From: shirley@fdr.uucp (Bill Shirley [CSC])
Subject: Objective-C  Frequently Asked Questions
Followup-To: comp.lang.objective-c
Summary: Answers to Frequently Asked Questions asked about the Objective-C
	programming language, an Object Oriented extension to the C.
	Please read this before posting to comp.lang.objective-c.
Supersedes: <objc_721077644@fdr.jsc.nasa.edu>
Organization: nasa-jsc
Date: Sun, 13 Dec 1992 09:00:31 GMT
Expires: 26 Jan 1993 09:00:01 GMT

Archive-name: Objective-C/faq
Last-modified: 1992/10/09 
Version: 1.0


				Answers to
			FREQUENTLY ASKED QUESTIONS
				concerning	
			       Objective-C



Clarification: I have used the word `ObjC' for `Objective-C', the proper
	and official name for the language discussed within, throughout
	this file.  Is is simply a time and space saver.



Questions & Answers
~~~~~~~~~~~~~~~~~~~
Q1. What is Objective-C?

 A1. An object oriented (OO) computer language based on the C language and
	influenced by SmallTalk.  It was designed by The Stepstone Corp. (and
	is a Trademark of Stepstone) and includes many of the OO things that
	make Smalltalk the "language to learn OOP with", and many of the
	great things about C that make it highly portable and very optimizable.
    It is NOT C++.


Q2. Who makes an Objective-C compiler, what does it cost, how do I order?

 A2.1	The Stepstone Corporation
	(203) 426-1875 - (800) BUY-OBJEct voice / (203) 270-0106 fax
	75 Glen Road
	Sandy Hook, CT 06482

	compilers and runtime

		IBM RISC System/6000 w/ AIX
		Sun 3, 4, SPARCstations w/ SunOS
		HP9000/300,400,700,800 w/ HP-UX
		DEC Stations w/ ULTRIX
		Data General AViiON w/ DG/UX
		SCO UNIX SYS V
		PS/2 w/ AIX or OS/2
		PC-AT w/ MS-DOS
		VAX w/ VMS
	      	MIPS
	 	NeXT
	      	Macs with MPW (Mac Programmers Workshop)

	class libraries available for a subset of the above

 A2.2	NeXT, Inc.

		Comes bundled with the extended OS release.
		(NeXTSTEP 3.0 - $295 on CD ROM) shipped Sept 92
		(NeXTSTEP '486 - $995) intended Q4 92

 A2.3	GNU Project

		Freely available
		GNU Experimental Tape 
		GCC 2 (the GNU C/C++/ObjC Compiler with many new
			features - doesn't include runtime - yet)
		prep.ai.mit.edu (18.71.0.38) pub/gnu/gcc-2.0.tar.Z
		(See sites at end of file)




Q3. Does GNU's gcc `do' Objective-C?

 A3. Didn't you read the above answer: It does as of release 2.0,
	but does not initially include the runtime system required
	to use ObjC.  A runtime has been completed and donated to
	the project.  It is currently (Sept 92) undergoing testing
	and some rewriting.
    Reports are: On the NeXT, which has a runtime system, programs
	compiled with gcc 2.0 (as available on the net) run when linked
	to NeXT's runtime library.
    A runtime has been completed and donated to the project.  It is
	currently (Sept 92) undergoing testing and some rewriting.
    A group has formed to help develop a library of Objective-C
	classes to be distributed freely by the GNU project.


Q4. What books concerning Objective-C are available?

 A4.1
    Object Oriented Programming: An Evolutionary Approach
	Author:		Brad J. Cox (, Andrew J. Novobilski second edition)
	Publisher:	Addison-Wesley
	ISBN#:		0-201-10393-1	August, 1986
	ISBN#:		0-201-54834-8	1991
	two editions

 A4.2
    Objective-C: Object Oriented Programming Techniques
	Authors:	Lewis J. Pinson, Richard S. Wiener
	Publisher:	Addison-Wesley, 1991
	ISBN#:		0-201-50828-1
	Abstract:	Includes many examples, discusses both Stepstone's
			and NeXT's versions of Objective-C, and the (minor)
			differences between the two.

 A4.3
    An Introduction to Object-Oriented Programming
	Author:		Timothy Budd
	Publisher:	Addison-Wesley
	ISBN#:		0-201-54709-0
	Abstract:	An intro to the topic of OOP, as well as a comparison
			of C++, Objective-C, Smalltalk, and Object Pascal

 A4.4
    NeXTSTEP Programming  Step 1: Object-oriented applications
	Availablility:	Fall, 1992
	Authors:	Simson L. Garfinkel, Michael K. Mahoney
	Publisher:	Springer-Verlag, 1992	(800)SPR-INGE
	Abstract:	It's updated to discuss NeXTSTEP 3.0 features
			(Project Builder, new development environment)
			but doesn't discuss 3DKit or DBKit


Q5. What is the difference between Objective-C and C?

 A5. ObjC is a superset of the ANSI C standard.  It is an object oriented 
	language that has extensions similar to Smalltalk applied to C.
	It includes a few more key words and constructs.
	New Keywords:
	 @interface
	 @implementation
	 @public
	 @selector
	Constructs:
	 [instance  method: param];  <- sending a message to an instance
	Types:
	 id	- basically a generic pointer to an object

	[****************]


Q6. What is the difference between Objective C and C++?

 A6. C++ has operator overloading, ObjC doesn't.  Some consider this
	to be 'syntactic sugar', and it is, but it can be a quite
	handy bit of syntactic sugar.
     C++ has multiple inheritance, ObjC doesn't.  There is much
	debate over the desirability and value of this.  There are
	several ways to 'get around' this in ObjC.
     The run-time binding mechanism differ quite a bit in their functionality.
	(Details?? [***])


Q7. What is the difference between NeXT's, Stepstone's and GNU's
	versions of Objective C?

