220 30195 <7d89ec43-c691-44d8-bcb5-74dcda9eb493@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: enum_cast proposal Date: Tue, 3 Jan 2017 11:59:46 -0800 (PST) Lines: 168 Approved: news@gmane.org Message-ID: <7d89ec43-c691-44d8-bcb5-74dcda9eb493@isocpp.org> References: <2998762.etaySSRi7N@tjmaciei-mobl1> <85e681ea-20cb-5d87-d337-6b472e35eb11@wanadoo.fr> <2225413.PkzHcSPZVx@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2653_46906257.1483473586093" X-Trace: blaine.gmane.org 1483473590 3674 195.159.176.226 (3 Jan 2017 19:59:50 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 3 Jan 2017 19:59:50 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBMUFWDBQKGQEC6RNXYQ@isocpp.org Tue Jan 03 20:59:45 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f70.google.com ([74.125.83.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cOVFH-0008SA-Fr for gclcip-std-proposals@m.gmane.org; Tue, 03 Jan 2017 20:59:43 +0100 Original-Received: by mail-pg0-f70.google.com with SMTP id g1sf1309167468pgn.3 for ; Tue, 03 Jan 2017 11:59:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=drTHkwxzxB+x1/mNOFORba68gMbMth7YgtywJubwdz0=; b=seqfgw7WRNg5e9vs9kjkZNhRgXlo4NWSWD8lmyz88MzmZgonYWJB2peCqA1B7cJbvK 2JGEOMF1Y9F3IWEKsKnLC2qUl+VTOWvEYZOjSecj2U1rSm8Tp2jOwqryhI8VEsjmJeVS jopdwWXREUVHsjKiRykcaBgPcVhW1UQ+mtkvvvKLJ4WEFhBb3Agi8v6Hyb9h/P33xxUb KoPbzZADmX44fYfWgT+EYp+aJVUWYi43T3F+RmiGuynac2MAWVvenkV+ZO16NGNgVT06 cA/TzR4dH7XHRdM5sEm18dp5/9GHuTa7wWgVJd/0XH3/LtVzyPbMK5Us44M/MqsaMHnA +0WA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=drTHkwxzxB+x1/mNOFORba68gMbMth7YgtywJubwdz0=; b=Xw9SIqBZwU174d2C4Z3QV7o4D9yTbKhaehun5xuFZjG4j37WohZ0e1niGYBbPTBr8U FvSZiivrQ/n1fbe4Dkhs+KZImVfoVGxnIA/HTSAA8Nss7vtsN0WduTl2kaOVabdfcOBp RR+kbDzzTC0Rz2WPd/M5T4ZIMbLcI9k/l+VfETMVRaDqg3LQE2GlVWkJuBI8L6yZzuIp KrdjmByf5q+/82x8KQMiycBe2Rj+xxG44g/VB4cd3H39tVXQNRKIg+Boz+CE1TOqH1O7 LFAwCxRy3pu14rZdUPR52zdfSN7AvGxfk4zTwBWpbgrnFuXYPt8xmLcaLebCzpPAdE0R clbg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=drTHkwxzxB+x1/mNOFORba68gMbMth7YgtywJubwdz0=; b=EZ0lLEpQheSN6ngyZw/xktB62Gg5XVV0FXO/nAV0PH4f4VIT7YPGPFXU0tupFLXsXd BdTYKK35lofuGViE1peno8etGwuu0Im5+gMWzRx4tQOyk98UwNXuhR67QH4gL+X+zic9 8mBa4Iyoprdtw8+GMk0pvTK+yoUMZW7DhgEfM0Gm9K2Oj+nWupbYhMvZtdtp/KZt1Eb2 kMqvt6+lm/SXkEjD70BRmvUugAlRl1XeiHvPtH7VH2e6QgCbmTlbiPFpOB7xtZEFUlZ6 NluwgrWH8M/4U7DN162vC1OgDYaXuRwIb9wYxwwAzMAEs9f+NE+kZZWeuQO2lygHEuq0 8Tiw== X-Gm-Message-State: AIkVDXKBzt0YVTEBwupPKmf0/RwoAaPpv1HwvoRzlb16nt+V4nMVQeRXYmRRbIIKDJFbRA== X-Received: by 10.99.171.67 with SMTP id k3mr34414480pgp.46.1483473587296; Tue, 03 Jan 2017 11:59:47 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.11.115 with SMTP id p48ls18816613otd.19.gmail; Tue, 03 Jan 2017 11:59:46 -0800 (PST) X-Received: by 10.157.35.89 with SMTP id k25mr2147444otd.11.1483473586558; Tue, 03 Jan 2017 11:59:46 -0800 (PST) In-Reply-To: <2225413.PkzHcSPZVx@tjmaciei-mobl1> X-Original-Sender: jmckesson@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:30195 Archived-At: ------=_Part_2653_46906257.1483473586093 Content-Type: multipart/alternative; boundary="----=_Part_2654_514702518.1483473586094" ------=_Part_2654_514702518.1483473586094 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Tuesday, January 3, 2017 at 2:09:19 PM UTC-5, Thiago Macieira wrote: > > On ter=C3=A7a-feira, 3 de janeiro de 2017 18:53:02 BRST Vicente J. Botet= =20 > Escriba=20 > wrote:=20 > > Le 03/01/2017 =C3=A0 16:56, Thiago Macieira a =C3=A9crit :=20 > > > On ter=C3=A7a-feira, 3 de janeiro de 2017 07:06:10 BRST m.ce...@gmail= ..com=20 > =20 > > > wrote:=20 > > >=20 > > > Also, where did that optional come from? If it's going to use=20 > optional,=20 > > > then it needs to be a library feature.=20 > >=20 > > I'm not for the optional return type, but we have already a lot of case= s=20 > > where syntactic sugar as range-based for loop and structure binding=20 > > depends on library.=20 > > So I'm not sure this could be an argument against optional.=20 > > Those are quite different things. In fact, they show how to *not* depend= =20 > on=20 > the library in the first place. Ranged fors and structured bindings only= =20 > require that functions called begin, end, and get exist. So I can add any= =20 > function of my own with those names and it will work. And structured=20 > bindings=20 > are very specifically *not* a std::tuple. They're something different.=20 > > So enum_cast could return an optional-like opaque type defined by the=20 > compiler=20 > (like structured bindings and lambdas) which can be converted to=20 > std::optional=20 > on construction. > First, a correction. Structured binding does not deal in an "opaque type=20 defined by the compiler". It deals in exactly whatever type the=20 initializing expression resolves to. So long as the type has the specified= =20 characteristics or provides the interface required, it will work. But the= =20 compiler is not inventing some type behind-the-scenes. Further, I think your analogy is rather wrongheaded. Ranges and product=20 types are categories of types provided by the user. The language is=20 defining mechanisms for the user to provide a type which can be consumed by= =20 the language. `enum_cast` is not like that at all. It *returns* a type,=20 based on basic values provided by the user. For the optional-version of it,= =20 that type would have all of the properties of `optional`. Lastly, having `enum_cast` work via some compiler-defined type is rather=20 extreme for something as simplistic as this, don't you think? There are=20 only so many ways to implement an "optional", and you can define implicit= =20 conversions from `std::optional` type to yours with little penalty. The=20 conversion can even be `constexpr`, just like `std::optional`, so you would= =20 suffer no loss of performance in constexpr contexts. --=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/7d89ec43-c691-44d8-bcb5-74dcda9eb493%40isocpp.or= g. ------=_Part_2654_514702518.1483473586094 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Tuesday, January 3, 2017 at 2:09:19 PM UTC-5, T= hiago Macieira wrote:
On ter=C3= =A7a-feira, 3 de janeiro de 2017 18:53:02 BRST Vicente J. Botet Escriba=20
wrote:
> Le 03/01/2017 =C3=A0 16:56, Thiago Macieira a =C3=A9crit :
> > On ter=C3=A7a-feira, 3 de janeiro de 2017 07:06:10 BRST m.ce...@gm= ail.com
> > wrote:
> >=20
> > Also, where did that optional come from? If it's going to= use optional,
> > then it needs to be a library feature.
>=20
> I'm not for the optional return type, but we have already a lo= t of cases
> where syntactic sugar as range-based for loop and structure bindin= g
> depends on library.
> So I'm not sure this could be an argument against optional.

