220 18733 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Factory functions as deducing constructors? (N4471) Date: Wed, 24 Jun 2015 17:44:52 +0800 Lines: 124 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2098\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_44A6BC3A-C329-4E6E-B722-B3EC9B0E10E1" X-Trace: ger.gmane.org 1435139113 29399 80.91.229.3 (24 Jun 2015 09:45:13 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 24 Jun 2015 09:45:13 +0000 (UTC) Cc: Richard Smith To: Mike Spertus , std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBIHYVGWAKGQERHCA32Y@isocpp.org Wed Jun 24 11:45:09 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yk0-f200.google.com ([209.85.160.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Z7hEw-0006XY-Dh for gclcip-std-proposals@m.gmane.org; Wed, 24 Jun 2015 11:45:07 +0200 Original-Received: by ykay124 with SMTP id y124sf32207494yka.2 for ; Wed, 24 Jun 2015 02:45:05 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:subject:date:message-id:cc:to :mime-version:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=AmgKPxnePbHD3xIwVRkqJ7zks8WQs04rgfeZdampMTA=; b=X+cTokonqoLvqU7vW6CL/hhvhVUDljbWLLgG5js3SMTBPI4qtV21x/T4rJLTPzI3Md rHL1rFasezzD4vb/c1YIiUvdGheoXMQMjRJ2vJTo1ptgLQ1XT72EscEeDr4IX8eSMkqE wtRxZ2G7nzFyn+N+Th1PKEO6vJEOU3VErk3kNaMnnMM7V5GZA5RS2RdqIRAYRwMv7kag 7bfbMmjYckG5u0bGK3PDaO4SMfzz10m8ksPSM/fwdYAYCMKzgqD+WenXetNiTMCDMrmj OLMxCAgAXB9Uf2crz+2FsU2VEFMQZ0gPKQyJ+U08nWfBLaKimsE0rbUoa6IqSSArSkx1 Y82Q== X-Gm-Message-State: ALoCoQlH3BMQXhs8gx+vfGHwFDCsTV9fxi5CBBw1d0JNyf9jaPGUe6knnl6fnNQQYmzBduMiMENB X-Received: by 10.13.241.68 with SMTP id a65mr10514829ywf.50.1435139105437; Wed, 24 Jun 2015 02:45:05 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.114.73 with SMTP id je9ls477441igb.38.gmail; Wed, 24 Jun 2015 02:45:04 -0700 (PDT) X-Received: by 10.66.66.46 with SMTP id c14mr79046755pat.71.1435139104539; Wed, 24 Jun 2015 02:45:04 -0700 (PDT) Original-Received: from mail-pa0-x22e.google.com (mail-pa0-x22e.google.com. [2607:f8b0:400e:c03::22e]) by mx.google.com with ESMTPS id so4si39045631pbc.76.2015.06.24.02.45.04 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 24 Jun 2015 02:45:04 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22e as permitted sender) client-ip=2607:f8b0:400e:c03::22e; Original-Received: by padev16 with SMTP id ev16so25963136pad.0 for ; Wed, 24 Jun 2015 02:45:04 -0700 (PDT) X-Received: by 10.70.48.68 with SMTP id j4mr67889205pdn.111.1435139104440; Wed, 24 Jun 2015 02:45:04 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.94]) by mx.google.com with ESMTPSA id im7sm26058215pbc.25.2015.06.24.02.44.59 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 24 Jun 2015 02:45:03 -0700 (PDT) X-Mailer: Apple Mail (2.2098) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22e as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:18733 Archived-At: --Apple-Mail=_44A6BC3A-C329-4E6E-B722-B3EC9B0E10E1 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Hi Mike, Most libraries already implement factory function templates, e.g. make_thin= g(). Why not just let them be named e.g. thing(), aliasing the class templa= te? No new deduction rules are needed. It=E2=80=99s a non-breaking change because such aliasing is currently disal= lowed. It differs from established practice only by renaming. The full flexibility of factory functions is preserved, including explicit = template arguments. It does not intrude into the class template implementation. For the sake of member templates, such a function template (or function) sh= ould still be found by a qualified-id preceded by the typename keyword. It = should also be illegal to take its address, or to do anything with its name= or template-id besides call it. The factory should defer to real construct= ors when the explicit template argument list satisfies the class template p= arameters. Require the factory to be declared after the initial declaration of the cla= ss template, and require its return type to be a specialization. Implementation model: Attach the overload set of factory functions to the c= lass template. When name lookup finds the class template, grab the factorie= s too. If template arguments are provided, treat the class template as an o= verload to the factories and run template argument deduction for explicit a= rguments ([temp.deduct]/1-5). If the class template has not suffered a dedu= ction failure, terminate deduction and restart overload resolution using it= s constructors. I=E2=80=99d also like to propose enforced copy elision, such that simple fa= ctory functions don=E2=80=99t require move construction. (Call it =E2=80=9C= perfect return.=E2=80=9D) This seems like a separate feature. However, it w= ould be nice to have both. Long ago, in some early C++ implementations at least, constructors were non= -member factory functions. Hopefully this won=E2=80=99t be seen as =E2=80= =9Ccoming full circle.=E2=80=9D It=E2=80=99s really not, since the factory = has no special abilities or status besides sharing a name. - D --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail=_44A6BC3A-C329-4E6E-B722-B3EC9B0E10E1 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
Hi= Mike,

Most libra= ries already implement factory function templates, e.g. make_thing(). Why not just let them be named e.g.&nbs= p;thing(), aliasing the class temp= late?

  • No new deduction rules are needed.
  • = It=E2=80=99s a non-breaking change because such aliasing is currently disal= lowed.
  • It differs from established practice only by rena= ming.
  • The full flexibility of factory functions is prese= rved, including explicit template arguments.
  • It does not= intrude into the class template implementation.

For the sake of member templates= , such a function template (or function) should still be found by a qualifi= ed-id preceded by the typename key= word. It should also be illegal to take its address, or to do anything with= its name or template-id besides call it. The factory should defer to real = constructors when the explicit template argument list satisfies the class t= emplate parameters.

Require the factory to be declared after the initial declaration of the = class template, and require its return type to be a specialization.

Implementation model: At= tach the overload set of factory functions to the class template. When name= lookup finds the class template, grab the factories too. If template argum= ents are provided, treat the class template as an overload to the factories= and run template argument deduction for explicit arguments ([temp.deduct]/= 1-5). If the class template has not suffered a deduction failure, terminate= deduction and restart overload resolution using its constructors.

I=E2=80=99d also like to = propose enforced copy elision, such that simple factory functions don=E2=80= =99t require move construction. (Call it =E2=80=9Cperfect return.=E2=80=9D)= This seems like a separate feature. However, it would be nice to have both= ..

Long ago, in so= me early C++ implementations at least, constructors were=  non-member factory functions. Hopefully this won=E2=80=99t be see= n as =E2=80=9Ccoming full circle.=E2=80=9D It=E2=80=99s really not, since t= he factory has no special abilities or status besides sharing a name.
=

- D

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