220 30516 <84cb007b-d3c2-4204-be97-27f7f0df0e7d@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: stefan.pantos@torstonetech.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Named constructors Date: Thu, 12 Jan 2017 05:07:35 -0800 (PST) Lines: 584 Approved: news@gmane.org Message-ID: <84cb007b-d3c2-4204-be97-27f7f0df0e7d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_152_744654487.1484226455130" X-Trace: blaine.gmane.org 1484226465 22742 195.159.176.226 (12 Jan 2017 13:07:45 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 12 Jan 2017 13:07:45 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCE7F7MVYEPRBGH73XBQKGQEYWP5TAQ@isocpp.org Thu Jan 12 14:07:35 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f198.google.com ([209.85.192.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cRf6K-0004yf-MS for gclcip-std-proposals@m.gmane.org; Thu, 12 Jan 2017 14:07:33 +0100 Original-Received: by mail-pf0-f198.google.com with SMTP id z128sf49428907pfb.4 for ; Thu, 12 Jan 2017 05:07:37 -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: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=noggB+rXIRLWJGK/ppdG7SY3Ilaa2QaLPgRpP4ybs9U=; b=Z6alh5cCZzDAcjqC4HjvnsKW6OVIsGf3VxvA8ciG7sSB0upHPFptfFDzs9ka++a0nj JZBrrNmiLOvpgbeFBIhKHwqMx0yGz0uBxQeceVs0gB3ZjKZo1Ld2EYWmnB6mOlXmzIwl E/5TqTYYUnass3Z6Sysop0kKS9SnSDz9E8/EHAGlFB7KEIn5jFaEaZT4Goq/74D05opz tnY8RwdlgZV8Xog53N4ZaoBaBnA2Udlr+P1nYkFI2wfDiCbCCVhWchZRDV7alCn8sDyZ 5buIZSRRy1llxUyVMxNQkhFeaYg3n84AxnVqNrMpW1PXlXXT8PGxM3ypdyXWdrKQ4l4/ S3sA== 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: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=noggB+rXIRLWJGK/ppdG7SY3Ilaa2QaLPgRpP4ybs9U=; b=C3jTtqvIMMXuU5UKcrNz/Y4mMTTzh4EM+Y1NXGYT8dTRsCZALfu2uOI6IYTEQ7NzAI LEO3FCS9fFlwrVWW4R+gxHmu3siQUcXzGbTmua5Piv6uPVdS1h74ntvZAPCH6XDuGZ3C XbsSsKUhqy28l9nyPefpr8W1/+o7udH77HD3U3Rs3KI/7B9lK0wxe35OB4W+7iRXGr2e AQQQoRYfNPhZA8UYzSUB8uByS2WFzMzLQ7Ap7587Q8nGNqOlXUPGGdyH7JFAZsnLJ/yH ZxPH13G2dUi/doNBEIH9d6WHh4qhS/H7NGruZQuqpfyvrcheLdkTOcd++SSUTK0C3fkx Antg== X-Gm-Message-State: AIkVDXLXmcUMOQdz0xW/A+Mky6Bmuo9l2B0j4om76h8jMGoRwOkggLRMaVnVSzyTzgw3lA== X-Received: by 10.99.99.65 with SMTP id x62mr5077952pgb.58.1484226456826; Thu, 12 Jan 2017 05:07:36 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.51.56 with SMTP id f53ls3375239otc.46.gmail; Thu, 12 Jan 2017 05:07:36 -0800 (PST) X-Received: by 10.157.35.89 with SMTP id k25mr1287892otd.11.1484226455961; Thu, 12 Jan 2017 05:07:35 -0800 (PST) X-Original-Sender: stefan.pantos@torstonetech.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:30516 Archived-At: ------=_Part_152_744654487.1484226455130 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I find my self regularly creating static factory functions on classes to=20 better describe how something is being constructed or to avoid having flags= =20 passed which may have some invalid permutations. Now some times this is=20 because of poor design(Lazy/efficient) but it does make me wonder if=20 constructers with names different to their class would be helpful. Take this contrived example: class Temperature { public: Temperature(float k); Temperature(float c, bool celsiusOrFahrenheit); //An abomination float inKelvin() const; float inCelsius() const; float inFahrenheit() const; private: float kelvin; }; auto zero =3D Temperature(0); auto zeroC =3D Temperature(=E2=88=92273.15, true); auto