220 31826 <20a47769-a0c4-4f14-97a7-7b506c2dba8e@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: This variable should not be named: an identifier (not) to remember Date: Tue, 28 Mar 2017 10:18:42 -0700 (PDT) Lines: 123 Approved: news@gmane.org Message-ID: <20a47769-a0c4-4f14-97a7-7b506c2dba8e@isocpp.org> References: <985b9b2a-c734-45eb-95f4-db4dc0d309a1@isocpp.org> <14211602.1P4LjlnQMh@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_14103_1962243281.1490721522457" X-Trace: blaine.gmane.org 1490721528 21861 195.159.176.226 (28 Mar 2017 17:18:48 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 28 Mar 2017 17:18:48 +0000 (UTC) Cc: zy@miator.net To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBB45V5LDAKGQEHPIP3TY@isocpp.org Tue Mar 28 19:18:44 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f69.google.com ([209.85.214.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1csulS-0004ds-6a for gclcip-std-proposals@m.gmane.org; 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Tue, 28 Mar 2017 10:18:43 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.42.168 with SMTP id e37ls10243617otb.3.gmail; Tue, 28 Mar 2017 10:18:42 -0700 (PDT) X-Received: by 10.157.45.133 with SMTP id g5mr377664otb.4.1490721522870; Tue, 28 Mar 2017 10:18:42 -0700 (PDT) In-Reply-To: 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:31826 Archived-At: ------=_Part_14103_1962243281.1490721522457 Content-Type: multipart/alternative; boundary="----=_Part_14104_1650800070.1490721522457" ------=_Part_14104_1650800070.1490721522457 Content-Type: text/plain; charset=UTF-8 On Tuesday, March 28, 2017 at 12:53:55 PM UTC-4, Zhihao Yuan wrote: > > > On Tue, Mar 28, 2017 at 11:43 AM, Thiago Macieira > wrote: > >> just like std::ignore, I don't expect __ here >> > to issue a call to get(obj), nor an access to a subobject. >> > I don't think the *preserving* semantics can fulfill the >> > *ignoring* demand. >> >> It has no difference, because the structured binding is actually a >> *binding*, >> so there's difference to object creation or retention. The result from >> get_tuple is assigned to an anonymous object and all its members are >> retained. >> Then it's decomposed into two elements: an anonymous one (can't be >> accessed) >> and a. >> > > Whether a call to get<0>(anon) being made > certainly has difference, because it can have > observable effect. And, just like I showed > in a previous discussion, the purpose of the > anonymous object is for decomposition. If > nothing is decomposition it, it needs not to be > created, therefore > > auto [__, __] = get_tuple(...); > > should be physically equivalent to > > get_tuple(...); > > thus, the result object is dropped on the floor. > No, it should not. It should create a hidden variable, initialize it with the return value of the expression, then extract nothing from it. The hidden variable will *always* be destroyed at the end of scope. The behavior of `auto [name, __]` should not be substantially different from that of `auto [__, __]`. What you're saying would violate that. -- 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/20a47769-a0c4-4f14-97a7-7b506c2dba8e%40isocpp.org. ------=_Part_14104_1650800070.1490721522457 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Tuesday, March 28, 2017 at 12:53:55 PM UTC-4, Z= hihao Yuan wrote:

On Tue, Mar 28, 2017 at 11:43 AM, T= hiago Macieira <thi...@macieira.org> wrote:
just like std::ignore, I don't e= xpect __ here
> to issue a call to get(obj), nor an access to a subobject.
> I don't think the *preserving* semantics can fulfill the
> *ignoring* demand.

It has no difference, because the structured binding is actually a *= binding*,
so there's difference to object creation or retention. The result from<= br> get_tuple is assigned to an anonymous object and all its members are retain= ed.
Then it's decomposed into two elements: an anonymous one (can't be = accessed)
and a.

Whether a call to get<0&g= t;(anon) being made
certainly has difference, because it can have
obs= ervable effect.=C2=A0 And, just like I showed
in a previous d= iscussion, the purpose of the
anonymous object is for decomposition.=C2= =A0 If
nothing is decomposition it, it needs not to be
created, there= fore

=C2=A0 auto [__, __] =3D get_tuple(...);

should be physically equivalent to

=C2=A0 get_tupl= e(...);

thus, the result object is dropped on the floor.<= br clear=3D"all">

No, it should not. It sh= ould create a hidden variable, initialize it with the return value of the e= xpression, then extract nothing from it. The hidden variable will always= be destroyed at the end of scope.

The behavior of `auto [name, = __]` should not be substantially different from that of `auto [__, __]`. Wh= at you're saying would violate that.

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
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/20a47769-a0c4-4f14-97a7-7b506c2dba8e= %40isocpp.org.
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