Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!spool.mu.edu!howland.reston.ans.net!europa.eng.gtefsd.com!uunet!mcsun!sun4nl!tuegate.tue.nl!krait.es.ele.tue.nl!tiggr From: tiggr@es.ele.tue.nl (Tiggr) Newsgroups: comp.lang.objective-c,comp.answers,news.answers Subject: comp.lang.objective-c FAQ, part 1/3: Answers Supersedes: Followup-To: comp.lang.objective-c Date: 17 Aug 1993 10:09:53 GMT Organization: Eindhoven University of Technology, the Netherlands Lines: 528 Approved: news-answers-request@mit.edu Expires: 26 Sep 1993 10:09:52 GMT Message-ID: Reply-To: tiggr@es.ele.tue.nl (Tiggr) NNTP-Posting-Host: krait.es.ele.tue.nl Summary: This first part of the comp.lang.objective-c FAQ postings tries to answer all your Objective-C questions. Originator: tiggr@krait.es.ele.tue.nl 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 @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 obj2 = obj1; // illegal, obj1 does not conform to the TargetAction protocol. id 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 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 lozinski@cup.portal.com 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 rms@gnu.ai.mit.edu. 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 (203) 426-1875 - (800) BUY-OBJEct voice / (203) 270-0106 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 +1-617-876-3296 General questions about the GNU Project can be asked to gnu@prep.ai.mit.edu. GNU CC comes with an Objective-C compiler and runtime library which includes the Object class. For information on how to order GNU software on tape, floppy, or cd-rom, check the file etc/DISTRIB in the GNU Emacs distribution, or e-mail a request to: gnu@prep.ai.mit.edu GNU software is available from the following sites: Asia: utsun.s.u-tokyo.ac.jp:/ftpsync/prep, ftp.cs.titech.ac.jp, cair.kaist.ac.kr:/pub/gnu Australia: archie.oz.au:/gnu Europe: ftp.informatik.tu-muenchen.de, src.doc.ic.ac.uk:/gnu, nic.funet.fi:/pub/gnu, ugle.unit.no, isy.liu.se, nic.switch.ch:/mirror/gnu, archive.eu.net, ftp.informatik.rwth-aachen.de:/pub/gnu, ftp.stacken.kth.se, ftp.win.tue.nl, ftp.denet.dk, ftp.eunet.ch, irisa.irisa.fr:/pub/gnu United States: wuarchive.wustl.edu, ftp.cs.widener.edu, uxc.cso.uiuc.edu, col.hp.com:/mirrors/gnu, gatekeeper.dec.com:/pub/GNU, ftp.uu.net:/systems/gnu 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: (510) 795-6086 fax: (510) 795-8077 email: lozinski@cup.portal.com 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, gnu-objc@gnu.ai.mit.edu, discussing this very topic. To subscribe to this list, send a mail with your request to `gnu-objc-request@gnu.ai.mit.edu.' 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 `tiggr@es.ele.tue.nl'. -- --Tiggr