220 4098 article Path: news.gmane.org!not-for-mail From: Hariharan Subramanian Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Default values in middle parameters in a function Date: Tue, 30 Apr 2013 13:27:07 +0530 Lines: 166 Approved: news@gmane.org Message-ID: References: <25d51458-9a2e-4607-9b1a-9d2a7be8df4e@isocpp.org> <7c79ca05-607f-40e8-bd51-467363f007ac@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=089e014939480adb8b04db8f5932 X-Trace: ger.gmane.org 1367308629 28883 80.91.229.3 (30 Apr 2013 07:57:09 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 30 Apr 2013 07:57:09 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCJZFFHTQQIBBVHS7WFQKGQEFUTOK5Y@isocpp.org Tue Apr 30 09:57:09 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ee0-f71.google.com ([74.125.83.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UX5Qz-0003lL-9K for gclcip-std-proposals@m.gmane.org; Tue, 30 Apr 2013 09:57:09 +0200 Original-Received: by mail-ee0-f71.google.com with SMTP id c13sf311236eek.6 for ; Tue, 30 Apr 2013 00:57:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:received-spf:mime-version :x-received:in-reply-to:references:date:message-id:subject:from:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-google-group-id:list-post :list-help:list-archive:list-subscribe:list-unsubscribe:content-type; bh=VaGWs5I/CByHTMVw3wQ6EcXjKbV5o7ekR8U46LQj0GY=; b=AQIZPngUwr6YpiNwMqPwXMWYR+XKLl1KdXzVrFH1r4JKO0z8o1gzgq054E0BFUd3E/ BtlUdDGIXqjt/Cu/82g82L4R+61k31Z4J488pDaRj7otso8qNHNufJ29RMXDgyC+sfX4 1b69RNvosI+efCPb0wpTyjWz07aP4Zgeu4GrqgFPF3lQBdUSDeWYyFQA58jpj+RVKu3w nFayH+wzGxMn2E5XdS52NTQnrwFVLwqPHFs8umo6I4IDs2g7GQtaVO9HRfAxM23bzJJ9 NYHfu9ajT7ezXVR5ekVLcTnP8qWnkyoCD4Rd3D3dQ9TnE/nslSz2SW3AUKnLgfSveRHU X-Received: by 10.180.75.133 with SMTP id c5mr8084260wiw.2.1367308628871; Tue, 30 Apr 2013 00:57:08 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.211.201 with SMTP id ne9ls63928wic.51.gmail; Tue, 30 Apr 2013 00:57:07 -0700 (PDT) X-Received: by 10.180.72.227 with SMTP id g3mr21434829wiv.1.1367308627615; Tue, 30 Apr 2013 00:57:07 -0700 (PDT) Original-Received: from mail-wi0-x22b.google.com (mail-wi0-x22b.google.com [2a00:1450:400c:c05::22b]) by mx.google.com with ESMTPS id d13si8451211wjx.87.2013.04.30.00.57.07 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 30 Apr 2013 00:57:07 -0700 (PDT) Received-SPF: pass (google.com: domain of tohari@gmail.com designates 2a00:1450:400c:c05::22b as permitted sender) client-ip=2a00:1450:400c:c05::22b; Original-Received: by mail-wi0-f171.google.com with SMTP id l13so3595922wie.10 for ; Tue, 30 Apr 2013 00:57:07 -0700 (PDT) X-Received: by 10.194.240.132 with SMTP id wa4mr47269906wjc.52.1367308627530; Tue, 30 Apr 2013 00:57:07 -0700 (PDT) Original-Received: by 10.194.59.207 with HTTP; Tue, 30 Apr 2013 00:57:07 -0700 (PDT) In-Reply-To: <7c79ca05-607f-40e8-bd51-467363f007ac@isocpp.org> X-Original-Sender: tohari@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of tohari@gmail.com designates 2a00:1450:400c:c05::22b as permitted sender) smtp.mail=tohari@gmail.com; dkim=pass header.i=@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:4098 Archived-At: --089e014939480adb8b04db8f5932 Content-Type: text/plain; charset=ISO-8859-1 > Personally, I don't think that default parameters are a "flawed feature". > That being said, the flaws discussed are *not* resolved by your > suggestion. > > Default parameters solve a simple, very specific problem. Function > overloading solves a larger, more general problem. I like that we have > both. And I think that the problem that you intend to solve, with some kind > of "default" parameter, is best solved with the larger, more general > solution. > > Also, with `optional`: > > void foo(int param, optional value1, optional value2) { > type1 val1 = value1.value_or(a1); > type2 val2 = value2.value_or(a2); > //implementation > } > > void usefoo() { > foo(0, nullopt, b2); > } > > That seems to take care of the problem. > > This does not work. //foo.h void foo(int a, type1 param1, type2 param2 = value2); //foo.cpp void foo(int a, type1 param1, type2 param2) { //Is param1 value1 or value3? } //usefoo1.cpp void foo(int a, type1 param1 = value1, type2 param2); void usefoo1() { foo(1); } //usefoo2.cpp void foo(int a, type1 param1 = value3, type2 param2); void usefoo2() { foo(1); } From the above example it is clear that the caller decides the default values with the function based on the latest declaration that is available to it. So foo() cannot decide the default values. Regarding the Lawrence' comment that default parameters being unnecessary: We need to define as many overloaded functions as the number of default parameters. Besides it will be hidden in the source and will not be available if only the headers are available. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. --089e014939480adb8b04db8f5932 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable

=
Personally, I don't think that default parameters are a "flaw= ed feature". That being said, the flaws discussed are not resol= ved by your suggestion.

Default parameters solve a simple, very spec= ific problem. Function overloading solves a larger, more general problem. I= like that we have both. And I think that the problem that you intend to so= lve, with some kind of "default" parameter, is best solved with t= he larger, more general solution.

Also, with `optional`:

<= span style=3D"color:rgb(0,0,136)">void foo(in= t param, optional<type1= > value1,= optional<t= ype2> value2) {
=A0 type1 val1 =3D value1.value_or(a= 1);
=A0 type2 val2 =3D value2.value_or(a= 2);
=A0 //implementation
}<= br>
void us= efoo() {
=A0 =A0 =A0 =A0foo(0, nullopt,= b2);=
}
=

That seems to take care of the problem.


This does not work.

<= div class=3D"gmail_extra" style>//foo.h
void foo(int a, type1 param1, type2 param2 =3D value2);

//foo.cpp
=
void foo(int a, type1= param1, type2 param2) {
=A0 =A0//Is param1 value1 or value3?=
}

//usefoo1.cpp
void fo= o(int a, type1 param1 =3D value1, type2 param2);
void usefoo1() {
=A0 =A0 =A0foo(1);
}
=
//usefoo2.cpp
void foo(int a, type1 param1 =3D value3, type2 param2);
void usefoo2() {
=A0 =A0 =A0foo(1);
}

From the above example it is clear that the calle= r decides the default values with the function based on the latest declarat= ion that is available to it. So foo() cannot decide the default values.

Regarding the Lawrence' comment that de= fault parameters being unnecessary: We need to define as many overloaded fu= nctions as the number of default parameters. Besides it will be hidden in t= he source and will not be available if only the headers are available.

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