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comp.lang.objective-c FAQ, part 1/3: Answers


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Archive-name: Objective-C/answers
Version: $Id: answers,v 3.64 1998/09/17 12:51:31 tiggr Exp $

See reader questions & answers on this topic! - Help others by sharing your knowledge
				 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 document answers the following questions:

 1   What is Objective-C?
 2   What exactly is it that makes Objective-C have `classes similar to
     Smalltalk', and what are the resulting capabilities of Objective-C?
 3   What are the differences between Objective-C and C++?
 4   What are the differences between Objective-C and Java?
 5   What are the `nice features' of Objective-C?
 6   What are some problems of the language and how can I work around them?
 7   What object encapsulation does Objective-C provide?
 8   What are protocols?
 9   Does Objective-C employ garbage collection?
 10  How do I debug Objective-C using an unaware gdb?
 11  I get this `Floating exception'...
 12  Why am I lectured by gcc about `#import'?
 13  What written information concerning Objective-C is available?
 14  History	
 15  Major compilers
 16  Compiler differences
 17  Objective-C support per platform
 18  What are the newsgroups or mailing lists to read?
 19  Are there any FTP sites with Objective C code?  Where?
 20  Is there any information on the Net concerning Objective-C?
 21  For more information...

(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.

    Objective-C includes, when compared to C, a few more keywords and
    constructs, a short description of which follows.  For a complete
    example see part 3 of this FAQ.

    `@interface' declares a new class.  It indicates the name of the class,
    the name of its superclass, the protocols adhered to (see Q7), the
    layout of the instance variables (similar to the definition of a struct,
    but including encapsulation information (see Q6)) and declares the
    methods implemented by this class.  A class' interface usually resides
    in a file called `<classname>.h'.

    `@implementation' defines a class.  The implementation is no more than a
    collection of method definitions.  Without an implementation, a class
    does not exist at run time.  The implementation of a class usually
    resides in a file called `<classname>.m'.

    A category is a named collection of method definitions which are added
    to an existing class, possibly at run time.  Thus, you use a category
    to add methods to a class.  Some Objective-C implementations allow a
    category to even replace methods.

    Objective-C includes the predefined type `id' which stands for a pointer
    to some object.  Thus, `id obj;' declares a pointer to an object.  The
    actual class of the object being pointed to is almost irrelevant, since
    Objective-C does run-time binding.

    `-message;' declares a method called `message'.  The `-' indicates that
    the message can be sent to objects.  A `+' instead indicates the message
    can be sent to class objects.  A method is similar to a function in that
    it has arguments and a return value.  The default return type is `id'.
    If a method has nothing useful to return, it returns `self', which is a
    pointer to the object to which the message was sent (similar to `this'
    in C++).

    [obj message], [obj message: arg1] and [obj message: arg1 with: arg2]
    are examples of sending a message to the object OBJ with 0, 1 and 2
    arguments respectively.  The name of the message is called the selector.
    In this example, the selectors are: `message', `message:' and
    `message:with:', respectively.


2   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:

      * Much of the syntax, i.e. both Smalltalk and Objective-C use method
	names like `a:method:name:'.  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: single inheritance with the `Object'
	class as the root of the tree.

      * Most method names in the Object class are the same like, for example,
	`respondsTo:'.  What is called `doesNotUnderstand:' in Smalltalk
	is unchanged in the POC, but it is called `doesNotRecognize:' in
	GNU and Apple Objective-C.

      * Smalltalk normally uses `doesNotUnderstand:' to implement
	forwarding, delegation, proxies, etc.  In [GNU, Apple] Objective-C,
	such functionality is implemented by `forward::'.

      * Objective-C has class objects and meta classes mostly like Smalltalk.

      * You can add or delete methods and classes at runtime.  A subclass
	can pose as its superclass.

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

      * 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'.)

    The big difference of course is that Objective-C is a hybrid between
    Smalltalk and C.  One can choose to represent a string as a `char *' or
    as an object, whereas in Smalltalk everything is an object.  This is one
    reason for Objective-C being faster.  On the other hand, if every bit of
    information in an Objective-C program would be represented by an object,
    the program would probably run at a speed comparable to Smalltalk and it
    would suffer from not having optimizations performed on the basic
    classes, like Smalltalk can do.


3   What are the differences between Objective-C and C++?

    C++ is a multi-paradigm language; Objective-C is object orientation on
    top of C.  In this answer, only the OO aspects of both languages will be
    discussed.

    In Objective-C, the value of a variable denoting an object is always a
    reference to the object.  In C++ it is, in addition, possible for the
    variable's value to be the actual object.

