220 28275 <0e27bc76-eada-4d70-80d7-b91b5a2d5767@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Something is better then nothing: Please, relax Initializer List use with operators! Date: Sun, 18 Sep 2016 06:08:08 -0700 (PDT) Lines: 189 Approved: news@gmane.org Message-ID: <0e27bc76-eada-4d70-80d7-b91b5a2d5767@isocpp.org> References: <0892f743-bca2-4f18-b023-d4a57cea60fb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_411_1138377791.1474204088556" X-Trace: blaine.gmane.org 1474204103 9822 195.159.176.226 (18 Sep 2016 13:08:23 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 18 Sep 2016 13:08:23 +0000 (UTC) Cc: mihailnajdenov@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBOND7K7AKGQE2OI3RAI@isocpp.org Sun Sep 18 15:08:19 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f197.google.com ([209.85.223.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1blbpJ-0000yL-9b for gclcip-std-proposals@m.gmane.org; Sun, 18 Sep 2016 15:08:09 +0200 Original-Received: by mail-io0-f197.google.com with SMTP id q92sf204623879ioi.3 for ; 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Sun, 18 Sep 2016 06:08:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.48.1 with SMTP id w1ls2847533iow.4.gmail; Sun, 18 Sep 2016 06:08:09 -0700 (PDT) X-Received: by 10.36.103.4 with SMTP id u4mr180404itc.9.1474204089359; Sun, 18 Sep 2016 06:08:09 -0700 (PDT) In-Reply-To: <0892f743-bca2-4f18-b023-d4a57cea60fb@isocpp.org> 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:28275 Archived-At: ------=_Part_411_1138377791.1474204088556 Content-Type: multipart/alternative; boundary="----=_Part_412_1619274301.1474204088557" ------=_Part_412_1619274301.1474204088557 Content-Type: text/plain; charset=UTF-8 On Sunday, September 18, 2016 at 8:41:39 AM UTC-4, mihailn...@gmail.com wrote: > > Hello, in a recent search about why is Initializer List limited so much > (cant be used in arithmetics, assignment, etc), I came to this post : > http://stackoverflow.com/a/11445905/362515 > > Basically the problem is, Initializer List is impossible to work in any > situation where operators are used - parsing such an expression will be to > complex. > > That is fine. > > However support in limited, but well defined scenarios is possible and I > believe the benefits will still outweigh the confusion why it is not > working in some contexts. > What is more *there is already a confusion why it is not working in > "similar contexts"!* > > For example *from the user perspective assignment and comparison are > "similar" contexts of use - one is "are you that thing" the other is > "become that thing". * > Needless to say the syntax is, as we know, dangerously close "==" vs "=" > *.* > > If for instance is allowed *only the type of the RHS in operator to be > deduced from an init-list* and expression is dissuaded to start with a > init-list, not only many if not all use cases will be possible, *but it > will be somewhat easier to teach why the other case is to available.* > > Back to the example - it is hard to explain why this is working: > Point p; > p = {-1, -1}; > > but this is not: > > if(p == {-1, -1}) return; > > Because one is assignment while the other is equality testing. They're two different things. Why is that hard to explain? > *But this is actually relatively easy to explain why it's not working*: > if({-1, -1} == p) return; > > Um, no. If we allow `X == Y` to work, then it makes sense that we allow `Y == X` to work. To disallow one and not the other is far more bizarre than allowing one operation but forbidding a completely different one. *Bonus:* > This must work, it really must work: > const auto e = car ? car->engine() : {}; > We know it does not but it should, no reason why not - first arg is > given, second arg is given, the the third must the same type as the second, > so we have everything to deduce it! > Again, that goes back to consistency. `!car ? {} : car->engine()` is conceptually the same, yet it wouldn't work under your scheme. -- 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/0e27bc76-eada-4d70-80d7-b91b5a2d5767%40isocpp.org. ------=_Part_412_1619274301.1474204088557 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Sunday, September 18, 2016 at 8:41:39 AM UTC-4, mihailn= ....@gmail.com wrote:
Hello, in a recent search about why is Initializer List limited so= much (cant be used in arithmetics, assignment, etc), I came to this post := =C2=A0http://stackoverflow.= com/a/11445905/362515

Basically the problem is,= Initializer List is impossible to work in any situation where operators ar= e used - parsing such an expression will be to complex.

That is fine.

However support in limited, bu= t well defined scenarios is possible and I believe the benefits will still = outweigh the confusion why it is not working in some contexts.=C2=A0
<= div>What is more there is already a=C2=A0confusion why it is not working= in "similar contexts"!=C2=A0

For example from the user perspective assignment and comparison are &qu= ot;similar" contexts of use - one is "are you that thing" th= e other is "become that thing".=C2=A0
Needless to s= ay the syntax is, as we know, dangerously close "=3D=3D" vs "= ;=3D".

If for instance is allow= ed only the type of =C2=A0the RHS in operator to be deduced from an init= -list=C2=A0and expression is dissuaded to start with a init-list, not o= nly many if not all use cases will be possible, but it will be somewhat = easier to teach why the other case is to available.

Back to the example - it is hard to explain why this is working:
Point p;
p =3D = {-1, -1};

but this is not:

if(= p =3D=3D {-1, -1}) return;


Because one is assignment while the other is equality t= esting. They're two different things.

Why is that hard to explai= n?


But this is actually relatively easy t= o explain why it's not working:
if({-1, -1} =3D=3D p) return;


U= m, no. If we allow `X =3D=3D Y` to work, then it makes sense that we allow = `Y =3D=3D X` to work. To disallow one and not the other is far more bizarre= than allowing one operation but forbidding a completely different one.
Bonus:
This must work, it really must work:<= div style=3D"background-color:rgb(250,250,250);border-color:rgb(187,187,187= );border-style:solid;border-width:1px;word-wrap:break-word">
const auto e =3D car ? car->engine() : {};
=C2=A0We know it does not but it should, no reason why not - first arg is = given, second arg is given, the the third must the same type as the second,= so we have everything to deduce it!

Again= , that goes back to consistency. `!car ? {} : car->engine()` is conceptu= ally the same, yet it wouldn't work under your scheme.

--
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