Those are quite different things. In fact, they show how to *not* depen= d on=20
the library in the first place. Ranged fors and structured bindings onl= y=20
require that functions called begin, end, and get exist. So I can add a= ny=20
function of my own with those names and it will work. And structured bi= ndings=20
are very specifically *not* a std::tuple. They're something differe= nt.

So enum_cast could return an optional-like opaque type defined by the c= ompiler=20
(like structured bindings and lambdas) which can be converted to std::o= ptional=20
on construction.

First, a correction. Structur= ed binding does not deal in an "opaque type defined by the compiler&qu= ot;. It deals in exactly whatever type the initializing expression resolves= to. So long as the type has the specified characteristics or provides the = interface required, it will work. But the compiler is not inventing some ty= pe behind-the-scenes.

Further, I think your analogy is rather wrongh= eaded. Ranges and product types are categories of types provided by the use= r. The language is defining mechanisms for the user to provide a type which= can be consumed by the language. `enum_cast` is not like that at all. It <= i>returns a type, based on basic values provided by the user. For the o= ptional-version of it, that type would have all of the properties of `optio= nal`.

Lastly, having `enum_cast` work via some compiler-defined type= is rather extreme for something as simplistic as this, don't you think= ? There are only so many ways to implement an "optional", and you= can define implicit conversions from `std::optional` type to yours with li= ttle penalty. The conversion can even be `constexpr`, just like `std::optio= nal`, so you would suffer no loss of performance in constexpr contexts.
=

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/7d89ec43-c691-44d8-bcb5-74dcda9eb493= %40isocpp.org.
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