    In Objective-C, all method invocations are dynamically bound.  In C++,
    invocations of virtual member functions are dynamically bound.  Other
    (non-virtual) member functions are more like scoped normal functions
    with a different syntax.  In Objective-C, the dynamic binding is safe:
    it will be signaled if an object does not implement the requested
    method; in C++, the results will be unpredictable.  In Objective-C, the
    safe dynamic binding mechanism underlies method forwarding and easy
    delegation.  The selector type and perform: methods further increase the
    usefulness of dynamic binding.

    Objective-C only allows single inheritance, with the Object class
    residing at the root of the inheritance tree.  C++ allows multiple
    inheritance where direct superclasses must be distinct and conspicuous
    use of `virtual' defines whether a repeatedly inherited superclass is
    shared or repeated in the subclass.

    In Objective-C, static typing is optional, though commonly used; in C++
    static typing is mandatory.

    In C++ (member) functions can be overloaded on argument types.
    Operators can be overloaded too.  Both are not possible in Objective-C.

    Other differences include C++' implicit invocation of constructor and
    destructor functions when objects enter and exit scope, respectively;
    and the concept of `friend' classes and functions.

    Objective-C protocols resemble C++ signatures.  In Objective-C, a class
    is an object like any other; C++ classes do not exist at run time.


4   What is the difference between Objective-C and Java?

    The most obvious difference is syntax: Java's syntax is based on C++'
    whereas Objective-C employs the C syntax (and semantics) with the syntax
    of object declaration and manipulation based on Smalltalk.

    The Java object model comes straight from Objective-C.  Nested classes
    do not change that since they are retrofitted, in Java 1.1, onto the
    original Java 1.0 object model for compatibility with 1.0 tools.

    Features that Java adds include mandatory typing, exception and thread
    support in the language, security managers, name spaces, predefined
    class libraries (java.lang, awt), etc.

    Features that Java misses include categories and the selector type.  No
    categories means that a class can not be amended to suit your needs (a
    problem suffered by more OO languages, including C++).  No selector type
    means that `forward::' and `perform:' methods do not exist, and that the
    possibilities of dynamic binding are limited; e.g. you can't tell an
    array to say `@selector (hello)' to all its elements, and the buttons in
    your GUI won't be able to use the target/action paradigm employed by the
    OPENSTEP AppKit (and which is why inner classes were invented in Java
    1.1).

    Java currently enjoys a lot of `compile-once run-anywhere' hype.  Even
    though Objective-C in this respect suffers the portability problems of
    C, OPENSTEP provides a foundation that could be described as
    `compile-once-for-each-platform run-on-each-platform': OPENSTEP enables
    application deployment on platforms ranging from MS Windows to Solaris.
    This is without modifications to the source, which is what
    `compile-once' is all about.


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

    The possibility to load class definitions at run time.  The possibility
    to extend a class at compile, link, or run time, through a category or
    class posing.

    Objects are dynamically typed: full type information (name and type of
    instance variables; name, argument and return types of methods, etc) is
    available at run time.

    Every method invocation is dynamically bound.  Add to this the selector
    type, forward:: and perform: methods, and you get flexibility not easily
    matched by other OO languages.  Object archiving and distributed objects
    (`remote method invocation' in Java speak) are easy, not just to
    implement but also to use.


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

    The underlying language is C: use Objective-C and you get all of C's
    problems for free.

    There are some `common problems':

	There is no innate 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).

	    To get around the absence of MI, you can create a compound
	    class, i.e. a class with instance variables that are references
	    to 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 the world slightly different in such a way that you
	    don't need multiple inheritance.

	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).

	    To get the effect of class variables for which each subclass has
	    a distinct value, more elaborate schemes are needed.  An often
	    used solution is to use a hash table to store the value of the
	    `variable', keyed on the class.


7   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.
		@protected	only methods belonging to this object's
				class or a subclass thereof have access to
				the instance variables.
		@private	only methods of this class may access the
				instance variables.  This excludes methods
				of a subclass.

	If not explicitly set, all instance variables are @protected.  Some
	people advocate that only @private should be used.  It is generally
	agreed upon that @public if prohibited.

	Note: Instance variable encapsulation is enforced at compile-time.
	At run-time, full typing information on all instance variables is
	available, which sort-of makes all variables @public again.

    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 employs static typing.  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.


8   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 derivatives, and
    GNU CC (since 2.4).

    In short, a protocol is a set of method declarations.  A protocol can
    inherit from multiple other protocols, and a class can inherit multiple
    protocols.  The protocol hierarchy is unrelated to the class hierarchy.
    The type of a variable can be `id <P>' to denote that the class of the
    object pointed to does not matter, as long as the class implements all
    methods prescribed by the protocol P.  Protocols provide `multiple
    inheritance of interface'.  They are matched by Java interfaces and C++
    signatures.

