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Archive-name: Objective-C/answers
Version: $Id: answers,v 2.7 1993/12/01 07:34:52 tiggr Exp $


				 Answers to

			 FREQUENTLY ASKED QUESTIONS

			   concerning Objective-C


This is the first in a series of three informational postings concerning
comp.lang.objective-c.  This first part answers FAQs; the second part lists
available class libraries and the third part is a simple sample Objective-C
program.

This posting answers the following questions:

 1   What is Objective-C?
 2   What is the difference between Objective-C and C?
 3   What is the difference between Objective-C and C++?
 4   What exactly is it that makes Objective-C have `classes similar to
     Smalltalk', and what are the resulting capabilities of Objective-C?
 5   What are the `nice features' of Objective-C?
 6   What are some of the common problems of the language and how can I work
     around them?
 7   What are Protocols?
 8   What is the difference between the NeXTSTEP, Stepstone and GNU CC
     versions of Objective-C?
 9   What written information concerning Objective-C is available?
 10  What kind of Objective-C support is provided by Stepstone?
 11  What kind of Objective-C support is provided by NeXT?
 12  What kind of Objective-C support is provided by GNU?
 13  What kind of Objective-C support is provided by BPG.
 14  What are the newsgroups to read or mailing lists to subscribe to in order
     to stay up-to-date on developments for GNU Objective-C?
 15  Are there any FTP sites with Objective C code?  Where?
 16  What class libraries are available for Objective C?
 17  So show me a program, a simple example.
 18  How can garbage collection be applied to Objective-C?
 19  What object encapsulation does Objective-C provide?

(To find a question search on the question number starting a line.)

1   What is Objective-C?

    Objective-C is an object oriented computer programming language.  It is
    a superset of ANSI C and provides classes and message passing similar to
    Smalltalk.

2   What is the difference between Objective-C and C?

    Objective-C is a superset of ANSI C.  It is an object oriented language
    that has extensions similar to Smalltalk that are integrated into C
    syntax without taking anything away.

    It includes a few more keywords and constructs.
	New Keywords:
	 @class
	 @interface
	 @implementation
	 @public
	 @selector
	 @protocol
	 @end
	Constructs:
	 [aList add: anObject];  // send a message to an instance
	Types:
	 id	// a generic pointer to an object

    [This is nowhere near to what I would like it to be.]

3   What is the difference between Objective-C and C++?

    C++ follows the Simula 67 school of OO programming, where Objective-C
    follows the Smalltalk school.  In C++ the static type of an object
    determine whether you can send it a message, in Objective-C the dynamic
    type determine it.  The Simula 67 school is safer, in that more errors
    are detected at compile time.  The Smalltalk school is more flexible, as
    some valid programs will execute correctly in Smalltalk, where they
    would be rejected by Simula 67.

    Stepstone's Objective-C allows you to chose between the dynamic and
    static binding, GNU and NeXT do not.  ANSI C++ allows you to use dynamic
    binding, but discourages you from doing so.

    In many ways, the difference between C++ and Objective-C is more a
    question of mindset than technical barriers.  Are you willing to offer
    some flexibility for some safety?  Advocates for the Simula 67 school
    claims that a well designed program doesn't need the extra flexibility
    (a lie), while advocates for the Smalltalk school claims that the errors
    are no problem in practice (another lie).

    Pragmatic differences between Objective-C and C++ include:

	C++ has operator overloading.  Some consider this to be `syntactic
	sugar', and it is, but it can be a quite handy bit of sugar.

	C++ has multiple inheritance.  There are several ways to `get
	around' this in Objective-C (see below).

	The added syntax and semantics of C++ is huge, while Objective-C is
	C plus just a small number of new features.

4   What exactly is it that makes Objective-C have `classes similar to
    Smalltalk', and what are the resulting capabilities of Objective-C?

    Objective-C is as close to Smalltalk as a compiled language allows.  The
    following is a list of the features `taken' from Smalltalk:

      * Objective-C is compiled---Smalltalk is only partially compiled.  The
	current Objective-C implementations are all *much* faster than any
	Smalltalk.  For example ParcPlace Smalltalk-80/4 is at least 3 times
	slower than both the GNU and NeXT Objective-C's.  (This was measured
	using the Self/Smalltalk benchmark suite available by FTP from
	`self.stanford.edu:pub/Self-2.0.1'.)

      * You may add or delete methods and classes at runtime.  (This is done
	via dynamic loading of object code.  This is currently supported by
	both NeXT and Stepstone Compilers [?].)

