220 4058 <25d51458-9a2e-4607-9b1a-9d2a7be8df4e@isocpp.org> article Path: news.gmane.org!not-for-mail From: Hariharan Subramanian Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Default values in middle parameters in a function Date: Mon, 29 Apr 2013 03:15:28 -0700 (PDT) Lines: 115 Approved: news@gmane.org Message-ID: <25d51458-9a2e-4607-9b1a-9d2a7be8df4e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_3792_936000.1367230528048" X-Trace: ger.gmane.org 1367230530 6574 80.91.229.3 (29 Apr 2013 10:15:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 29 Apr 2013 10:15:30 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCJZFFHTQQIBBQ4Q7GFQKGQEH6CZJLQ@isocpp.org Mon Apr 29 12:15:34 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f71.google.com ([209.85.219.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UWl7M-0008TK-Om for gclcip-std-proposals@m.gmane.org; Mon, 29 Apr 2013 12:15:33 +0200 Original-Received: by mail-oa0-f71.google.com with SMTP id o17sf38433350oag.6 for ; Mon, 29 Apr 2013 03:15:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:date:from:to:message-id:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-google-group-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=UgPcaozpLpJl05BHV3AusNNrrSG0d7y5HoP7s+rXMdQ=; b=gI6D9gkSfj0/hmNQ+Z69COnkKh3DItqHhlnpEcVEQBABmx7wFpZp9LGrzgRzgAl7hr iUI3EuhrxZWfn3hd+jJVkfcmpqf+G2aP4ZvGIqXGeIVJe9S+jd8am4mbavn0fFcg4NuY sdYfpzH0Pw5mW/X6AAxxPeuyS2QOkkNseawiqCMtSPjNbB4C7ePcoZrol1ta5qygl6eF 3K3ltIThp10DJpYbOQt/3INzHxJFkOkgvadXBWWiucvQNiGSoMWm5hzqxjMv4R89A98q 5VWulBhmS4fYxCcQmQdhctnT5n2gzEAz8Q4xbHnmcCQTpjd/Yvoonwu/Z3lmSTz4tEej +guA== X-Received: by 10.50.7.74 with SMTP id h10mr1121120iga.0.1367230531486; Mon, 29 Apr 2013 03:15:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.22.99 with SMTP id c3ls176400igf.11.canary; Mon, 29 Apr 2013 03:15:29 -0700 (PDT) X-Received: by 10.50.25.33 with SMTP id z1mr121308igf.16.1367230529411; Mon, 29 Apr 2013 03:15:29 -0700 (PDT) X-Original-Sender: tohari@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:4058 Archived-At: ------=_Part_3792_936000.1367230528048 Content-Type: text/plain; charset=ISO-8859-1 Problem: Change the default value of a default parameter in a function might result in a surprising behaviour. The problem is described below. Consider the following function declaration & definition //file1.h void foo(int param, type1 value1 = a1, type2 value2 = b1); //file1.cpp #include "file.h" void foo(int param, type1 value1, type2 value2) { //implementation } //file2.cpp #include "file1.h" void usefoo() { foo(0, a1, b2); //i want to pass b2 and hence i am passing the default value a1 also } It is often the case that during the lifecycle of the software the default value is changed to a2 from a1. This necessitates a change in the function usefoo(). As there is no compilation error it is very easy to miss this error. Even if the person knows that it should be changed it is a cumbersome manual process (change it in all the places) and hence error prone. This kind of change is not uncommon at all. Days of work gets wasted due to this. Solution: Introduce the default keyword during the function call. The function call becomes void usefoo() { foo(0, default, b2); } Now, no matter what the change in the default value the usefoo function will not produce any surprise. It might seem that the mirror image of the problem exists. If a user wants to pass a1 then changing the default value to a2 will result in a problem. The problem can be solved like this below. foo(0, default, b2); // if the user wants to pass whatever the default value is foo(0, a1, b2); // if the user wants to pass a1 no matter what the default value is Addition of this functionality matches the user intent. -- --- 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. ------=_Part_3792_936000.1367230528048 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
Problem:
Change the default value of a default parameter in = a function might result in a surprising behaviour. The problem is described= below.
Consider the following function declaration & definit= ion

//file1.h
void foo(int param, type1 value= 1 =3D a1, type2 value2 =3D b1);

//file1.cpp
#i= nclude "file.h"
void foo(int param, type1 value1, type2 value2) {=
        //implementation
}

//file2.cpp
#include "file1.h"

void usefoo() {
       foo(0, a1, b2= ); //i want to pass b2 and hence i am passing the default value a1 also
}

It is often the case that during the life= cycle of the software the default value is changed to a2 from a1. This nece= ssitates a change in the function usefoo(). As there is no compilation erro= r it is very easy to miss this error. Even if the person knows that it shou= ld be changed it is a cumbersome manual process (change it in all the place= s) and hence error prone. This kind of change is not uncommon at all. Days = of work gets wasted due to this.

Solution:
Introduce the default keyword during the function call. The function cal= l becomes

void usefoo() {
  &n= bsp;    foo(0, default, b2);
}

Now, no matter what the change in the default value the usefoo funct= ion will not produce any surprise. It might seem that the mirror image of t= he problem exists. If a user wants to pass a1 then changing the default val= ue to a2 will result in a problem. The problem can be solved like this belo= w.

foo(0, default, b2); // if the user wants to pa= ss whatever the default value is
foo(0, a1, b2);     &n= bsp; // if the user wants to pass a1 no matter what the default value is

Addition of this functionality matches the user inte= nt.

--
 
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