    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;


9   Does Objective-C employ garbage collection?

    No.  Some libraries conventionally use a reference counting scheme, but
    since such a scheme is implemented outside the language, it needs a lot
    of extra typing (by the programmer, on the keyboard) with all the usual
    consequences: errors caused by not properly maintaining an object's
    reference count.

    Hans Boehm's garbage collector can be used with Objective-C.  Depending
    on the Objective-C compiler and library you use, integration is
    automatic or needs some work.

	Garbage Collection in an Uncooperative Environment

	    Garbage collection of chunks of memory obtained through (its
	    replacement of) malloc(3).  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 `ftp://parcftp.xerox.com/pub/gc/gc4.10.tar.gz'
	    (check the directory for a newer version).


10  How do I debug Objective-C using an unaware gdb.

    On August 20 1996, Michael Snyder of NeXT posted patches to GDB 4.16
    to make it Objective-C aware for GNU Objective-C code.  This was
    tested on HP-UX, Solaris and MS Windows.  As he did not supply an URL
    for these patches, I've made the postings available as
    ftp://ftp.ics.ele.tue.nl/pub/users/tiggr/objc/gdb-gnu-objc.diff.gz.uue
    and ftp://ftp.ics.ele.tue.nl/pub/users/tiggr/objc/gdb-gnu-objc.README.

    Debugging Objective-C using a non-Objective-C aware gdb has been
    documented by Martin Cracauer <cracauer@wavehh.hanse.de> on
    http://www.cons.org/cracauer/objc-hint-gdb.html.  It comes down to
    understanding that you can very well look at Objective-C from the C
    perspective, a language very well understood by gdb.


11  I get this `Floating exception'...

    This answer describes possible solutions for the floating exception
    problem when using GNU Objective-C on i386 machines.  [This problem
    existed some time ago; it should not exist any more with recent
    versions of the libraries/software.]  If the problem you're having
    does not fit either description, try applying the hack described in
    ftp://ftp.ics.ele.tue.nl/pub/users/tiggr/objc/README.387, which has
    been reported to work for Linux, FreeBSD, and DJGPP (2.7.2).  If it
    does not solve the problem, ask.

    If you're running Linux, adding `-lieee' to the linker invocation could
    help.  Thomas March <amadeus@bga.com> reported that on several
    occasions, on systems running Linux ELF, with libc.so.5.0.9 and
    libm.so.5.?.?, the problem was reproducible and adding `-lieee' did not
    solve the problem.  However, switching to a newer libc (libc.so.5.2.18
    and libm.so.5.0.5) both solved the problem and removed the need for
    `-lieee' (i.e. a back to normal situation).

    If you're using DJGPP, you might want to take a look at the DJGPP FAQ,
    especially http://www.delorie.com/djgpp/v2faq/faq082.html.

12  Why am I lectured by gcc about `#import'?

    GNU CC issues the following multi-line warning about the how the use
    of `#import' is discouraged (output from GNU CC 2.7.0):

	foo.m:1: warning: using `#import' is not recommended
	The fact that a certain header file need not be processed more than once
	should be indicated in the header file, not where it is used.
	The best way to do this is with a conditional of this form:

	  #ifndef _FOO_H_INCLUDED
	  #define _FOO_H_INCLUDED
	  ... <real contents of file> ...
	  #endif /* Not _FOO_H_INCLUDED */

	Then users can use `#include' any number of times.
	GNU C automatically avoids processing the file more than once
	when it is equipped with such a conditional.

    In short, use `-Wno-import' as an argument to gcc to stop it from
    producing this.  Another possibility is to compile gcc after having
    changed the line reading `static int warn_import = 1' into `static
    int warn_import = 0' in `cccp.c' (line 467 in GNU CC 2.7.1); this way,
    `-Wno-import' is the default setting.

    Whether or not using `#import' is desirable (obviously) has to do with
    how to prevent multiple inclusions of the same file.  Most include
    files, when included multiple times, either do nothing new (possibly
    due to guards being used) or (without the guards) cause the emission
    of C code on which the compiler will choke (due to, for instance,
    repeated typedefs).  Thus, if everybody were to use `#import'
    everybody would be happy, since it does not seem to matter.  However,
    a notable exception to this rule is `assert.h', which changes the
    definition of the `assert' macro depending on the setting of the
    NDEBUG macro.

    There is one point to be made in favor of the warning: if the _user_
    of an include file uses `#include' instead of `#import', the guards
    will be necessary.  Thus, actually, the warning should be issued when
    a file is imported that appears not to be guarded.