      * Much of the syntax, i.e. Smalltalk uses method names like
	`a:method:name:', as does Objective-C.  In Objective-C, the message
	sending construct is enclosed in square brackets, like this:
	`[anObject aMessage: arg]' whereas Smalltalk uses something like
	`anObject aMessage: arg'.

      * The basic class hierarchy, that is, having class `Object' in the very
	top, and letting most other classes inherit from it.

      * Most method names in class object is the same.  E.g. `respondsTo:'.
	What is called `doesNotUnderstand:' in Smalltalk is called
	`doesNotRecognize:' in Objective-C.

      * Smalltalk normally uses `doesNotUnderstand:' to implement
	forwarding, delegation, proxies etc.  In Objective-C, these tasks
	are different:

	    forwarding/delegation: `forward::' can be overridden to
	    implement forwarding.  On the NeXT, `forward::' is even used
	    when passing to super.

	    proxies: (Next) An instance of the NXProxy class forwards all
	    methods and their arguments to the remote object via Mach
	    messages.

      * Objective-C has meta classes mostly like Smalltalk.

      * Objective-C does not have class variables like Smalltalk, but pool
	variables and globals are easily emulated via static variables.

5   What are the `nice features' of Objective-C?

    The possibility to load class definitions and method definitions
	(which extend a class) at run time.

	Objects are dynamically typed: Typing information is available at
	run time.  A prime example of this is NeXTSTEP's `-[Application
	loadNibSection:owner:]'

	Persistence [...].

	Remote objects [...].

	Delegation and target/action protocols [...].

6   What are some of the common problems of the language and how can I work
    around them?

    There are some `common problems':

	There is no innate multiple inheritance (of course some see this as
	a benefit).

	    To get around it you can create a compound class, i.e. a class
	    with instance variables that are ids of other objects.
	    Instances 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.)  [Of course, this is not `getting around the problem
	    of not having multiple inheritance', but just modeling your
	    world slightly different in such a way that you don't need
	    multiple inheritance.]

	    Protocols address the absence of multiple inheritance (MI) to
	    some extent: Technically, protocols are equivalent to MI for
	    purely "abstract" classes (see the answer on `Protocols' below).

	    [How does Delegation fit in here?  Delegation is extending a
	    class' functionality in a way anticipated by the designer of
	    that class, without the need for subclassing.  One can, of
	    course, be the delegate of several objects of different
	    classes.  ]

	There are no class variables.

	    You can get around this by defining a static variable in the
	    implementation file, and defining access methods for it.  This
	    is actually a more desirable way of designing a class hierarchy,
	    because 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).

	    [The question remains what the exact syntax of class variables
	    should be: Should a class object A be seen as an instance of its
	    meta-class MA, which has a super class MB being the meta-class
	    of A's super, B, and, as such, should A have seperate instances
	    of class variables defined for B?  Or not?]

7   What are Protocols?

    Protocols are an addition to Objective-C that allows you to organize
    related methods into groups that form high-level behaviors.  Protocols
    are currently available in NeXTSTEP (since 3.0) and GCC (since 2.4).

    Protocols address the MI issue.  When you design an object with multiple
    inheritance, you usually don't want *all* the features of both A and B,
    you want feature set X from A and feature set Y from B.  If those
    features are methods, then encapsulating X and Y in protocols allows you
    to say exactly what you want in your new object.  Furthermore, if
    someone changes objects A or B, that doesn't break your protocols or
    your new object.  This does not address the question of new instance
    variables from A or B, only methods.

    Protocols allow you to get type-checking features without sacrificing
    dynamic binding.  You can say "any object which implements the messages
    in Protocol Foo is OK for this use", which is usually what you want -
    you're constraining the functionality, not the implementation or the
    inheritance.

    Protocols give 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: (Stream *) stream;
	-write: (Stream *) 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];

	// OK: obj1 conforms to the Archiving protocol.
	id <Archiving> obj2 = obj1;

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

    Another use of protocols is that you can declare an ID to conform to
    some protocol in order to help the compiler to resolve method name
    conflicts:

	@interface Foo: Object
	-(int) type;
	@end

	@protocol Bar
	-(const char *) type;
	@end

	-blah1: d
	{
	  id t = [d someMethod];
	  do_something_with ([t type]);
	}

	-blah2: d
	{
	  id <Bar> t = [d someMethod];
	  do_something_with ([t type]);
	}

    In this example, there are two kinds of the `-type' method.  In the
    method `-blah1:', the compiler doesn't know what return type to expect
    from `[t type]', since it has seen both declarations of `-type'.  In
    method `-blah2:', it knows that `t' conforms to the `Bar' protocol and
    thus that `t' implements the `-type' method returning a `const char *'.

