Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!bbn.com!nic!chaos.cs.brandeis.edu!chaos!phils
From: phils@chaos.cs.brandeis.edu (Phil Shapiro)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Casting in Think C
Message-ID: <PHILS.90Dec3175623@chaos.cs.brandeis.edu>
Date: 3 Dec 90 22:56:23 GMT
References: <6167@munnari.oz.au>
Sender: @chaos.cs.brandeis.edu
Organization: Symantec Corp.
Lines: 38
In-Reply-To: caw@munnari.oz.au's message of 3 Dec 90 13:29:16 GMT

In article <6167@munnari.oz.au> caw@munnari.oz.au (Chris Wright) writes:
   Why do you have to cast gApplication in ThinkC e.g.:
   extern	CApplication	*gApplication;

   void main()
   {
        gApplication = new(CEditApp);
        ((CEditApp *)  gApplication)->IEditApp();
        gApplication->Run();
        gApplication->Exit();
   }

   if you remove the cast and try gApplication -> iEditApp(), the compiler
   complains.
   new is meant to "automatically assign a class to the newly created object"

There's really a couple things going on here.

When you use new() to create an object, the object is assigned a
number based on the type that's passed to new().  This is called the
Class ID.  When you call a method, this is the information that's used
to determine which method (if it's overridden) to call.  This is also
the information that bless() changes.

At compile time, the compiler must be happy that the method call (or
instance variable fetch) is correct.  This may or may not have
anything to do with the Class ID that's used at runtime.  This is how
a subclass can override a method, and have the overriding method
executed, even if the type the object is declared as is not of the
subclass's type <whew!>.

I tried mailing, but it bounced :-(

	-phil
--
   Phil Shapiro                           Technical Support Analyst
   Language Products Group                     Symantec Corporation
		Internet: phils@chaos.cs.brandeis.edu