    Apart from the more-or-less religious (and thus useless) debate
    whether `#import' or `#include'-with-guards is better, it has been
    observed that `#import' does not catch re-reading a linked and/or
    duplicated file, whereas the guards do.  However, this is, of course,
    a highly unlikely and probably undesirable situation for which neither
    was designed to catch.

    The reason for the existence of `#import' probably is historical: the
    first implementation of Objective-C (by Stepstone) was as a preprocessor
    to C, run after a modified cpp.  `#import' was the include-once
    directive to that cpp.  Since it is part of the Objective-C language, it
    has made it into GNU CC's cpp.

13  What written information concerning Objective-C is available?

    Books on Objective-C, or object oriented programming in general:

	Brad J. Cox, Andrew J. Novobilski: `Object Oriented Programming:
	An Evolutionary Approach', Addison-Wesley Publishing Company,
	Reading, Massachusetts, 1991.  ISBN: 0-201-54834-8 (Japanese:
	4-8101-8046-8).

	    This is the original book on Objective-C, which actually is a
	    book on object oriented system development using Objective-C.

	Object-Oriented Programming and the Objective-C Language.
	Originally published as `Nextstep Object-Oriented Programming and
	the Objective-C Language' by Addison-Wesley in 1993 with ISBN
	0-201-63251-9 (Japanese: 4-7952-9636-7).  In its current [Tue Aug
	26 1997] incarnation it is available on-line from
	http://devworld.apple.com/dev/SWTechPubs/Documents/OPENSTEP/ObjectiveC/objctoc.htm

	    This is a good book on OO programming in general, which focuses
	    on Objective-C as the implementation language.  It is also the
	    definitive source of information for the Objective-C language as
	    implemented by NeXT (and GNU).

	Lewis J. Pinson, Richard S. Wiener: Objective-C: Object Oriented
	Programming Techniques.  Addison-Wesley Publishing Company, Reading,
	Massachusetts, 1991. ISBN 0-201-50828-1 (Japanese: 4-8101-8054-9).

	    Includes many examples, discusses Stepstone's and NeXT's
	    versions of Objective-C, and their differences.

	Timothy Budd: An Introduction to Object-Oriented Programming.
	Addison-Wesley Publishing Company, Reading, Massachusetts.
	ISBN 0-201-54709-0 (Japanese: 4-8101-8048-4).

	    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).

	    It's updated to discuss NeXTSTEP 3.0 features (Project Builder,
	    new development environment) but doesn't discuss 3DKit or DBKit.

    Articles

	`Why I need Objective-C', by Christopher Lozinski.
	Journal of Object-Oriented Programming (JOOP) September 1991.
	Contact info@bpg.com for a copy and subscription to the BPG
	newsletter.

	    This article discusses the differences between C++ and
	    Objective-C in great detail and explains that 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.

	    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 info@bpg.com.

	    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.


14  History

    Objective-C was developed by Brad Cox, who founded the Stepstone
    corporation in 1983 to develop and support the language, a compiler,
    and supporting libraries.  Stepstone never really made it big,
    fostering Objective-C in a niche similar to that of Smalltalk.

    In 1985, Steve Jobs left Apple and started NeXT, a company that
    developed m68k machines and the NeXTSTEP operating system.  The user
    interface of these machines was provided by Display PostScript and the
    AppKit, which, written in Objective-C, made Objective-C the language of
    choice on NeXT computers.

    Brad Cox and Andrew Novobilski write `Object Oriented Programming: An
    Evolutionary Approach' in 1986, the first book on Objective-C.

    NEXTSTEP 1.0a was released in 1989.  The Objective-C compiler is based
    on gcc 1.34.

    In 1991, the Objective-C related modifications by NeXT to gcc find
    their way back into the FSF GNU CC distribution.  By version 1.99, gcc
    (the compiler) supports Objective-C.

    NEXTSTEP 3.0 is released in 1992.  The compiler supports Objective-C++,
    and the Objective-C language has been extended with `@protocol'.
    In the same year, gcc 2.0 comes supplied with an Objective-C runtime
    library.

    In 1993, gcc supports protocols by version 2.4.  NeXT stops the
    production of hardware.  With NEXTSTEP 3.1 they include support for
    PC's.  NeXT starts creating other products not bound to a particular
    operating system, such as Portable Distributed Objects (PDO), which
    comes with its own Objective-C compiler and debugger.

    OPENSTEP, an Objective-C API, is made public in 1994.

    NEXTSTEP 3.3 is released in 1995; it adds support for HP hppa and Sun
    sparc machines.  Also in this year, NeXT acquires all rights to the
    Objective-C programming language and trademark from Stepstone.

