220 32508 <1c728d4e-7f8e-4528-23a7-5a9ba198ebbc@honermann.net> article Path: news.gmane.org!.POSTED!not-for-mail From: Tom Honermann Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allow Structured Binding Outside of Variable Declaration Date: Tue, 16 May 2017 19:43:21 -0400 Lines: 186 Approved: news@gmane.org Message-ID: <1c728d4e-7f8e-4528-23a7-5a9ba198ebbc@honermann.net> References: <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> <20170516164245.GA16348@fukushima.lysator.liu.se> <586c2633-4586-12d8-35ae-75ebab176ff8@honermann.net> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------8C013CBA4CC1370326F81341" X-Trace: blaine.gmane.org 1494978206 32670 195.159.176.226 (16 May 2017 23:43:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 16 May 2017 23:43:26 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDBNXSHG6UDBBGU553EAKGQEKFSYHJQ@isocpp.org Wed May 17 01:43:19 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f197.google.com ([209.85.192.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dAm7a-0008Hi-Ro for gclcip-std-proposals@m.gmane.org; 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[108.166.43.83]) by mx.google.com with ESMTPS id q144si331743iod.126.2017.05.16.16.43.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 May 2017 16:43:22 -0700 (PDT) Received-SPF: neutral (google.com: 108.166.43.83 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=108.166.43.83; Original-Received: from smtp11.relay.ord1c.emailsrvr.com (localhost [127.0.0.1]) by smtp11.relay.ord1c.emailsrvr.com (SMTP Server) with ESMTP id 388C3A00CC for ; Tue, 16 May 2017 19:43:22 -0400 (EDT) X-Auth-ID: tom@honermann.net Original-Received: by smtp11.relay.ord1c.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id 065ECA01A1 for ; Tue, 16 May 2017 19:43:21 -0400 (EDT) X-Sender-Id: tom@honermann.net Original-Received: from [192.168.1.7] (pool-173-53-30-126.rcmdva.fios.verizon.net [173.53.30.126]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA) by 0.0.0.0:587 (trex/5.7.12); Tue, 16 May 2017 19:43:22 -0400 In-Reply-To: X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 108.166.43.83 is neither permitted nor denied by best guess record for domain of tom@honermann.net) smtp.mailfrom=tom@honermann.net 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:32508 Archived-At: This is a multi-part message in MIME format. --------------8C013CBA4CC1370326F81341 Content-Type: text/plain; charset="UTF-8"; format=flowed On 05/16/2017 06:41 PM, Arthur O'Dwyer wrote: > On Tue, May 16, 2017 at 2:53 PM, Tom Honermann > wrote: > > On 05/16/2017 05:12 PM, Bengt Gustafsson wrote: >> Why not let a variable declaration inside a structured binding be >> a variable declaration? > I eluded to this being a sound design in an earlier reply. Is > there good motivation for being able to declare a variable rather > than a binding? I find the current design highly motivating > specifically because a binding is effectively an alias. > > > Speaking of "being able to declare a variable instead of a binding", > this morning I ran across this example due to Tony van Eerd: > > struct Foo { > int x = 0; > std::string str = "world"; > ~Foo() { std::cout << str; } > }; > > int main() { > auto [i, s] = Foo(); > std::cout << "hello "; > s = "structured bindings"; > // Foo's destructor prints "hello structured bindings"! > } > > ...which is the nail in the coffin of "ever using C++17 structured > bindings in production code" AFAIC. Do you find the fact that the following code exhibits the same behavior surprising? int main() { auto foo = Foo(); std::cout << "hello "; foo.s = "structured bindings"; // Foo's destructor prints "hello structured bindings"! } Tom. -- 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/1c728d4e-7f8e-4528-23a7-5a9ba198ebbc%40honermann.net. --------------8C013CBA4CC1370326F81341 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On 05/16/2017 06:41 PM, Arthur O'Dwyer wrote:
On Tue, May 16, 2017 at 2:53 PM, Tom Honermann <tom@honerma= nn.net> wrote:
O= n 05/16/2017 05:12 PM, Bengt Gustafsson wrote:
Why not let a variable declaration inside a structured binding be a variable declaration?
 I eluded to this being a sound design in an earlier reply.=C2=A0 Is there good motivation for being abl= e to declare a variable rather than a binding?=C2=A0 I find t= he current design highly motivating specifically because a binding is effectively an alias.

Speaking of "being able to declare a variable instead of a binding", this morning I ran across this example due to Tony van Eerd:

struct Foo {<= /div>
=C2=A0 =C2=A0 int x = =3D 0;
=C2=A0 =C2=A0 std::s= tring str =3D "world";
=C2=A0 =C2=A0 ~Foo()= { std::cout << str; }
};

int main() {<= /div>
=C2=A0 =C2=A0 auto [= i, s] =3D Foo();
=C2=A0 =C2=A0 std::c= out << "hello ";
=C2=A0 =C2=A0 s =3D = "structured bindings";
=C2=A0 =C2=A0 // Foo= 's destructor prints "hello structured bindings"!
}

...which is the nail in the coffin of "ever using C++17 structured bindings in production code" AFAIC.

Do you find the fact that the following code exhibits the same behavior surprising?

int main() {
=C2=A0 =C2=A0 auto foo =3D Foo= ();
=C2=A0 =C2=A0 std::cout <&l= t; "hello ";
=C2=A0 =C2=A0 foo.s =3D "struc= tured bindings";
=C2=A0 =C2=A0 // Foo's destruc= tor prints "hello structured bindings"!
}

Tom.

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