976" ------=_Part_17783_967905290.1514414251976 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wednesday, December 27, 2017 at 10:15:35 AM UTC-8, Nicol Bolas wrote: > > There is clearly intended to be a dividing line between C++ attributes an= d=20 > C++ keywords. > > Herb Sutter outlined one version of this division=20 > .= =20 > As he states: > > > [[attributes]] are specifically designed to be ignorable and shouldn=E2= =80=99t=20 > be used for things having language semantic meaning. > > P0840=20 > =20 > offers a different but similar division: > > > compiling a valid program with all instances of a particular attribute= =20 > ignored must result in a correct interpretation of the original program > > I submit that these dividing lines are both unsustainable and *have=20 > already been violated*. > I submit that the current situation is fine. We have guidelines, and we=20 have smart people in charge of making sure that reality follows those=20 guidelines as closely as possible (but no closer). The original source of the "Attributes should not change semantics"=20 guideline was N2761 "Towards support for attributes in C++ (R6)", in 2008.= =20 Here is the complete text of the guidelines at the time, with hyperlinks to= =20 various original documents, which I collated the last time this topic came= =20 up. https://groups.google.com/a/isocpp.org/d/msg/sg14/IvKFg6-9_Xo/PNj1kAntCQAJ IMO, these guidelines have held up very well. They are still quite=20 sensible. I don't think N2761 has any embarrassing conflicts with the=20 actual history of C++ since 2008. What makes this unsustainable is this: what we eventually want to do is=20 > make [[fallthrough]]* mandatory*. That is, to make it ill-formed to fall= =20 > through without explicitly asking for it. The problem is that we can't. S= o=20 > long as we stick to the above rules about attributes, we cannot have the= =20 > standard declare the lack of use of an attribute to be ill-formed. > I think you're mixing senses of the word "we". *Individual programmers*=20 may want to make [[fallthrough]] mandatory or may not, depending on their= =20 degree of paranoia and/or degree of spending time maintaining C code from= =20 the 1980s. *Individual vendors* are effectively free to make=20 [[fallthrough]] mandatory today (and have generally already done so, in the= =20 recommended -W -Wall -Werror mode of invocation). *The ISO C++ Committee*= =20 indeed cannot declare the lack of use of an attribute to be ill-formed, as= =20 long as the ISO C++ Committee sticks (strictly) to the above guidelines;=20 but fortunately, I don't think the ISO C++ Committee has much interest in= =20 sticking strictly to any guideline ever. Heck, they don't even stick to=20 the *official ISO language standard* for more than three years at a time=20 anymore. ;) Something similar happens with [[noreturn]]: if we ever want to make=20 > functions that fail to return ill-formed, we would have to make such a ru= le=20 > take into account calls to [[noreturn]] functions. But we can't do that,= =20 > because attributes cannot be used to make something ill-formed. > As observed in many a previous thread: sure they can. Look at=20 [dcl.attr.fallthrough]/1, specifically the sentence beginning "The program= =20 is ill-formed if..." The Committee follows its self-imposed guidelines *as closely as possible,= =20 but no closer*. [...] > But we should not separate out attributes from keywords in terms of their= =20 > relation to overall program behavior. That is, we should ditch the idea= =20 > that attributes should be considered "ignorable", that the lack of an=20 > attribute should never cause a program to be ill-formed. > I have good news for you. ;) =E2=80=93Arthur --=20 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 e= mail 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/b46ccd49-43c8-424e-90c9-3053128f6560%40isocpp.or= g. ------=_Part_17783_967905290.1514414251976 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Wednesday, December 27, 2017 at 10:15:35 AM UTC-8, Nico= l Bolas wrote:
There is clearly intended to be a dividing line between C++ attribute= s and C++ keywords.

Herb Sutter outlined one version of this div= ision. As he states:

>=C2=A0[[attributes]] = are specifically designed to be ignorable and shouldn=E2=80=99t be used for= things having language semantic meaning.

P0840 offer= s a different but similar division:

>=C2=A0comp= iling a valid program with all instances of a particular attribute ignored must result in a correct interpretation of the original program
<= br>
I submit that these dividing lines are both unsustainable and= have already been violated.

=
I submit that the current situation is fine. We have guidelines, and w= e have smart people in charge of making sure that reality follows those gui= delines as closely as possible (but no closer).

Th= e original source of the "Attributes should not change semantics"= guideline was N2761 "Towards support for attributes in C++ (R6)"= , in 2008. Here is the complete text of the guidelines at the time, with hy= perlinks to various original documents, which I collated the last time this= topic came up.
IMO, these guidelin= es have held up very well. =C2=A0They are still quite sensible. =C2=A0I don= 't think N2761 has any embarrassing conflicts with the actual history o= f C++ since 2008.


What makes this unsustainable = is this: what we eventually want to do is make [[fallthrough]] mandatory= . That is, to make it ill-formed to fall through without explicitly ask= ing for it. The problem is that we can't. So long as we stick to the ab= ove rules about attributes, we cannot have the standard declare the lack of= use of an attribute to be ill-formed.
I think you're mixing senses of the word "we".=C2= =A0=C2=A0Individual programmers may want to make [[fallthrough]] man= datory or may not, depending on their degree of paranoia and/or degree of s= pending time maintaining C code from the 1980s.=C2=A0=C2=A0Individual ve= ndors=C2=A0are effectively free to make [[fallthrough]] mandatory today= (and have generally already done so, in the recommended -W -Wall -Werror m= ode of invocation). =C2=A0The ISO C++ Committee indeed cannot declar= e the lack of use of an attribute to be ill-formed, as long as the ISO C++ = Committee sticks (strictly) to the above guidelines; but fortunately, I don= 't think the ISO C++ Committee has much interest in sticking strictly t= o any guideline ever. =C2=A0Heck, they don't even stick to the offic= ial ISO language standard for more than three years at a time anymore. = ;)


Something similar happens with [[n= oreturn]]: if we ever want to make functions that fail to return ill-formed= , we would have to make such a rule take into account calls to [[noreturn]]= functions. But we can't do that, because attributes cannot be used to = make something ill-formed.

As o= bserved in many a previous thread: sure they can. Look at [dcl.attr.fallthr= ough]/1, specifically the sentence beginning "The program is ill-forme= d if..."
The Committee follows its self-imposed guidelines <= i>as closely as possible, but no closer.

[...]=
Bu= t we should not separate out attributes from keywords in terms of their rel= ation to overall program behavior. That is, we should ditch the idea that a= ttributes should be considered "ignorable", that the lack of an a= ttribute should never cause a program to be ill-formed.

I have good news for you. ;)

=E2=80=93Arthur

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