8   What is the difference between the NeXTSTEP, Stepstone and GNU CC
    versions of Objective-C?

    NeXT extended Stepstone's definition of the language to include new
    constructs, such as protocols, which are touted to deal with some
    aspects of multiple inheritance.

    Stepstone supports static _binding_, whereas NeXTSTEP and GNU CC don't.
    All implementations do support static _typing_.

    Stepstone has a standard set of Foundation class libraries that work
    across all supported machines, including NeXTSTEP.  NEXTSTEP comes with
    its own set of libraries (called `kits').  GNU libobjc.a currently only
    includes the `Object' class, though people are busy on a Real library
    (see part two of this FAQ (The ClassWare Listing) for details).

    The `Object' class of all implementations differ.

    NeXTSTEP and GNU CC support Categories, Stepstone doesn't.

    NeXT has a native language debugger, Stepstone and GNU don't.  [This is
    not really true, since NeXT's debugger is gdb, the GNU debugger, and
    their extensions are available.  I think I've seen them on the Fall 1992
    Edu CD.  Their extensions haven't appeared in the official FSF
    distribution yet.]

    NeXTSTEP (from version 3.0) and GCC (from version 2.4) support protocols
    and forward declarations of classes, Stepstone currently does not.

9   What written information concerning Objective-C is available?

    Books:

	Brad J. Cox, Andrew J. Novobilski: Object Oriented Programming: An
	Evolutionary Approach.  Addison-Wesley Publishing Company, Reading,
	Massachusetts, 1991.  ISBN: 0-201-54834-8.

	abstract:	The first book on Objective-C.



	Lewis J. Pinson, Richard S. Wiener: Objective-C: Object Oriented
	Programming Techniques.  Addison-Wesley Publishing Company, Reading,
	Massachusetts, 1991. ISBN 0-201-50828-1.

	abstract:       Includes many examples, discusses both Stepstone's
	                and NeXT's versions of Objective-C, and the
	                differences between the two.


	Timothy Budd: An Introduction to Object-Oriented Programming.
	Addison-Wesley Publishing Company, Reading, Massachusetts.
	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


	Simson L. Garfinkel, Michael K. Mahoney: NeXTSTEP Programming Step
	ONE: Object-Oriented Applications.  TELOS/Springer-Verlag, 1993
	(tel: (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.


	NeXTSTEP Object Oriented Programming and the Objective C Language.
	Addison-Wesley Publishing Company, Reading, Massachusetts, 1993.
	ISBN 0-201-63251-9.

	abstract: 	This book describes the Objective-C language as it
			is implemented for NeXTSTEP.  While clearly targeted
			at NeXTSTEP, it is a good first-read to get to learn
			Objective-C.

    Articles

	`Why I need Objective-C', by Christopher Lozinski.
	Journal of Object-Oriented Programming (JOOP) September 1991.

	Abstract:	This article discusses the differences between C++
			and Objective-C in great detail and explains why
			Objective-C is a better object oriented language.

	`Concurrent Object-Oriented C (cooC)', by Rajiv Trehan et. al.
	ACM SIGPLAN Notices, Vol. 28, No 2, February 1993.

	Abstract:	This article discusses cooC, a language based on the
			premise that an object not only provides an
			encapsulation boundary but should also form a
			process boundary.  cooC is a superset of
			Objective-C.

	`Porting NEXTSTEP Applications to Microsoft Windows',
	by Christopher Lozinski.  NEXTWORLD EXPO Conference Proceedings,
	San Francisco, CA, May 25-27, 1993.  Updated version of the article
	available from the author.  Contact lozinski@cup.portal.com

	Abstract:	This article describes how to develop Objective-C
			applications for both Microsoft Windows and
			NEXTSTEP.

    GNU Documentation

	The GNU project needs a free manual describing the Objective-C
	language features.  Because of its cause, GNU cannot include the
	non-free books in the GNU system, but the system needs to come with
	documentation.

	Anyone who can write good documentation, please think about giving
	it to the GNU project.  Contact rms@gnu.ai.mit.edu.

