220 19870 <86717349-1fce-4247-9372-08eedce4cd30@isocpp.org> article Path: news.gmane.org!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: default arguments Date: Mon, 17 Aug 2015 15:52:24 -0700 (PDT) Lines: 294 Approved: news@gmane.org Message-ID: <86717349-1fce-4247-9372-08eedce4cd30@isocpp.org> References: <736eb378-52a4-43ba-b3b5-a73719ba70af@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1740_1780373017.1439851944288" X-Trace: ger.gmane.org 1439851948 31387 80.91.229.3 (17 Aug 2015 22:52:28 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 17 Aug 2015 22:52:28 +0000 (UTC) Cc: bop@gmb.dk To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDLZJYWNDQIKTS6JVYCRUBGV33VLU@isocpp.org Tue Aug 18 00:52:27 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f71.google.com ([209.85.220.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZRTGV-00035J-4J for gclcip-std-proposals@m.gmane.org; Tue, 18 Aug 2015 00:52:27 +0200 Original-Received: by paom9 with SMTP id m9sf344974721pao.3 for ; Mon, 17 Aug 2015 15:52:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type: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=CoOp5NhzBCUHez4enT/qRG9+jt56i0HS0s37LV7qZV0=; b=HpxtWnLCnGyu5u5seEfkDhmozj/zNXKa0Vpxs1Fn7PzzZHOm+QTOv2SbMt4H4oxb0Z hSA7/yaewoE9v7T0bOD+SZ892vcpvejVIPOLAy34yEvcvnuhhLnhpKt0HgTIZlRsyZdw JxzpjruKQwX1Zt6z9MEYSnmPY9tfjEXfmxVAFz4QLO1dlvqVGz/ZUIZCGTNw0t+Hko+U rhB4WCOoHhgiR3EWohexcaHsJgBg8Lt089fYKIQe9bapSgQtofK9yJZA38ICpjAAmk82 GcTUmXTRVE5l2SIQIBVjXsoKnn3g8IKDdEkWYP08xJHWedLOdAKwBE30dPfvRZsk31m+ OnKg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type: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=CoOp5NhzBCUHez4enT/qRG9+jt56i0HS0s37LV7qZV0=; b=Iu0B9bwfHSoN7yQM0V+mrNbsfYFnAvqf5sg/UY4PCHPzdcd+XcUbX0KDV2KtbdtbLT n8no3sOjJ1L1MVSD5C7GfSrNxAzWE2GLHrMUjbTFrhkY0WlcBYmnkAdXAH0+h+kMQ+PR BypkHiMXW84u619BWp68bU4jgRJL2izLdRCgjkSyTiLsogw3pkDJarsw0oZR9tvtlVl7 zsngy3t+FaYmZJzCf+gWZH0Z/3zRaqmSyLH2KAcGC5TiDymYPPeBA+uEJsNKmJdCqmMY HoWpcDn3y3cZjZjub+PzUYmFPduk9MUJxkYr1woRNAkuOS++kUbtrK4k+H76tBA4RdCN l0TA== X-Gm-Message-State: ALoCoQlbyRxkmrtaWwM5R33EIW06cTZMII+GbPNVJjYwfInKbpOwQQTPdwrcs/ai00nQNL7eepTd X-Received: by 10.66.237.69 with SMTP id va5mr3170865pac.48.1439851946087; Mon, 17 Aug 2015 15:52:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.39.140 with SMTP id n134ls1899046ion.51.gmail; Mon, 17 Aug 2015 15:52:25 -0700 (PDT) X-Received: by 10.50.137.7 with SMTP id qe7mr285217igb.17.1439851945305; Mon, 17 Aug 2015 15:52:25 -0700 (PDT) In-Reply-To: X-Original-Sender: arthur.j.odwyer@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:19870 Archived-At: ------=_Part_1740_1780373017.1439851944288 Content-Type: multipart/alternative; boundary="----=_Part_1741_413134508.1439851944288" ------=_Part_1741_413134508.1439851944288 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I work on a codebase where this feature would have lots of use-cases,=20 because we have lots of classes whose constructors have defaulted=20 parameters: C(int buffer_size =3D 1024, int timeout =3D 10, int tries =3D 3) :=20 buffer_size_(buffer_size), timeout_(timeout), tries_(tries) {} auto cptr =3D new C(); // usually the defaults are okay auto c2ptr =3D new C(2048); // use a bigger buffer auto c3ptr =3D new C(1024, 1); // use the default buffer size but with= a=20 shorter timeout auto c4ptr =3D new C(2048, 10, 5); // testing: bump up the number of= =20 tries It would be marginally more convenient (and refactor-proof) to be able to= =20 write auto cptr =3D new C(); // usually the defaults are okay auto c2ptr =3D new C(2048); // use a bigger buffer auto c3ptr =3D new C(default, 1); // use the default buffer size but= =20 with a shorter timeout auto c4ptr =3D new C(2048, default, 5); // testing: bump up the number= =20 of tries However, I consider defaulted parameters in general to be a huge=20 antipattern, and this constructor pattern in particular to be even worse. I would MUCH rather just rewrite all these constructors so that they could= =20 be used as auto cptr =3D new C(); // usually the defaults are okay auto c2ptr =3D new C().withBufferSize(2048); // use a bigger buffer auto c3ptr =3D new C().withTimeout(1); // use the default buffer size= =20 but with a shorter timeout auto c4ptr =3D new C().withBufferSize(2048).withTries(5); // testing:= =20 bump up the number of tries or auto cptr =3D new C(); // usually the defaults are okay auto c2ptr =3D new C(C::BufferSize(2048)); // use a bigger buffer auto c3ptr =3D new C(C::Timeout(1)); // use the default buffer size bu= t=20 with a shorter timeout auto c4ptr =3D new C(C::BufferSize(2048), C::Tries(5)); // testing: bu= mp=20 up the number of tries or (the most conservative way to rewrite our existing code) static constexpr int default_buffer_size =3D 1024; static constexpr int default_timeout =3D 10; static constexpr int default_tries =3D 3; C() : buffer_size_(default_buffer_size), timeout_(default_timeout),=20 tries_(default_tries) {} C(int buffer_size, int timeout, int tries) : buffer_size_(buffer_size),= =20 timeout_(timeout), tries_(tries) {} auto cptr =3D new C(); // usually the defaults are okay auto c2ptr =3D new C(2048, C::default_timeout, C::default_tries); // u= se=20 a bigger buffer auto c3ptr =3D new C(C::default_buffer_size, 1, C::default_tries); //= =20 use the default buffer size but with a shorter timeout auto c4ptr =3D new C(2048, C::default_timeout, 5); // testing: bump up= =20 the number of tries I don't really want to make it *easier* for people to write the "function= =20 taking tons of defaulted parameters" antipattern. Let's teach people how to avoid the use of defaulted parameters, instead. =E2=80=93Arthur On Monday, August 17, 2015 at 11:58:41 AM UTC-7, Bo Persson wrote: > > On 2015-08-17 20:37, Arthur Tchaikovsky wrote:=20 > > I would really appreciate some feedback from people, or is this proposa= l=20 > > so uninteresting that there is no point in wasting keyboard on it?=20 > > You might wonder how widely used it will be?=20 > > If not all parameters are of the same type, you can already get some of= =20 > the effect with a few overloads:=20 > > void f(int a =3D 0, string b =3D "1",=20 > complex c =3D 2.0, type4 d =3D 3);=20 > > inline void f(int a, complex c)=20 > { f(a, "1", c, 3); }=20 > > inline void f(string b, complex c)=20 > { f(0, b, c, 3); }=20 > > assuming that d is put last because you hardly ever want to change the=20 > default.=20 > > Just saying that you can already do this without a language change.=20 > > > Bo Persson=20 > > > > >=20 > > On Saturday, 15 August 2015 17:49:50 UTC+1, Arthur Tchaikovsky wrote:= =20 > >=20 > > Hi,=20 > > Does anyone see helpfulness of such solution:=20 > > Having fnc:=20 > >=20 > > void f(int a =3D 0, int b =3D 1, int c =3D 2, int d =3D 3);=20 > >=20 > > I think that it would be nice to be able to say whilst calling it= =20 > > and requiring only some of those args to be non-default:=20 > >=20 > > f(default,2,default,4);=20 > >=20 > > Thoughts?=20 > >=20 > >=20 > > > --=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/. ------=_Part_1741_413134508.1439851944288 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I work on a codebase where this feature would have lots of= use-cases, because we have lots of classes whose constructors have default= ed parameters:

