220 32048 <57ddf392-ae57-bfd2-1ae6-15da6fce5aa9@scylladb.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Avi Kivity Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Non-last default arguments Date: Fri, 7 Apr 2017 12:36:25 +0300 Lines: 311 Approved: news@gmane.org Message-ID: <57ddf392-ae57-bfd2-1ae6-15da6fce5aa9@scylladb.com> References: <91d734b5-d1f2-773a-b5c2-3b96696a6f0f@scylladb.com> <23242c11-a20b-9dfe-42fc-7a1ae3195849@scylladb.com> <6ee7b168-7b23-4368-a587-1f4042903db9@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------13372B8BD8DCF0F697C29281" X-Trace: blaine.gmane.org 1491557796 8125 195.159.176.226 (7 Apr 2017 09:36:36 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 7 Apr 2017 09:36:36 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHJFOWD7YCRBIN3TXDQKGQEV7D6SIA@isocpp.org Fri Apr 07 11:36:29 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f72.google.com ([209.85.215.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cwQJg-0001Hw-Tf for gclcip-std-proposals@m.gmane.org; Fri, 07 Apr 2017 11:36:28 +0200 Original-Received: by mail-lf0-f72.google.com with SMTP id f66sf11603595lfe.23 for ; Fri, 07 Apr 2017 02:36:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to: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=9/PpGhs0wWvKo0vCg0eQOZpDiw647pRGw3lQBfF8C+g=; b=wH2in7Dw65QeN8eAdL/aEjkvCJ8opTK2Cbd9sPVK71Ec8JuGjq0pkmDUDyttHcSb0T zhmNp/d6+Y7SPtzIu8+44BNPkmJsU/x6xEZmqSczk4yqyt6UwrYTEoTnOhgEh1GEAwlg MTvHcqSF0QuY4Zmi2qekFLr7cTWK6M8I6HE/7ySUSMXOh5dRCbWhgxEXKzhzm8yh09Yc MUgO5VYAD3IyPU5VoQmWNYTWDOBXXl+ihVEUzdRBx/oIUo996DK0RrxPaJLGv2nWSYCS AFi30J5XGRgkLTlaquMEOc1DuYsL+hXPGtoE2KNT+N5FTWTxKj3pzTC20IPySrZhU0Z5 oVZQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to: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=9/PpGhs0wWvKo0vCg0eQOZpDiw647pRGw3lQBfF8C+g=; b=ucrvdZAB4wZybdWo3nGmod+xFQwFwl1CywnpxMeka9gq1ogvdNK/O1seLpavbY8M+I +VCXtQqDjyEwUHK9Nj+w5ztdnL2vUNRVfrDu6Sy0wyx4w8Hq06bSIkvfHrPGOPYZJlUT x3Qq16I0Gsb43Tg2U3y8pUvlbylcxS92t0YQ0VNDTdeFNXX3gWulCOgMFJFdEvqAAuIy 82qBnjidk0T5xi3kk/9Ek0lWDf8pLOZRazH1dvneSi0DecVL9y3DMmr5V06v+IJ/UWpg vfFrDy9PDK/JWjeQZuBWVZZmcG7jEHlDZKwQx5xQ4jt9pMOMYIjxxUMtQ8BYEXX/znaN rfLQ== X-Gm-Message-State: AN3rC/7Pv2jVgrU9pGpVol3HuSVfkFyy0+ib8izyGwb8OIuIa62ZSXbp6sGr4HMyl3Vv3Q== X-Received: by 10.25.215.220 with SMTP id q89mr452687lfi.28.1491557794941; Fri, 07 Apr 2017 02:36:34 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.45.135 with SMTP id t129ls93859wmt.20.gmail; Fri, 07 Apr 2017 02:36:33 -0700 (PDT) X-Received: by 10.223.147.98 with SMTP id 89mr31153063wro.99.1491557793322; Fri, 07 Apr 2017 02:36:33 -0700 (PDT) Original-Received: from mail-wr0-x230.google.com (mail-wr0-x230.google.com. [2a00:1450:400c:c0c::230]) by mx.google.com with ESMTPS id v124si7378218wmd.89.2017.04.07.02.36.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Apr 2017 02:36:33 -0700 (PDT) Received-SPF: pass (google.com: domain of avi@scylladb.com designates 2a00:1450:400c:c0c::230 as permitted sender) client-ip=2a00:1450:400c:c0c::230; Original-Received: by mail-wr0-x230.google.com with SMTP id c55so25455283wrc.3 for ; Fri, 07 Apr 2017 02:36:33 -0700 (PDT) X-Received: by 10.223.134.203 with SMTP id 11mr2259040wry.149.1491557792958; Fri, 07 Apr 2017 02:36:32 -0700 (PDT) Original-Received: from [10.0.0.3] (bzq-79-176-149-103.red.bezeqint.net. [79.176.149.103]) by smtp.googlemail.com with ESMTPSA id y66sm5303375wrb.39.2017.04.07.02.36.31 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Apr 2017 02:36:31 -0700 (PDT) In-Reply-To: <6ee7b168-7b23-4368-a587-1f4042903db9@isocpp.org> X-Original-Sender: avi@scylladb.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@scylladb-com.20150623.gappssmtp.com; spf=pass (google.com: domain of avi@scylladb.com designates 2a00:1450:400c:c0c::230 as permitted sender) smtp.mailfrom=avi@scylladb.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:32048 Archived-At: This is a multi-part message in MIME format. --------------13372B8BD8DCF0F697C29281 Content-Type: text/plain; charset=UTF-8; format=flowed On 04/06/2017 07:44 PM, Nicol Bolas wrote: > > > On Thursday, April 6, 2017 at 12:28:33 PM UTC-4, Avi Kivity wrote: > > > > On 04/06/2017 06:26 PM, Nicol Bolas wrote: >> On Thursday, April 6, 2017 at 11:17:42 AM UTC-4, dgutson wrote: >> >> see the date: >> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1466.pdf >> >> >> >> I'm with Stroustrup: "Nahh...don't complicate default parameters. >> We have overloading instead." > How can I overload f(int = 1, int = 2, int = 3, int = 4) to allow > any combination of parameters to be overloaded? > > Named arguments would be better though, but I understand that's > dead (and why). > > > But it's not dead. Designated initializers will apparently exist in > C++20, which gives you the effective equivalent of named parameters: > > | > structf_args > { > inta =1;intb =2;intc =3;intd =4; > }; > > voidf(f_args args); > > f({.b =1}); > | > > Equally importantly... why do you /need/ to give people all of this > complexity? If your function has that many arguments, and that many of > them have default values, and none of those parameters are more likely > to be defaulted than others, then it seems very likely that your > function is doing too much work or otherwise taking too many parameters. Sometimes functions are indeed complicated, and sometimes they evolve this way. I hope C++ is not meant for perfect software only. > > Furthermore, can you provide a real-world example of a case where the > user using this feature would make the code less complex? And by that, > I mean less complex than just living with the limitation, not less > complex than jumping through a bunch of hoops to enforce it. I have a > hard time reading`some_func(default, value, value, default, value)` > and understanding what that function is doing. I too prefer a struct for passing large number of configuration items. If we have designated initializers, then that's a superior solution to the problem. The typical pattern I see start with a constructor with a few (maybe no) parameters grow parameters overtime, defaulting them to preserve compatibility give up in disgust and replace with a config struct, adding much boilerplate to call sites Designated initializers aren't perfect for this, but they're pretty good (real named arguments would be better). -- 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/57ddf392-ae57-bfd2-1ae6-15da6fce5aa9%40scylladb.com. --------------13372B8BD8DCF0F697C29281 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On 04/06/2017 07:44 PM, Nicol Bolas wrote:


On Thursday, April 6, 2017 at 12:28:33 PM UTC-4, Avi Kivity wrote:



On 04/06/2017 06:26 PM, Nicol Bolas wrote:
On Thursday, April 6, 2017 at 11:17:42 AM UTC-4, dgutson wrote:

I'm with Stroustrup: "Nahh...don't complicate default parameters. We have overloading instead."
How can I overload f(int =3D 1, int =3D 2, int =3D 3, int =3D 4= ) to allow any combination of parameters to be overloaded?

Named arguments would be better though, but I understand that's dead (and why).

But it's not dead. Designated initializers will apparently exist in C++20, which gives you the effective equivalent of named parameters:

struct f_args
{
=C2=A0 int a =3D 1; int b =3D 2; int c =3D 3; int d =3D 4;
};

void f(f_arg= s args);

f({.b =3D 1});

Equally importantly... why do you need to give people all of this complexity? If your function has that many arguments, and that many of them have default values, and none of those parameters are more likely to be defaulted than others, then it seems very likely that your function is doing too much work or otherwise taking too many parameters.

Sometimes functions are indeed complicated, and sometimes they evolve this way.=C2=A0 I hope C++ is not meant for perfect software onl= y.


Furthermore, can you provide a real-world example of a case where the user using this feature would make the code less complex? And by that, I mean less complex than just living with the limitation, not less complex than jumping through a bunch of hoops to enforce it. I have a hard time reading`some_func(default, value, value, default, value)` and understanding what that function is doing.

I too prefer a struct for passing large number of configuration items.=C2=A0 If we have designated initializers, then that's a superior solution to the problem.


The typical pattern I see

=C2=A0 start with a constructor with a few (maybe no) parameters
=C2=A0 grow parameters overtime, defaulting them to preserve compatibility
=C2=A0 give up in disgust and replace with a config struct, adding much boilerplate to call sites

Designated initializers aren't perfect for this, but they're pretty good (real named arguments would be better).

--
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