zeroF =3D Temperature(=E2=88=92459.67, false); class Temperature { public: struct F{}; struct C{}; struct K{}; Temperature(float k, K); Temperature(float c, C); Temperature(float f, F); float inKelvin() const; float inCelsius() const; float inFahrenheit() const; private: float kelvin; }; auto zero =3D Temperature(0, Temperature::K()); auto zeroC =3D Temperature(-273.15, Temperature::C()); auto zeroF =3D Temperature(=E2=88=92459.67, Temperature::F()); class Temperature { public: static Temperature fromKelvin(float k); static Temperature fromCelsius(float c); static Temperature fromFahrenheight(float f); float inKelvin() const; float inCelsius() const; float inFahrenheit() const; private: float kelvin; }; auto zero =3D Temperature::fromKelvin(0); auto zeroC =3D Temperature::fromCelsius(-273.15); auto zeroF =3D Temperature::fromFahrenheight(=E2=88=92459.67); First. I think most people wouldn't like the first. Second. Helpful for meta programming and I quite like it in general but=20 this becomes more complex the more arguments involved. (wish I came up with= =20 a better example now.). Third. You cannot construct the temperature on the heap. But is as clear as= =20 the second.=20 Would it be nice to do something like: class Temperature { public: constructor fromKelvin(float k); constructor fromCelsius(float c); constructor fromFahrenheight(float f); float inKelvin() const; float inCelsius() const; float inFahrenheit() const; private: float kelvin; }; auto zero_ptr =3D new Temperature::fromKelvin(0); auto zeroC_ptr =3D new Temperature::fromCelsius(-273.15); auto zeroF_ptr =3D new Temperature::fromFahrenheight(=E2=88=92459.67); Please don't hold this against me but it does have some similarities to=20 Objective-C's init methods or other languages where the argument names are= =20 used to select the call. Thanks, Stefan Pantos --=20 The contents (including attachments) of this email are strictly=20 confidential, intended for the addressee(s) only and may be legally=20 privileged. If you are not the intended recipient, please contact the=20 sender immediately and delete all copies of this email without distributing= =20 it further in any way. Torstone Technology Limited, its affiliates and=20 staff do not accept any liability for the contents of the message, and the= =20 sending of this message is not intended to form a contract with the=20 recipient. Torstone Technology Limited is registered in England and Wales= =20 with company number 07490275 and registered office at 8 Lloyd's Avenue,=20 London EC3N 3EL, United Kingdom. --=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/84cb007b-d3c2-4204-be97-27f7f0df0e7d%40isocpp.or= g. ------=_Part_152_744654487.1484226455130 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I find my self regularly creating static factory functions= on classes to better describe how something is being constructed or to avo= id having flags passed which may have some invalid permutations. Now some t= imes this is because of poor design(Lazy/efficient) but it does make me won= der if constructers with names different to their class would be helpful.
Take this contrived example:
class = Temperatur= e
{
=C2=A0 =C2=A0= public:
=C2=A0 =C2=A0 =C2=A0 =C2= =A0 Temper= ature(float k);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Temperature(float c, bool celsiusOrFahrenheit); //An abomination