    Sun includes support for Objective-C++ in their SparcCompiler in 1996.
    They release OPENSTEP for Solaris.  NeXT releases OPENSTEP 4.0 for PC
    hardware.  Support for hppa and sparc has vanished.  OPENSTEP for
    Windows NT is released.

    Early 1997, Apple acquires NeXT and starts work on the next Macintosh
    operating system, code named Rhapsody, based on the technology they got
    from NeXT.  In the same year, Sun fosters the success of Java and
    subsequently kills OPENSTEP for Solaris.

    [Entry updated Tue Feb 17 1998.  Credits to Norihiro Ito, Hajime Murao,
    KATO Tsuguru, and Hironobu Suzuki.]

15  Major compilers

    There are 4 major Objective-C compilers, provided by Apple, GNU,
    Stepstone, and Portable Object Compiler.  The Stepstone and POC are
    preprocessors that emit vanilla C code, which makes them very
    portable.  The GNU compiler compiles directly to assembly, as does
    Apple's, which is based on the GNU compiler.

    Apple

	The Apple (formerly NeXT) Objective-C compiler comes bundled with
	OpenStep Developer, current Rhapsody seeds, and various other
	products like Portable Distributed Objects (PDO) and WebObjects.

	Bundled with the operating systems is the FoundationKit general
	class library and the AppKit GUI library.

	OpenStep is available for NeXT hardware and PC compatible Intel
	machines.  Rhapsody is the current name for the next-generation
	Apple Macintosh operating system; it is based on OpenStep.  Rhapsody
	will be available for PowerPC-based Macintoshes and Intel-based PC
	compatibles.

	Apple Computer, Inc.
	1 Infinite Loop
	Cupertino, CA 95014
	voice: 408-996-1010
	www: http://www.apple.com/

	[Entry up to date as of Sun Sep 21 1997.]

    GNU

	GNU CC, since version 2, comes with an Objective-C compiler.  The
	current distribution of GNU CC (version 2.8.0) includes an
	Objective-C compiler and runtime library.  The latter includes the
	`Object' and `NXConstantString' classes.  Some people are working
	on GNU libraries, see part 2 of this FAQ (The ClassWare Listing)
	for details or visit http://www.gnustep.org/.

	Free Software Foundation
	59 Temple Place -- Suite 330
	Boston, MA   02111
	+1-617-542-5942

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

	For information on how to order GNU software on tape or cd-rom,
	and printed GNU manuals, check the file etc/ORDERS in the GNU
	Emacs distribution, ftp the file /pub/gnu/GNUinfo/ORDERS on prep,
	or e-mail a request to: gnu@prep.ai.mit.edu

	By ordering your GNU software from the FSF, you help us continue
	to develop more free software.  Media revenues are our primary
	source of support.  Donations to FSF are deductible on US tax
	returns.

	The above software will soon be at these ftp sites as well.
	Please try them before prep.ai.mit.edu as prep is very busy!

	  thanx -gnu@prep.ai.mit.edu

        ASIA: ftp.cs.titech.ac.jp, tron.um.u-tokyo.ac.jp/pub/GNU/prep
	 cair-archive.kaist.ac.kr/pub/gnu, ftp.nectec.or.th/pub/mirrors/gnu
        AUSTRALIA: archie.au/gnu (archie.oz or archie.oz.au for ACSnet)
        AFRICA: ftp.sun.ac.za/pub/gnu
        MIDDLE-EAST: ftp.technion.ac.il/pub/unsupported/gnu
        EUROPE: irisa.irisa.fr/pub/gnu, ftp.univ-lyon1.fr:pub/gnu,
	 ftp.mcc.ac.uk, unix.hensa.ac.uk/mirrors/uunet/systems/gnu,
	 src.doc.ic.ac.uk/gnu, ftp.ieunet.ie:pub/gnu, ftp.eunet.ch,
	 nic.sunsite.cnlab-switch.ch/mirror/gnu, ftp.win.tue.nl/pub/gnu,
	 ftp.nl.net, ftp.informatik.rwth-aachen.de/pub/gnu,
	 ftp.informatik.tu-muenchen.de, ftp.etsimo.uniovi.es/pub/gnu,
	 ftp.funet.fi/pub/gnu, ftp.denet.dk, ftp.stacken.kth.se,
	 isy..se, ftp.luth.se/pub/unix/gnu,
	 ftp.sunet.se/pub/gnu, archive.eu.net
        SOUTH AMERICA: ftp.inf.utfsm.cl/pub/gnu, ftp.unicamp.br/pub/gnu
        WESTERN CANADA: ftp.cs.ubc.ca/mirror2/gnu
        USA: wuarchive.wustl.edu/systems/gnu, labrea.stanford.edu,
	 ftp.digex.net/pub/gnu, ftp.kpc.com/pub/mirror/gnu,
	 f.ms.uky.edu/pub3/gnu, jaguar.utah.edu/gnustuff,
	 ftp.hawaii.edu/mirrors/gnu, uiarchive.cso.uiuc.edu/pub/gnu,
	 ftp.cs.columbia.edu/archives/gnu/prep,
	 archive.cis.ohio-state.edu/pub/gnu, gatekeeper.dec.com/pub/GNU,
	 ftp.uu.net/systems/gnu

	[Entry up to date as of Sun Sep 21 1997.  GNU FTP mirror site list
	 up to date as of Tue Jan 13 1997.]