10  What kind of Objective-C support is provided by Stepstone?

	Compilers and runtime for: Apple Macintosh (running Mac Programmers
	Workshop), DEC Stations (ULTRIX), Data General AViiON (DG/UX),
	HP9000/300,400,700,800 (HP-UX), IBM RISC System/6000 (AIX), MIPS,
	NeXT, PC-AT (MS-DOS), PS/2 (AIX or OS/2), SCO/NCR UNIX SYS V, Sun 3, 4,
	SPARCstations (SunOS or Solaris), Silicon Graphics INDIGO and VAX(VMS).
	Other ports available by market demands or consulting services.

	ICpak101 Foundation Class Library is avaliable on all the above.
	ICpak201 GUI Class Library is available on platforms that support
	XWindows, Motif, OpenWindows and SunView.

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

11  What kind of Objective-C support is provided by NeXT?

	The Objective-C compiler and libraries come bundled with the
	NEXTSTEP Developer CD.  The compiler essentially is GNU CC.  For
	information on the Kits which are part of NEXTSTEP, see the
	ClassWare Listing (part 2 of this FAQ).

	Products are:

	    NEXTSTEP 3.1 for NeXT or Intel Computers

	    ObjectWare Catalogue (lists available classes, both commercial
	    and those freely available).

	NeXT Computer, Inc.
	900 Chesapeake Drive
	Redwood City, CA 94063
	tel: 800 848 NEXT
	fax: 415 780 2801
	email: NeXTanswers@NeXT.COM

12  What kind of Objective-C support is provided by GNU?

    GNU CC, since version 2, comes with an Objective-C compiler.  The
    current distribution of GNU CC (version 2.5.0) includes an Objective-C
    compiler and runtime library.  The latter includes the `Object' class.
    Some people are working on GNU libraries, see part two of this FAQ (The
    ClassWare Listing) for details.

    If you haven't switched to a GNU CC as recent as 2.4 yet, here's one
    reason to do so: The new runtime (as of 2.4) is more than 3 times as
    fast as the old runtime (pre 2.4) w.r.t. method invocation.

	Free Software Foundation
	675 Massachusetts Avenue
	Cambridge, MA  02139
	+1-617-876-3296

    General questions about the GNU Project can be asked to
    gnu@prep.ai.mit.edu.

	GNU CC comes with an Objective-C compiler and runtime library which
	includes the Object class.

	For information on how to order GNU software on tape, floppy or
	cd-rom, check the file etc/DISTRIB in the GNU Emacs distribution, or
	e-mail a request to: gnu@prep.ai.mit.edu

	GNU software is available from the following sites:

	    Asia: utsun.s.u-tokyo.ac.jp:/ftpsync/prep,
	      ftp.cs.titech.ac.jp, cair.kaist.ac.kr:/pub/gnu
	    Australia: archie.oz.au:/gnu
	    Europe: ftp.informatik.tu-muenchen.de,
	      src.doc.ic.ac.uk:/gnu, nic.funet.fi:/pub/gnu,
	      ugle.unit.no, isy.liu.se, nic.switch.ch:/mirror/gnu,
	      archive.eu.net, ftp.informatik.rwth-aachen.de:/pub/gnu,
	      ftp.stacken.kth.se, ftp.win.tue.nl, ftp.denet.dk,
	      ftp.eunet.ch, irisa.irisa.fr:/pub/gnu
	    United States: wuarchive.wustl.edu, ftp.cs.widener.edu,
	      uxc.cso.uiuc.edu, col.hp.com:/mirrors/gnu,
	      gatekeeper.dec.com:/pub/GNU, ftp.uu.net:/systems/gnu

13  What kind of Objective-C support is provided by BPG.

    BPG provides the Borland Extensions to Objective-C which allows the
    Objective-C translator to be used with the Borland Compiler, and makes
    it easy to develop Objective-C application for Microsoft Windows.

    BPG provides the Smalltalk Interface to Objective-C which makes
    Objective-C objects look like Smalltalk Objects.  It can be used to
    build Graphical User Interface on portable Objective-C objects, or to
    sell Objective-C libraries to Smalltalk developers.

    BPG provides the Objective-C Message Bus which sends Objective-C messages
    across heterogeneous computer platforms.

    BPG has a library of objects for modelling Objective-C programs.  A browser
    application has been built on this library.  Other potential applications
    include adding class variables to Objective-C, adding runtime information
    about instance variables, and method argument types, generating object
    versions, and eventually building a browser/translator.