=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inKelvin() const;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inCelsius() const;
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 float inFahrenheit() cons= t;<= span style=3D"color: #000;" class=3D"styled-by-prettify">
=C2=A0 =C2=A0<= /span>private:
=C2=A0 =C2=A0 =C2= =A0 =C2=A0= float kelv= in;=
};

auto zero =3D Temperature(0)= ;
auto zeroC =3D Temperature(=E2=88=92273.15, true);
auto<= /span> zeroF =3D Temperature(=E2=88=92459.67, false);

class Tem= perature{=
=C2=A0 =C2=A0= public:
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 struct F<= /span>{};<= span style=3D"color: #000;" class=3D"styled-by-prettify">
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 struct= C{};
=C2=A0 =C2= =A0 =C2=A0 =C2=A0 struct K{};<= /span>

=C2= =A0 =C2=A0 =C2=A0 =C2=A0 Temperature(float k, K);
=C2=A0 =C2=A0 =C2=A0 =C2=A0= Temperatu= re(= float c, C);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Temperature(float f, F);

=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inKelvin() const;
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 float inCelsius() const;
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 float inF= ahrenheit(= ) <= span style=3D"color: #008;" class=3D"styled-by-prettify">const;
=C2=A0 =C2=A0private:
=C2=A0 =C2=A0 =C2=A0 =C2=A0float kelvin;
};


auto zero =3D Temperature(0, Temperature::K());
auto zeroC =3D Tempera= ture(-273.15<= span style=3D"color: #660;" class=3D"styled-by-prettify">, Temperature::C());
auto zeroF =3D Temperature(=E2=88=92459.67, Temperature::F());

class Temperature
{<= /span>
=C2=A0 = =C2=A0publ= ic:=
=C2=A0 =C2=A0= =C2=A0 =C2=A0 static Temperat= ure fromKe= lvin(float k);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 static<= span style=3D"color: #000;" class=3D"styled-by-prettify"> Temperature fromCelsius(float c);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 static Temperature fromFahrenheight(float f);

=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inKelvin() const;=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inCelsius() con= st;=
=C2=A0 =C2=A0= =C2=A0 =C2=A0 float= inFahrenheit() const;
=C2=A0 =C2=A0private:
=C2=A0 =C2=A0 =C2=A0 =C2=A0float= kelvin;
};


auto zero =3D Temperature::fromKelvin(0);
auto zeroC =3D Temperature::fromCelsius(-273.15);auto zeroF = =3D Temperature::fromFahrenheight(=E2=88=92459.67);


First. I think most people wouldn't like the first.

Second. Helpful for meta programming and I quite like it = in general but this becomes more complex the more arguments involved. (wish= I came up with a better example now.).

Third. You= cannot construct the temperature on the heap. But is as clear as the secon= d.=C2=A0

Would it be nice to do something like:
class Temperature
{
=C2=A0 =C2=A0public:
=C2=A0 =C2=A0 =C2=A0 =C2=A0 constructor fromKelvin(float k);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 constructor fromCelsius(float c);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 constructor= fromFahrenheight(fl= oat f);

=C2=A0 =C2=A0 = =C2=A0 =C2=A0 float = inKelvin()= const;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <= /span>float inCelsius() const;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inFahrenheit() const;
=C2=A0 =C2=A0private:
=C2=A0 =C2=A0 =C2=A0 =C2=A0float kelvin;
};


auto zero_ptr =3D new Temperature::fromKelvin(0);
auto zeroC_ptr =3D new Temperature::fromCelsius(-273.15);
auto zeroF_ptr =3D new Temperature::fromFahrenheight(=E2=88=92459.67);

=
Please don't hold this against me but it does have som= e similarities to Objective-C's init methods or other languages where t= he argument names are used to select the call.

Tha= nks,
Stefan Pantos

The contents (including attachments) of this email are strictly confidential, intended f= or the addressee(s) only and may be legally privileged.=C2=A0 If you are not t= he intended recipient, please contact the sender immediately and delete all copies of t= his email without distributing it further in any way.=C2=A0 Torstone Technology Limited, its affiliates and staff do not accept any liability for the conte= nts of the message, and the sending of this message is not intended to form a contract with the recipient. =C2=A0Torstone Technology Limited is registere= d in England and Wales with company number 07490275 and registered office a= t 8 Lloyd's Avenue, London EC3N 3EL, United Kingdom.

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