    GNU/EGCS

	EGCS (at http://www.cygnus.com/egcs/) is a variation of GCC.  From
	the EGCS FAQ:

	In brief, the three biggest differences between egcs and gcc2 are
	these:

	    More rexamination of basic architectual decisions of gcc and
	    an interest in adding new optimizations;

	    working with the groups who have fractured out from gcc2 (like
	    the Linux folks, the Intel optimizations folks, Fortran folks)
	    including more front-ends; and finally

	    An open development model (see below) for the development
	    process.

	Like GCC, EGCS supports Objective-C.

    Stepstone

	Steptone provides Objective-C compilers and runtime for: Apple
	Macintosh (running Mac Programmers Workshop), DEC (Open VMS,
	OSF/1, ULTRIX), HP9000/700, 800 (HPUX), IBM RISC System/6000
	(AIX), MIPS, NeXT, PC (MS-DOS, OS/2, MS Windows 3.1/95/NT, SCO
	Unix), Sun 3/4/SPARC (SunOS, Solaris), Silicon Graphics INDIGO
	(IRIS).  Other ports available by market demands or consulting
	services.

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

   	The Stepstone Corporation
	75 Glen Road
	Sandy Hook, CT 06482
	voice: 203 426 1875
	voice: 800 BUY OBJEct
	fax:   203 270 0106
	www: http://www.stepstn.com/

	[Entry up to date as of Sun Aug 31 1997.]

    Portable Object Compiler

	The Portable Object Compiler is available on Windows 95, Windows
	NT, OS/2, Macintosh MPW, Amiga, BeOS, Linux, Linux/Alpha, MkLinux,
	FreeBSD, NetBSD, AIX, SunOS, Solaris, NeXTstep, HP-UX, SGI IRIX,
	OSF/1 Digital UNIX, Apple Rhapsody, and CRAY Unicos.

	It's an Objective C compiler, developed under the GNU Library
	General Public License, compatible with several C compiler
	environments, including GNU cc, lcc, SGI cc, MIPSpro cc, HP-UX cc,
	AIX cc, DEC cc, SUN acc, DJGPP gcc, Cray cc, pgcc, WATCOM wcc,
	Microsoft Visual C, Metrowerks MWCPPC, Metrowerks/BeOS mwcc
	and IBM Visual Age icc.

	The Portable Object Compiler supports C and C++ as base languages,
	and comes with an extensive set of collection class objects,
	including objects for arbitrary precision arithmetic.  Most of the
	collection class objects compile on, and work with, Stepstone
	Objective C, GNU Objective C and NeXT Objective C.

	Additional interesting features of the compiler are support for
	Objective-C blocks, an option for garbage collection and an inline
	cache, pure C messenger.

	The Portable Object Compiler is obtainable from a few sites,

	 http://www.can.nl/~stes/compiler.html
	 http://cage.rug.ac.be/~stes/compiler.html
	 http://hydrogen.ch.ntu.edu.tw:8080/~david/OBJC/software.html.

	[Entry up to date as of Sun Jan 25 1998.]


16  Compiler differences

    This section lists differences between the various Objective-C
    compilers that are available.  The following compilers are compared:

    --------------------------------------------------------------------------
    Compiler	Description
    --------------------------------------------------------------------------
    gcc		GNU CC, version 2.8.0 or later
    apple	The (GNU CC based) compiler supplied with Rhapsody (previously
		known as the NeXT compiler supplied with NextStep/OpenStep).
    stepstone	Stepstone Objective-C compiler
    objc	Portable Object Compiler
    --------------------------------------------------------------------------

    The following table lists features of only the compiler and runtime
    library.  Any other libraries normally supplied or associated with the
    compiler are not taken into account.