=C2=A0 =C2=A0 C(int buffer_size =3D 1024,= int timeout =3D 10, int tries =3D 3) : buffer_size_(buffer_size), timeout_= (timeout), tries_(tries) {}

=C2=A0 =C2=A0 auto cpt= r =3D new C(); =C2=A0// usually the defaults are okay
=C2=A0 =C2= =A0 auto c2ptr =3D new C(2048); =C2=A0// use a bigger buffer
=C2=A0 =C2= =A0 auto c3ptr =3D new C(1024, 1); =C2=A0// use the default buffer size but= with a shorter timeout
=C2=A0 =C2=A0 auto c4ptr =3D new C(2048, = 10, 5); =C2=A0// testing: bump up the number of tries

<= div>It would be marginally more convenient (and refactor-proof) to be able = to write

=C2=A0 =C2=A0 auto cptr =3D new C();= =C2=A0// usually the defaults are okay
=C2=A0 =C2=A0 auto c2ptr = =3D new C(2048); =C2=A0// use a bigger buffer
=C2=A0 =C2=A0 auto c3ptr = =3D new C(default, 1); =C2=A0// use the default buffer size but with a shor= ter timeout
=C2=A0 =C2=A0 auto c4ptr =3D new C(2048,=C2=A0default= , 5); =C2=A0// testing: bump up the number of tries