 A7. NeXT has defined and distributed its own class libraries, which
	are very integral to the NeXTstep environment.
     NeXT also extended Stepstones definition of the language to include
	new constructs, such as protocols, which are touted to deal with
	some aspects of multiple inheritance.
     NeXT's implementation does *not* support static binding, Stepstone's
	does, and GNU's (does?/doesn't?).
     Stepstone has a standard set of class libraries that work across
	all supported machines.
     GNU gcc (C/C++/ObjC) does not have a runtime system for
	resolving message passing, or include any classes. (yet)
     The Object (root) class of NeXT's and Stepstone's are slightly
	different; there are several Object methods and runtime C
	functions that are not part of Stepstone's.
     NeXT (and GNU?) supports Categories, Stepstone doesn't.
     NeXT has a native language debugger, Stepstone and GNU don't.
     NeXT (3.0) supports Protocols and forward declarations of classes,
	Stepstone (does/n't?), GNU (does/n't?).
     [****************]


Q8. What are some of the common problems of the language and how can
	I work around them? (forward declarations of classes, class variables)

 A8.1 There is no innate multiple inheritance (of course some see this as a
	benefit).
      To get around it you can create a compound class ( a class with
	instance variables that are /id/s of other objects.  They can
	specifically redirect messages to any combination of the
	objects they are compounded of. (It isn't *that* much of a 
	hassle and you have direct control over the inheritance
	logistics)
      Protocols address this to some extent. [***]
      [** how does Delegation fit in here? **]

 A8.2 ObjC has no class variables.
      You can get around this by defining a static variable in the .m
	file, and defining access methods for it.  This is actually a
	more desirable way of designing a class hierarchy, because it
	allows subclasses shouldn't access superclass storage (this would
	cause the subclass to break if the superclass was reimplemented),
	and allows the subclass to override the storage (if the classes
	access all their own variables via methods)

 A8.3 problem X [****************]


Q9. What class libraries are available for Objective C?

 A9.  See the related FAQ file -  _Objective C - a matter of Class_


Q10.  Are there any FTP sites with Objective C code?  Where?

 A10.  Yes.
	There are NeXT related sites at
	 sonata.cc.purdue.edu
	 cs.orst.edu
	And non-NeXT related sites at
	 ?


Q11.  I'm an emacs junkie.  Are there any .el files somewhere to support
	Objective-C?

 A11.  Yes.  Try sonata.cc.purdue.edu /pub/next/misc/objc.tar.Z


Q12.  So show me a program, a simple example.

 A12.  See the companion file "A S[ia]mple Objective-C Program" or get
	all of the code from music.sie.arizona.edu:pub/ObjC/Sample.tar.Z


Q13.  What are Protocols?

 A13.  Protocols are an addition to (currently only NeXT's 3.0+ version of)
	ObjC that allow you to organize related methods into groups
	that form high-level behaviors.  This gives library builders
	a tool to identify sets of standard protocols, independent of
	the class hierarchy.  Protocols provide language support for
	the reuse of design, whereas classes support the reuse of code.
	Well designed protocols can help users of an application
	framework when learning or designing new classes.  Here is a
	simple protocol definition for archiving objects:

		@protocol Archiving
		- read:(NXTypedStream *)stream;
		- write:(NXTypedStream *)stream;
		@end

	Once defined, protocols can be referenced in a class interface
	as follows:

		// MyClass inherits from Object and
		// conforms to the Archiving protocol
		@interface MyClass : Object <Archiving>
		@end

	Unlike copying methods to/from other class interfaces, any
	incompatible change made to the protocol will immediately be
	recognized by the compiler (the next time the class is
	compiled).  Protocols also provide better type checking without
	compromising the flexibility of untyped, dynamically bound objects.

		MyClass *obj1 = [MyClass new];

		// legal, obj2 conforms to the Archiving protocol.
		id <Archiving> obj2 = obj1;

		// illegal, obj1 does not conform to the TargetAction
		// protocol.
		id <TargetAction> obj3 = obj1;


		



Terminology
~~~~~~~~~~~
C:		A programming language.
FTP:		File Transfer Protocol.
GNU:		A project making freely available software.
message:	A "call" to an implementation of a method
method:		An implementation of an ObjC "function"
factory method:	A method which creates a new instance of a class
class method:	A method which acts on or with the whole of a class rather
		than an instance of a class.  Often used interchangeably 
		with _factory method_, but there is a subtle difference.
instance method:A method witch acts on or with a specific instance of a
		class.
OOP:		Object Oriented Programming
ObjC:		An abbreviation of Objective-C
Object:		The base {class} of Objective-C.
Objective-C:	An Object Oriented programming language extended from
		  the C language, and what this whole file is about.
		It is also a registered trademark of The Stepstone
		Corporation.




