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From: AS01MEF@VM.TCS.TULANE.EDU (Jeff E Mandel MD MS)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Annoying MPW C++ warning
Message-ID: <5509@rex.cs.tulane.edu>
Date: 21 Dec 90 15:11:04 GMT
References: <450014@hpnmdla.HP.COM> <450015@hpnmdla.HP.COM>
Sender: news@rex.cs.tulane.edu
Organization: Tulane University School of Medicine
Lines: 48

>In comp.sys.mac.programmer, Bruce.Hoult@bbs.actrix.gen.nz writes:
>
>
>>    >Does anyone know how to eliminate this annoying warning message in
>>    >MPW C++?
>>    > 
>>    >void foo()
>>    >{
>>    >    Rect r, r2;
>>    > 
>>    >    r.left = 1;
>>    >    r.right = 2;
>>    >    r.bottom = 3;
>>    >    r.top = 4;
>>    > 
>>    >    r2 = r;
>>    >}
>>    > 
>>    >CFront gives a "variable 'r' used before set" warning for this code.

Sorry to post this to the entire list, but evidently my mail to Bruce failed to
reach him. The problem has to do with assignment operations in C++.
Essentially, as I understand it (which is not very well), C++ creates a
temporary read-only copy of the Rect prior to making the assignment. For
reasons which are not clear to me, it is completely succesful, but still needs
to kvetsch about it. The solution, depending on whether you are doing an
assignment or construction, is to define two methods for your class Rect; an =
operator, and a constructor, each of which take readonly (e.g. const) Rect
arguments. To Wit:

Rect::Rect(const Rect& theRect) {[useful code goes here]}
Rect& Rect::operator = (const Rect& theRect) {[useful code goes here]}

This should clear up your problem. While you are at it, define a constructor
which takes the four coordinates for your rect, and you can write the far more
readable

Rect r(1,2,3,4);
Rect r2=r;

Jeff E Mandel MD MS
Asst Professor of Anesthesiology
Tulane University School of Medicine
New Orleans, LA

Disclaimer: "Anyone who lets an anesthesiologist teach them C++ programming
should be equally willing to let a computer science professor give them an
anesthetic"