	Christopher Lozinski
	BPG
	35032 Maidstone Court
	Newark, CA 94560
	Tel: (510) 795-6086
	fax: (510) 795-8077
	email: lozinski@cup.portal.com

14  What are the newsgroups to read or mailing lists to subscribe to in order
    to stay up-to-date on developments for GNU Objective-C?

    Read comp.lang.objective-c, which is bound to discuss current events.
    There is also a mailing list, gnu-objc@gnu.ai.mit.edu, discussing this
    very topic.  To subscribe to this list, send a mail with your request to
    `gnu-objc-request@gnu.ai.mit.edu.'

15  Are there any FTP sites with Objective C code?  Where?

    NeXTSTEP sites:
	sonata.cc.purdue.edu	128.210.15.30
	cs.orst.edu		128.193.32.1
	ftp.stack.urc.tue.nl	131.155.2.71
	ccrma-ftp.stanford.edu	36.49.0.93	(MusicKit)

    See also part 2 of this FAQ.

16  What class libraries are available for Objective C?

    See the related FAQ file Objective-C/classes.

17  So show me a program, a simple example.

    See the related FAQ file Objective-C/sample.

18  How can garbage collection be applied to Objective-C?

    Currently, there are two implementations of garbage collection which can
    be used in Objective-C programs [that I'm aware of].  Both methods use a
    radically different approach.

	Garbage Collection in an Uncooperative Environment

	    This implements garbage collection of chunks of memory obtained
	    through (its replacement of) malloc(3).  It works for C, C++,
	    Objective-C, etc.

	    @article{bw88,
	    title="Garbage Collection in an Uncooperative Environment",
	    author="Hans J\"urgen Boehm and Mark Weiser",
	    journal="Software Practice and Experience",
	    pages=807-820,volume=18,number=9,month=sep,year=1988}

	    Available as `iesd.auc.dk:/pub/ObjC/gc3.0.tar.z'.

	Garbage Collection through Class Abstraction

	    This implements garbage collection through class abstraction
	    (and hence is Objective-C specific).  Anything to be garbage
	    collectible must be an object (instance of a subclass of a
	    specific class) or have such an object for a wrapper.

	    Available as `ftp.es.ele.tue.nl:/pub/tiggr/tobjc.tar.gz'

    Apart from the obvious radical difference, another difference currently
    is also noteworthy: The first method automatically protects objects
    pointed to from the stack, bss or data segments; the second doesn't.

19  What object encapsulation does Objective-C provide?

    Object encapsulation can be discerned at two levels: encapsulation of
    instance variables and of methods.  In Objective-C, the two are quite
    different.

    Instance variables:

	The keywords @public, @private and @protected are provided to secure
	instance variables from prying eyes to some extent.

		@public		anyone can access any instance variable.
		@private	only methods belonging to this object's
				class or a subclass thereof have access to
				the instance variables.
		@protected	only methods of this class may access the
				instance variables.  This excludes methods
				of a subclass.

	If not explicitly set, all instance variables are @private.  (Note
	that full typing information is available through the runtime,
	making all variables @public again.  However, this does solve the
	problem of instance variables moving in an object because of
	recompilation.  Such instance variable lookup is performed on
	NeXTSTEP by the `loadNibSection...' methods, making it completely
	safe.)

    Methods:

	To the Objective-C runtime, all methods are @public.  The programmer
	can only show his/her intention of making specific methods not
	public by not advertising them in the class' interface.  In
	addition, so-called private methods can be put in a category with a
	special name, like `secret' or `private'.

	However, these tricks do not help much if the method is declared
	elsewhere, unless one reverts to indicating the object's type at
	compile time.  And the runtime doesn't care about all this and any
	programmer can easily circumvent the tricks described.  Thus, all
	methods really are always @public.

The early version of this FAQ was compiled by Bill Shirley, with the aid of
many people.  The current version is being maintained by Tiggr, aided by a
lot of people, including: Per Abrahamsen, Paul Burchard, Brad Cox,
Christopher Lozinski, Mike Mahoney, Paul Sanchez, Lee Sailer, Bill Shirley,
Subrata Sircar, Ted Slupesky, Richard Stallman and Kersten Krab Thorup,

Any text in between `[' and `]' is a comment.  Comments indicate `problems'
with this FAQ, which should be solved.  Send your suggestions, additions,
bug reports, comments and fixes to `tiggr@es.ele.tue.nl'.

    The information in this file comes AS IS, WITHOUT ANY WARRANTY.  You may
    use the information contained in this file or distribute this file, as
    long as you do not modify it, make money out of it or take the credits.