---
All [**********] designate incompleteness of the FAQ.  If you have any
questions, corrections, comment, suggestions pass them along.  I can be
reached directly at <shirley@fdr.jsc.nasa.gov> or you can post to
the news group (comp.lang.objective-c).

Disclaimer:  I am not related to any company or group mentioned above.
This file was created to provide information to interested people, and
not advertizing for anything listed above.

gnu ftp site, prep.ai.mit.edu:pub/gnu/etc/DISTRIB
 (prep UK/Europe mirror) src.doc.ic.ac.uk (???)
 (prep Netherlands mirror) ftp.win.tue.nl (131.155.70.100)
 (prep UK mirror) src.doc.ic.ac.uk (146.169.3.7)
 (prep Japan mirror) utsun.s.u-tokyo.ac.jp (133.11.11.11)
 (Australian site) archie.au (IP number may change)


My list of thanks has gotten a bit long, so instead of leaving
 anyone out, I'll just say thanks to all who have given feedback.
This information is what YOU make it.
---
-- 
 Bill Shirley
 shirley@fdr.jsc.nasa.gov
-- 
	``One lonesome body,		Bill Shirley
	  one lonesome song.		shirley@fdr.jsc.nasa.gov
	  No lonesome body,
	  no lonesome song.'' - throwing muses
Newsgroups: comp.lang.objective-c