Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!sun-barr!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!swrinde!ucsd!ucbvax!bloom-beacon!vaxeline!bootsie!olson
From: olson@bootsie.UUCP (Eric Olson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Casting in Think C
Keywords: Casts ThinkC
Message-ID: <40@bootsie.UUCP>
Date: 7 Dec 90 02:33:19 GMT
References: <6167@munnari.oz.au> <3028@skye.cs.ed.ac.uk>
Reply-To: olson@endor.harvard.edu (Eric Olson)
Organization: Lexington Software Design, Lexington, MA
Lines: 55

In article <3028@skye.cs.ed.ac.uk> nick@lfcs.ed.ac.uk writes:
>
>As it should. I'm not sure what the argument to new() actually is/does, but
>my guess is that it just provides a size (a la "sizeof") to determine how
>much store to claim.
>
>gApplication is still a CApplication, and hence doesn't have an IEditApp()
>method unless you cast it.
>
[The next line is not writen by nick -EKO]
>> new is meant to "automatically assign a class to the newly created object"
>
>Now, I've never understood what this kind of thing means, ditto for Bless()
>and all the rest. Presumably it does something automagical to the method
>dispatcher. Anybody care to comment what "the class of an object" actually
>means (as opposed to the static type of the object pointer)?
>

The object class types (like "CBorder") are a5-relative addresses, stored
at the first longword of the Handle allocated by new().  a5 is subtracted
off before the class type is stored by bless() or new(), and added in
in the function below (so that comparisons will work right).

You have to coerce the value returned by GetObjectClass() to (void *) before
passing it to some other functions.  As far as I know, functions can not
return (void *) .

/*=============================================================================
 GetObjectClass

            Get the type of an object.
            member(object, GetObjectClass(object)) always returns nonzero.
 =============================================================================*/

long GetObjectClass(
       CObject *               obj)
{
       asm {
         movea.l obj,a1
         movea.l (a1),a1                         ;  <<indirect>>
         move.w  (a1),d0                         ;  D0.W = class ID
         ext.l   d0                              ;  Sign extend the class ID
         add.l   a5,d0                           ;  Add in a5
  }
}

Cheers!

-Eric

-- 
Eric K. Olson, Editor, Prepare()       NOTE: olson@bootsie.uucp will not work!
Lexington Software Design              Internet: olson@endor.harvard.edu
72A Lowell St., Lexington, MA 02173    Usenet:   harvard!endor!olson
(617) 863-9624                         Bitnet:   OLSON@HARVARD