However, I consider defaulted parameters in general to be a huge antipatt= ern, and this constructor pattern in particular to be even worse.
I would MUCH rather just rewrite all these constructors so that they could= be used as

=C2=A0 =C2=A0 auto cptr =3D new C(); = =C2=A0// usually the defaults are okay
=C2=A0 =C2=A0 auto c2ptr = =3D new C().withBufferSize(2048); =C2=A0// use a bigger buffer
=C2=A0 = =C2=A0 auto c3ptr =3D new C().withTimeout(1); =C2=A0// use the default buff= er size but with a shorter timeout
=C2=A0 =C2=A0 auto c4ptr =3D n= ew C().withBufferSize(2048).withTries(5); =C2=A0// testing: bump up the num= ber of tries

or

=C2= =A0 =C2=A0 auto cptr =3D new C(); =C2=A0// usually the defaults are okay
=C2=A0 =C2=A0 auto c2ptr =3D new C(C::BufferSize(2048)); =C2=A0// u= se a bigger buffer
=C2=A0 =C2=A0 auto c3ptr =3D new C(C::Timeout(1)); = =C2=A0// use the default buffer size but with a shorter timeout
= =C2=A0 =C2=A0 auto c4ptr =3D new C(C::BufferSize(2048), C::Tries(5)); =C2= =A0// testing: bump up the number of tries

o= r (the most conservative way to rewrite our existing code)

=C2=A0 =C2=A0 static constexpr int default_buffer_size =3D 1024;
=C2=A0 =C2=A0 static constexpr int default_timeout =3D 10;
=C2=A0 =C2=A0 static constexpr int default_tries =3D 3;
=
=C2=A0 =C2=A0 C() : buffer_size_(default_buffer_size), time= out_(default_timeout), tries_(default_tries) {}
=C2= =A0 =C2=A0 C(int buffer_size, int timeout, int tries) : buffer_size_(buffer= _size), timeout_(timeout), tries_(tries) {}

=C2=A0 =C2=A0 auto cptr =3D new C(); =C2=A0// usually the = defaults are okay
=C2=A0 =C2=A0 auto c2ptr =3D new C(204= 8, C::default_timeout, C::default_tries); =C2=A0// use a bigger buffer
= =C2=A0 =C2=A0 auto c3ptr =3D new C(C::default_buffer_size, 1, C::default_tr= ies); =C2=A0// use the default buffer size but with a shorter timeout
=
=C2=A0 =C2=A0 auto c4ptr =3D new C(2048, C::default_timeout, 5); =C2= =A0// testing: bump up the number of tries

I= don't really want to make it easier for people to write the &qu= ot;function taking tons of defaulted parameters" antipattern.
Let's teach people how to avoid the use of defaulted parameters, inst= ead.

=E2=80=93Arthur


=
On Monday, August 17, 2015 at 11:58:41 AM UTC-7, Bo Persson wrote:On 2015-08-17 20:37, Arthur Tchai= kovsky wrote:
> I would really appreciate some feedback from people, or is this pr= oposal
> so uninteresting that there is no point in wasting keyboard on it?

You might wonder how widely used it will be?

If not all parameters are of the same type, you can already get some of= =20
the effect with a few overloads:

=C2=A0 =C2=A0 =C2=A0void f(int a =3D 0, string b =3D "1",
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 complex<double> c =3D 2= ..0, type4 d =3D 3);

=C2=A0 =C2=A0 =C2=A0inline void f(int a, complex<double> c)
=C2=A0 =C2=A0 =C2=A0{ f(a, "1", c, 3); }

=C2=A0 =C2=A0 =C2=A0inline void f(string b, complex<double> c)
=C2=A0 =C2=A0 =C2=A0{ f(0, b, c, 3); }

assuming that d is put last because you hardly ever want to change the= =20
default.

Just saying that you can already do this without a language change.


=C2=A0 =C2=A0 Bo Persson



>
> On Saturday, 15 August 2015 17:49:50 UTC+1, Arthur Tchaikovsky wro= te:
>
> =C2=A0 =C2=A0 Hi,
> =C2=A0 =C2=A0 Does anyone see helpfulness of such solution:
> =C2=A0 =C2=A0 Having fnc:
>
> =C2=A0 =C2=A0 void f(int a =3D 0, int b =3D 1, int c =3D 2, int d = =3D 3);
>
> =C2=A0 =C2=A0 I think that it would be nice to be able to say whil= st calling it
> =C2=A0 =C2=A0 and requiring only some of those args to be non-defa= ult:
>
> =C2=A0 =C2=A0 f(default,2,default,4);
>
> =C2=A0 =C2=A0 Thoughts?
>
>


--

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