    --------------------------------------------------------------------
    Feature				gcc	apple	stepstn	objc
    --------------------------------------------------------------------
    supports
      blocks (1)			no	no	no	yes
      categories			yes	yes	no	yes
        replacing methods		yes	yes	?	?
      garbage collection		no (3)	no	no	yes (2)
      Objective-C++			no	yes	no	yes
      protocols				yes	yes	no	no
      static method binding		no	no	yes	?
      static/automatic allocation	no	no	yes	?
      supports =-style class defs	no	no	?	yes
      method decls in implementation	no	no	?	yes
      multithreading			yes	yes	?	yes
      +initialize method		yes	yes	yes	yes
      +load method			yes	yes	?	?
      @"string objects"			yes	yes	no	no
      @class forward declarations	yes	yes	?	?
    source available			yes	no	no	yes
    method binding mechanism		sparse	class	?	inline
					array	cache		cache
    exception handling mechanism	yes	yes	?	yes
    --------------------------------------------------------------------

    notes
    1: Described as `Action Expressions' in
       http://www.virtualschool.edu/cox/CoxTaskMaster.html
    2: Using Hans Boehm's garbage collector (see question 9).
    3: Work in progress; some next release of gcc/ecgs will
       have support for using Boehm's garbage collector.


17  Objective-C support per platform

    Below is a list of compilers supporting Objective-C, and which are not
    NeXT's, Stepstone's or plain GCC.

    DOS, Windows, OS/2

	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 modeling 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: +1 510 795-6086
		fax: +1 510 795-8077
		email: info@bpg.com

	DJGPP

	    DJGPP includes Objective-C support [though I do not know to
	    which extent].

	    From the DJGGP homepage at http://www.delorie.com:

		DJGPP is a complete 32-bit C/C++ development system for
		Intel 80386 (and higher) PCs running DOS. It includes
		ports of many GNU development utilities. The development
		tools require a 80386 or newer computer to run, as do the
		programs they produce. In most cases, the programs it
		produces can be sold commercially without license or
		royalties.

	GCC/EMX

	    The EMX port of GCC implements Objective-C, and with RSX (or
	    RSXNT) it runs on DOS/DPMI boxes (or NT) too.

	    EMX is available for anonymous ftp on at following locations:

		ftp://ftp.uni-stuttgart.de/pub/systems/os2/leo/devtools/emx+gcc/
		ftp://ftp-os2.cdrom.com/pub/os2/lang/emxtools/
		ftp://ftp.leo.org/pub/comp/os/os2/devtools/emx+gcc/
		ftp://src.doc.ic.ac.uk/packages/os2/ftp.cdrom.com/lang/emxtools/

	    RSX is available from
	    ftp://ftp.uni-bielefeld.de/pub/systems/msdos/misc/
	    Without RSX EMX is limited to DOS/VCPI and OS/2 >=2.0)

	    [Thanks to <huug@poboxes.com> for this information.]

	GCC for Win32

	    Hyungjip Kim <hjkim@namo.co.kr> provides GCC 2.7.2 binaries
	    for Win32.  This GCC is modified to generate CodeView
	    compatible debug information, to enable the use of existing
	    Windows debuggers.  Visit http://www.darkland.co.kr/~hjkim/objc/
	    for more information, and the binaries.

	GNU-Win32

	    The GNU-Win32 project aims at providing the GNU tools
	    (including GCC) for Windows NT/95.  [I do not know to which
	    extent Objective-C is supported.]  Below is a description of
	    the project, taken from http://www.cygnus.com/misc/gnu-win32:

		The GNU-Win32 tools are ports of the popular GNU
		development tools to Windows NT/95 for the x86 and powerpc
		processors. Applications built with these tools have
		access to the Microsoft Win32 API as well as the Cygwin32
		API which provides additional UNIX-like functionality
		including UNIX sockets, process control with a working
		fork and select, etc...

		With these tools installed, it is now possible to:

		write Win32 console or GUI applications that make use of
		the standard Microsoft Win32 API and/or the Cygwin32 API.

		easily configure and build many GNU tools from source
		(including rebuilding the gnu-win32 development tools
		themselves under x86 NT).

		port many other significant UNIX programs to Windows NT/95
		without making significant changes to the source code.

		have a fairly full UNIX-like environment to work in, with
		access to many of the common Unix utilities (from both the
		bash shell and command.com).

	    The GNU-Win32 project is run by Cygnus; for more information
	    see http://www.cygnus.com.

    Macintosh

	Metrowerks

	    Preliminary Objective-C support for the MacOS will be included
	    in CodeWarrior Professional 1, to be released at the end of
	    May 1997.  [Entry written Thu May 15 1997.]

	    Metrowerks Corporation
	    2201 Donley Drive
	    Austin, TX 78758
	    http://www.metrowerks.com/
	    phone: 1-800-377-5416, +1 512 873 8313
	    email: info@metrowerks.com

	Tenon

	    Tenon CodeBuilder supports Objective-C.  This product is based
	    on GNU CC and targeted at the Power Macintosh.

	    http://www.tenon.com/products/codebuilder/


    Sun SPARC

	Sun

	    Sun's SPARCcompiler C++ 4.1 understands Objective-C, though it
	    does not come with the relevant include files and libraries.
	    At one time, Sun provided OpenStep on Solaris; it now seems
	    that they are trying very hard to forget that: Sun's WWW site
	    no longer provides any relevant information on either OpenStep
	    or Objective-C.


