Nebula 1995 August/NEBULA.mdf/Documents/FAQ/Objective-C-faq/answers

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From: [e-mail] (Tiggr)
Newsgroups: comp.lang.objective-c,comp.answers,news.answers
Subject: comp.lang.objective-c FAQ, part 1/3: Answers
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Date: 17 Aug 1993 10:09:53 GMT
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Summary: This first part of the comp.lang.objective-c FAQ postings
        tries to answer all your Objective-C questions.
Originator: [e-mail]
Xref: senator-bedfellow.mit.edu comp.lang.objective-c:1503 comp.answers:1638 news.answers:11484

Archive-name: Objective-C/answers
Version: $Id: answers,v 1.6 1993/08/06 12:50:51 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.

Q   What is Objective-C?

A   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.

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

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

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

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

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

A   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.

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

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

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

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

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

      * The basic class hierachy, 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.  This is handled via special
    methods `forward::' and a special class (on NeXT named NXProxy)
    which must be used for such purposes.

      * 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.

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

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

    Typing information is available at run time.  [Prime example:
    NeXTSTEP's `-[Application loadNibSection:owner:]'.]

    Persistence [...].

    Remote objects [...].

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

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

A   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 .m
    file, and defining access methods for it.  This is actually a more
    desirable way of designing a class hierarchy, because it allows
    subclasses shouldn't access superclass storage (this would cause the
    subclass to break if the superclass was reimplemented), and allows
    the subclass to override the storage (if the classes access all
    their own variables via methods).

    [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?]

    You can't forward-declare classes.

    In NeXTSTEP 3.0 and GNU CC 2.4, you can:

        @class Foo;

    declares the class Foo.  [Stepstone?]

Q   What are Protocols?

A   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 3.0 and GCC 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: (NXTypedStream *) stream;
    -write: (NXTypedStream *) stream;
        @end

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

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

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

    MyClass *obj1 = [MyClass new];

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

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

    [The `illegal' remark in the comment is a bit far-sought.  The real use
    of declaring an ID to conform to some protocol is that it can help the
    compiler to resolve method name conficts:

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

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

    -blah1: d
    {
      id t = [d someMethod];

      do_something_with ([t type]);
    }

    -blah2: d
    {
      id <Barf> 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 to expect from `[t
    type]', since it has seen both declarations of `-type'.  In method
    `-blah2:', it knows that `t' conforms to the `Barf' protocol and thus
    that `t' implements the `-type' method returning a `const char *'.]

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

A   NeXT extended Stepstones 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 class libraries that work across all
    supported machines.  NeXTSTEP comes with its own set of libraries
    (called `kits').  GNU libobjc.a currently only includes the `Object'
    class.

    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]

    NeXTSTEP 3.0 and GCC 2.4 support protocols and forward declarations of
    classes.  [And Stepstone?]

Q   What written information concerning Objective-C is available?

A   Books:

    Object Oriented Programming: An Evolutionary Approach

        Author:         Brad J. Cox (, Andrew J. Novobilski second edition)
        Publisher:      Addison-Wesley
        ISBN#:          0-201-10393-1   August, 1986
        ISBN#:          0-201-54834-8   1991

    Objective-C: Object Oriented Programming Techniques

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

    An Introduction to Object-Oriented Programming

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

    NeXTSTEP Programming  Step ONE: Object-Oriented Applications

        Authors:        Simson L. Garfinkel, Michael K. Mahoney
        Publisher:      TELOS/Springer-Verlag, 1993   (800)SPR-INGE
        Availablility:  First printing (6000 copies) sold out in 3 months.
        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 progrects 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].

Q   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 UNIX SYS V
            Sun 3, 4, SPARCstations (SunOS)
            VAX (VMS)

        Class libraries are available for a subset of the above.
        [Which?]

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

Q   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 3.0
        NEXTSTEP 3.1 for NeXT Computers

        NEXTSTEP 3.1 for Intel Processors
        Released on May 25 1993.
        User                $ 795
        Developer            $1895
        Educational (U+D)        $ 295
        Evaluation Kit
          (U+D, no docs/upgrade)    $ 295

        ObjectWare Catalogue of available classes, both commercial and
        freely available.

    NeXT, Inc.
    [address]

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

A   The current distribution of GNU CC (version 2.4.5) includes an
    Objective-C compiler and runtime library.  The latter includes the
    `Object' class.  Some people are working on the GNU libobjc.

    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

Q.  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]

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

A   Read comp.lang.objective-c, which is bound to discuss current events.
    There is also a mailinglist, [e-mail], discussing this
    very topic.  To subscribe to this list, send a mail with your request to
    `[e-mail].'

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

A   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]

    [What about other sites?]

Q   What class libraries are available for Objective C?

A   See the related FAQ file objc-faq/classes.

Q   So show me a program, a simple example.

A   See the related FAQ file objc-faq/sample.

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

Any text in between `[' and `]' is a comment.  Comments indicate problems
with this FAQ, which should be solved.  Send your suggestions, additions,
bug reports, comments and fixes to `[e-mail]'.
--
--Tiggr