Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!dtgcube!ed
From: ed@DTG.COM (Edward Jung)
Newsgroups: comp.sys.next
Subject: Re: Lisp <-> Objective C
Message-ID: <1989Dec8.220555.3912@uunet!dtgcube>
Date: 8 Dec 89 22:05:55 GMT
References: <11996@phoenix.Princeton.EDU>
Sender: ed@uunet!dtgcube (Edward Jung)
Reply-To: ed@DTG.COM (Edward Jung)
Distribution: na
Organization: The Deep Thought Group, L.P.
Lines: 44
In-Reply-To: djlinse@phoenix.Princeton.EDU (Dennis Linse)

In article <11996@phoenix.Princeton.EDU>, djlinse@phoenix (Dennis Linse) writes:
>
>An associate is trying to use the Allegro Common Lisp - Objective C
>interface provided on the NeXT.  Nearly everything works fine, except
>(you knew that was coming) when trying to access an objective-c class
[code deleted]
>
>Included in SimulatorApp.m is a call to Expert (i.e. [Expert new]).
>When this is loaded into Lisp, the class is created and printed out just
>fine.  The loading of the .o file begins, but then fails with an error
>something like "Symbol .objc_class_name_Expert undefined".  He was only
>able to get this far by creating an .h file with an @interface to a
>dummy Expert class.  I can see how this is incorrect, but I can't find
>any other way to fix it.  What appears to be needed is an extern
>declaration for the class, so that the Objective-C can be compiled
>(without the dummy @interface, it won't even compile because Expert is
>unknown).

The "[Expert new]" is compiled into code that looks for an Objective-C
symbol for Expert, that is, ".objc_class_name_Expert".  Since that is
not in the image, the link fails.

You should use the following instead:

	#import <objc/objc.h>

	[objc_getClass("Expert") new];

Alternatively, you can "cache" the class (factory) object:

	id Expert;

	Expert = objc_getClass("Expert");

	my_expert = [Expert new];

Both examples insure that the symbol is extracted from the run-time
image rather than the compiled (static) information.

-- 
Edward Jung                             The Deep Thought Group, L.P.
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        UUCP: uunet!dtgcube!ed          Internet: ed@dtg.com