18  What are the newsgroups or mailing lists to read?

    Read comp.lang.objective-c, which is bound to discuss current events.
    Also, threads on comp.sys.next.programmer are Objective-C specific on
    occasion.

    There is a mailing list on GNU Objective-C: gnu-objc@gnu.ai.mit.edu.
    To subscribe to this list, send a mail with your request to
    `gnu-objc-request@gnu.ai.mit.edu.'

    There's a mailing list for Portable Object Compiler users. Send email to
    'objc-users-request@peti.gmd.de' to subscribe.

    GNUstep has several newsgroups devoted to it: gnu.gnustep.announce and
    gnu.gnustep.discuss to name but a few.

    Furthermore, the various kits that are being developed each come with
    their own mailing list.  See part 2 of this FAQ for more information.


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

    All Rhapsody/NextStep/OpenStep sites carry Objective-C material.
    To name a few:

	ftp://next-ftp.peak.org/pub/next/	(NEXTSTEP)
	ftp://ftp.leo.org/pub/comp/platforms/next/
	ftp://ftp.informatik.uni-muenchen.de
	ftp://ftp.gnustep.org/pub/		(GNUStep)
	ftp://ccrma-ftp.stanford.edu/pub/NeXT/	(MusicKit a.o.)

    See also part 2 of this FAQ.


20  Is there any information on the Net concerning Objective-C?

    Basic and related Objective-C (and/or NeXTSTEP) information is available
    at the following places:

    Apple Enterprise Group at http://www.enterprise.apple.com/ with the
    book at http://gemma.apple.com/techinfo/techdocs/rhapsody/apple.html

    Steve deKorte's Objective-C page at
    http://www.slip.net/~dekorte/Objective-C/,

    Brad Cox's Objective-C page at
    http://www.virtualschool.edu/mon/Cox/ObjectiveC.html,

    the GNUStep project at http://www.gnustep.org/, with a mirror at
    http://www.nmr.embl-heidelberg.de/GNUstep

    the libobjects FAQ at
    ftp://ftp.cs.rochester.edu/pub/u/mccallum/libobjects/Gnustep-FAQ.html,

    the NEXTSTEP/OpenStep Information Service at http://www.stepwise.com/,

    the eduStep initiative at http://www.nmr.embl-heidelberg.de/eduStep/,

    Cetus Links (http://www.rhein-neckar.de/~cetus/) has many links on OO;
    Objective-C is on http://www.rhein-neckar.de/~cetus/oo_objective_c.html,

    Nelson Minar's Objective-C page at
    http://www.santafe.edu/projects/swarm/ObjC/objective-c.html

    Tiggr's Objective-C page at http://www.ics.ele.tue.nl/~tiggr/objc/,

    Portable Object Compiler homepage at http://www.can.nl/~stes/compiler.html

    Norihiro Itoh's page at http://www.fsinet.or.jp/~nito/index-j.html,

    and of course the HTML versions of this FAQ and associated information
    at the addresses listed below.


21 For more information...

    Visit one of the places mentioned in #20, or see part 2 of this FAQ,
    Objective-C/classes a.k.a. the ClassWare Listing, for an [incomplete!]
    overview of available Objective-C classes and libraries.  See part 3 of
    this FAQ, Objective-C/sample a.k.a. the Simple Sample Program, for an
    example Objective-C program.


A World Wide Web hypertext version of this FAQ is maintained by Steve Dekorte
<dekorte@suite.com> at http://www.slip.net/~dekorte/Objective-C/.
A Japanese language version of this FAQ, maintained by Norihiro Itoh
<nito@fsinet.or.jp>, resides at http://www.fsinet.or.jp/~nito/index-j.html.

The early version of this FAQ was compiled by Bill Shirley, with the aid of
many people.  The current version is being maintained by Pieter Schoenmakers
<tiggr@gerbil.org>, aided by input from a lot of people, including: Per
Abrahamsen, Paul Burchard, Brad Cox, Christopher Lozinski, Mike Mahoney, Jon
F. Rosen, Paul Sanchez, Lee Sailer, David Stes, Paul Sanchez, Bill Shirley,
Subrata Sircar, Ted Slupesky, Richard Stallman, and Kersten Krab Thorup.

Send your suggestions, additions, bug reports, comments and fixes to
tiggr@gerbil.org.

    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.

User Contributions:

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