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

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Pieter Schoenmakers

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Jul 17, 1998, 3:00:00 AM7/17/98

Archive-name: Objective-C/answers
Version: $Id: answers,v 3.62 1998/07/07 11:19:55 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 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/objc/gdb-gnu-objc.diff.gz.uue, accompanied
by ftp://ftp.ics.ele.tue.nl/pub/objc/gdb-gnu-objc.README.

Debugging Objective-C using a non-Objective-C aware gdb has been
documented by Martin Cracauer <crac...@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/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 <ama...@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 in...@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 in...@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 r...@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
g...@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: g...@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 -g...@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)
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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, NeXT, HP-UX, IRIX, OSF/1
Digital UNIX 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: in...@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 <hu...@poboxes.com> for this information.]

GCC for Win32

Hyungjip Kim <hj...@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: in...@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-...@gnu.ai.mit.edu.
To subscribe to this list, send a mail with your request to
`gnu-objc...@gnu.ai.mit.edu.'

There's a mailing list for Portable Object Compiler users. Send email to
'objc-user...@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.)