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Subject: comp.lang.objective-c FAQ, part 1/3: Answers
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Summary: This first part of the comp.lang.objective-c FAQ postings
    tries to answer all your Objective-C questions.
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Archive-name: Objective-C/answers
Version: $Id: answers,v 2.11 1994/01/26 15:21:25 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 exactly is it that makes Objective-C have `classes similar to
     Smalltalk', and what are the resulting capabilities of Objective-C?
 4   What are the `nice features' of Objective-C?
 5   What are some of the common problems of the language and how can I work
     around them?
 6   What object encapsulation does Objective-C provide?
 7   What are Protocols?
 8   How can garbage collection be applied to Objective-C?
 9   What is the difference between the NeXTSTEP, Stepstone and GNU CC
     versions of Objective-C?
 10  What written information concerning Objective-C is available?
 11  What kind of Objective-C support is provided by Stepstone?
 12  What kind of Objective-C support is provided by NeXT?
 13  What kind of Objective-C support is provided by GNU?
 14  What kind of Objective-C support is provided by BPG.
 15  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?
 16  Are there any FTP sites with Objective C code?  Where?
 17  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 comlete example
    of the application of the constructs, 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 Q19)) 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.  A category is not allowed to redefine a
    class' existing 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 type checking.

    `-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.

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.

3   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'.)

    The big difference of course is that Objective-C does hybrid typing:
    one can choose to represent a string as a `char *' or as an object,
    whereas in Smalltalk, everything is an object.  This is a 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.

      * You may add or delete methods and classes at runtime.  (On GNU and
    NeXT one can load new classes and categories.  On Stepstone, which
    lacks categories, the only way to add methods is to load a subclass
    which then does a `+poseAs:' of the class to have methods added.
    This is less flexible, but it sort-of does the trick of just adding
    methods.)

      * 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.

4   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: Full type information (name and type
    information of methods and instance variables and type information
    of method arguments) is available at run time.  A prime example of
    application of this feature is `-loadNibSection:owner:' method of
    NEXTSTEP's Application class.

    Persistence [...].

    Remote objects [...].

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

5   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?]

6   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 @protected.
    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.  This
    information is for instance used to do instance variable lookup by
    NeXTSTEP's `loadNibSection:owner:' method, 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.

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   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.

9   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.

10  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 [e-mail]

    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 [e-mail].

11  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
    [phone] - (800) BUY-OBJEct voice / [phone] fax
    75 Glen Road
    Sandy Hook, CT 06482

12  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 3.2 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: [phone]
    email: [e-mail]

    [Note: The World of NEXTSTEP is moving rapidly: HP-PA NEXTSTEP has
    been demoed (available mid 94), Sparc NEXTSTEP is being worked on
    (available late 94?).  OpenStep has been defined and made available.
    This specification includes Objective-C, DPS, PDO, PB/IB.  OpenStep
    will be available on Solaris (when?).]

13  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 2.5.8) 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
    [phone]

    General questions about the GNU Project can be asked to
    [e-mail].

    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: [e-mail]

    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

14  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: [phone]
    fax: [phone]
    email: [e-mail]

15  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, [e-mail], discussing this
    very topic.  To subscribe to this list, send a mail with your request to
    `[e-mail].'

16  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.

17  For more information...

    See part 2 of this FAQ, Objective-C/classes a.k.a. the ClassWare
    Listing, for an 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.

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, Jon F. Rosen, Paul Sanchez, Lee Sailer,
Bill Shirley, Subrata Sircar, Ted Slupesky, Richard Stallman and Kersten
Krab Thorup,

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