220 30523 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Named constructors Date: Thu, 12 Jan 2017 07:23:54 -0800 (PST) Lines: 345 Approved: news@gmane.org Message-ID: References: <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/mixed; boundary="----=_Part_730_1838115120.1484234634883" X-Trace: blaine.gmane.org 1484234645 6142 195.159.176.226 (12 Jan 2017 15:24:05 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 12 Jan 2017 15:24:05 +0000 (UTC) Cc: stefan.pantos@torstonetech.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBDF733BQKGQEC35573Y@isocpp.org Thu Jan 12 16:24:01 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f199.google.com ([209.85.213.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cRhEG-0000PD-8R for gclcip-std-proposals@m.gmane.org; Thu, 12 Jan 2017 16:23:52 +0100 Original-Received: by mail-yb0-f199.google.com with SMTP id o65sf11301654yba.3 for ; Thu, 12 Jan 2017 07:23:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references: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=XjjIB8ukPavIxijo9OHWtdOXIOmu6YUB7kCZzT66Fdc=; b=OCRww8NB547IVXu8WRz8q6vgmV2Bq7kBzEBGrKnHsGZN0dX0UDzvgBjNpDilQXVHG+ fKeiNDuuvVv/CjpzhdXO1vPkdFydCZEt+7OGY1dVAOIbIHMnaKPbs1pT5gF9bip7APxh lON8s+pxlszjYDWUVCFar+HfXEDAPgLAvDcTUVcpMre9KqCX5q/dwddStJ6cCZ8/Pkcd 1g+xo6dyUfgQVH876BXWdGzjD4X5LUhKMM8cr3rKbgb8m//I/UhONdZ7pvAti4+Qyek5 A6zhEu0GmMDMiDO44gxDMZw+OG2lxgM5SdcqZHfsXhQkC/6a3PNoF4LMBgC8UFKZwhB+ LtuA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references: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=XjjIB8ukPavIxijo9OHWtdOXIOmu6YUB7kCZzT66Fdc=; b=YePGf7SI2sWhxSjc3Ie5uket3HLVgXxcWju1Yu2wD62HbKWc63vfX0SAvW/b4gkqgs /6pm84knnkzwfgT40t6oyUcJKE1yROqCp5I19rjS0YO9zOp5Lc08qIAZhWUniaxwTcet cERiEFal6Ox/ffAjAqkus99hWFva9T7/oTIJrpi3aRUzZ+SJg/tg11hLuxrVxhRj9+TY WIstZS5IToogaqPZ6a9RJGK6tzdhEhOT1c5jHQgxibP4tG0xRbAG8UM+C0l5YN60wKiF EZtdY9uFyUvGCoQfitWQPEY6tVw8YjIci23P3DqB4qTfvV1rQvNfFIJcY0MQK1z0i7Oe /H3A== 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:cc:message-id:in-reply-to :references: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=XjjIB8ukPavIxijo9OHWtdOXIOmu6YUB7kCZzT66Fdc=; b=st7/IAndVhG/VodJ9CR5b4l8iUpuyUjuqQ2sBKCjFsBQHCnv4lRpopg6unYlprULXp 2Nd8YIGLgcuyMJYSbKjqs7od+rRk9Gi2E/8IwbZuqJ/rhMQC5fmeAsXKWiMqXgbrV7CU OBgjwQUWiRob6/R4fPbkAAYYGcMoAGPeRt2xORISqroVmEmwuRun69fyuv+bYU8CQIUm VSsDdMQ6YQV1Qyg4Zd1hSawW+sXzulpEcbifSNZjXkfskUw2LT9KdX0+WOvApvTgA6Wo uy6kipg17oAXwn1iPCmgxf8QUN2Xz3hdP5SX6c31NfgI2cp9ZUaKHMIXhItDCz+yuuFl Vmqw== X-Gm-Message-State: AIkVDXKCUrfYKlfFkbicBl9DmM6FFeYlb9ynxprD6jeG01TiURLrA4yG6aepEXmwjcMtbQ== X-Received: by 10.129.98.194 with SMTP id w185mr4172931ywb.72.1484234636721; Thu, 12 Jan 2017 07:23:56 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.41.177 with SMTP id n46ls6515976otb.37.gmail; Thu, 12 Jan 2017 07:23:55 -0800 (PST) X-Received: by 10.157.61.131 with SMTP id l3mr1365096otc.18.1484234635919; Thu, 12 Jan 2017 07:23:55 -0800 (PST) In-Reply-To: <84cb007b-d3c2-4204-be97-27f7f0df0e7d@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:30523 Archived-At: ------=_Part_730_1838115120.1484234634883 Content-Type: multipart/alternative; boundary="----=_Part_731_557467787.1484234634884" ------=_Part_731_557467787.1484234634884 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Thursday, January 12, 2017 at 8:07:35 AM UTC-5,=20 stefan...@torstonetech.com wrote: > > I find my self regularly creating static factory functions on classes to= =20 > better describe how something is being constructed or to avoid having fla= gs=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 wi= th=20 > a better example now.). > > Third. You cannot construct the temperature on the heap. But is as clear= =20 > as the second. > Watch me: new auto(Temperature::fromCelsius(-273.15)); Oh look, it's constructed on the heap now. The system allocates memory,=20 passes it as the return value to `fromCelsius`, who calls the right=20 constructor of `Temperature` in a prvalue that gets returned. No copying.= =20 No moving. Behold the wonders of guaranteed elision ;) --=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/fec0c273-1b76-4ff7-a323-6d2e4a0d5233%40isocpp.or= g. ------=_Part_731_557467787.1484234634884 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Thursday, January 12, 2017 at 8:07:35 AM UTC-5,= stefan...@torstonetech.com wrote:
I find my self regularly creating static factory fun= ctions on classes to better describe how something is being constructed or = to avoid having flags passed which may have some invalid permutations. Now = some times this is because of poor design(Lazy/efficient) but it does make = me wonder if constructers with names different to their class would be help= ful.

Take this contrived example:
class Temperature
{
=C2=A0 =C2=A0publi= c:
= =C2=A0 =C2=A0 =C2=A0 =C2=A0 Temperature(float k);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Temperature(float c, bool celsiusOrFahrenhe= it); //An abomination

=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inKelvin() const;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 float= inCelsius() c= onst;=C2=A0 =C2=A0 =C2=A0 =C2=A0 float inFahrenheit= () cons= t;
= =C2=A0 =C2=A0private:
=C2=A0 =C2=A0 =C2=A0 = =C2=A0float kelvin;
};

auto zero =3D Temperature(0);
auto zeroC =3D Temperature(=E2=88=92= 273.15,= true);
auto zeroF =3D Temperature(=E2= =88=92459.67, = false);=

class Temperature
{
=C2=A0 =C2=A0public:
=C2=A0 =C2= =A0 =C2=A0 =C2=A0 struct F{};
=C2=A0 =C2=A0 =C2=A0 =C2=A0 struct C{};
=C2=A0 =C2=A0 =C2=A0 =C2= =A0 struct K{};

=C2=A0 =C2=A0 =C2=A0 =C2=A0 Temp= erature(float k, K);<= /span>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Temperature(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 inFahrenheit() const;
=C2=A0 =C2=A0private:
=C2=A0 =C2= =A0 =C2=A0 =C2=A0float kelvin;
};


auto zero =3D= Temperature(0= , Temperature::K());<= span style=3D"color:#000">
auto= zeroC =3D Temperatur= e(-273.= 15, Temperature::C())= ;
au= to zeroF =3D T= emperature(=E2=88=92459.67, Temperature::F());

class Temperature
{
=C2=A0 = =C2=A0public:
=C2=A0 =C2=A0 =C2=A0 =C2=A0 static Temperature fromKelvin(float k);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <= /span>static <= /span>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() const;
=C2=A0 =C2=A0 =C2=A0= =C2=A0 float inFahrenheit() const;
=C2=A0 =C2=A0= private:
=C2=A0 =C2=A0 =C2=A0 =C2=A0float kelvin;
};


auto zero =3D Temperature::= fromKelvin(0);=
aut= o zeroC =3D Te= mperature::fromCelsius(-273.15);
auto zeroF =3D Temperature::fromFahrenheig= ht(=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 thi= s 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 second.

Watch me:

new auto(Temp= erature::<= /span>fromCelsius= (-<= span style=3D"color: #066;" class=3D"styled-by-prettify">273.15));

Oh look, it's constructed on the heap now. The system alloca= tes memory, passes it as the return value to `fromCelsius`, who calls the r= ight constructor of `Temperature` in a prvalue that gets returned. No copyi= ng. No moving.

Behold the wonders of guaranteed elision ;)

--
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/fec0c273-1b76-4ff7-a323-6d2e4a0d5233= %40isocpp.org.
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