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Archive-name: eiffel-faq Posting-Frequency: approximately monthly Last-modified: 04 Apr 1997 EIFFEL: FREQUENTLY ASKED QUESTIONS ---------------------------------- This question-and-answer list is posted monthly to the Usenet newsgroups comp.lang.eiffel, comp.answers and news.answers. Please send corrections, additions and comments to Conrad Taylor (cwtaylor@tasc.com). This information is abstracted and condensed from the posts of many contributors to comp.lang.eiffel, supplemented by information from vendors. No guarantees are made regarding its accuracy. This compilation is by Conrad Taylor. Distribution over the Internet or by electronic mail is unrestricted. Other use requires my permission. You can receive the latest copy by anonymous file transfer from: ftp://ftp.cm.cf.ac.uk/pub/eiffel/eiffel-faq ftp://rtfm.mit.edu/pub/usenet/news.answers/eiffel-faq or by sending an email message to mail-server@rtfm.mit.edu with this message body: send /pub/usenet/news.answers/eiffel-faq ---------- CONTENTS Changes since the last posting: What's New: N01) Announce Visual Eiffel for Windows 95/NT N02) ISE Eiffel 4 for Windows N03) The long awaited "Object Oriented Software Construction 2ed" by Bertrand Meyer Frequently Asked Questions: Q01) What is Eiffel? Q02) Where did Eiffel come from? Q03) What Eiffel products are available? Q04) Is Eiffel available for free or as shareware? Q05) Is there an archive of the comp.lang.eiffel newsgroup? Q06) What is Sather? How does it compare to Eiffel? Q07) What books are available for learning about Eiffel? Q08) Are any magazines or newsletters available concerning Eiffel? Q09) Where can I find Eiffel on the World-Wide-Web? Q10) Where can I get an Eiffel editor or emacs-mode? Q11) What is BON? Q12) How large are typical Eiffel executables? Q13) Are there standards for the Eiffel language? Q14) How fast do Eiffel applications run? Q15) Are there any Eiffel user groups? Q16) Where can I get Eiffel products and services? Q17) Are there any conferences for Eiffel users? Q18) Why do most Eiffel implementations compile to C? Language Issues: L01) What features does Eiffel have? L02) What changes have been made to the Eiffel language definition? L03) What libraries come with Eiffel? L04) What's the big deal about preconditions and postconditions? L05) Please explain and discuss covariance vs. contravariance. L06) Is it true that there are "holes" in the Eiffel type system? L07) Is there support for concurrency in Eiffel? L08) Why doesn't Eiffel allow function overloading? L09) Why are there no procedural types in Eiffel? L10) Why are there no class attributes in Eiffel? L11) How can I call the parent-class version of a redefined routine? L12) Where can I find a comparison between Eiffel and C++? L13) Are there any destructors in Eiffel? N01) Announce Visual Eiffel for Windows 95/NT The Visual Eiffel for Windows 95/NT is current available for downloading for a limited evaluation from http://www.eiffel.com. If you have any questions, please send e-mail to info@eiffel.com or call (805) 685-1006. ---------- N02) ISE Eiffel 4 for Windows Here it is! ISE Eiffel 4 for Windows is now available for downloading. Just get it from our site or one of the mirrors, try it for free. You'll love it! What you will get ISE Eiffel 4 is the latest in the acclaimed ISE Eiffel product line. In the downloadable Windows version you will get: o The acclaimed EiffelBench environment for fast incremental recompilation (melting), C code generation, finalization of optimized executables, graphical browsing, automatic documentation tools (including generation of HTML, RTF, MML and other formats), debugging, object inspection etc. o Extensive on-line documentation. o Extensive reusable libraries (several hundred classes!): EiffelBase, the most thoroughly designed library covering fundamental structures of software engineering, from stacks and queues to files and iterators; WEL, the Windows Eiffel Library, for building robust graphical applications taking full advantage of the Windows API. These libraries come precompiled. o C and C++ interface. o The Eiffel Resource Bench: generate a Windows graphical application, using WEL, from a resource file produced by a Windows GUI builder such as Microsoft Developer's Studio. If you have any questions, please send e-mail to info@eiffel.com or call (805) 685-1006. ---------- N03) The long awaited "Object Oriented Software Construction 2ed" by Bertrand Meyer Overview Object-Oriented Software Construction, second edition is the comprehensive reference on all aspects of object technology, from design principles to O-O techniques, Design by Contract, O-O analysis, concurrency, persistence, abstract data types and many more. Written by a pioneer in the field, contains an in-depth analysis of both methodological and technical issues. Comes with a CD-ROM containing: the complete hyperlinked text, for easy reference; software to read the text on major industry platforms; supplementary material (reusable components, mathematical complements); and a complete graphical O-O development environment supporting the concepts of the book. Ordering Information Title: Object-Oriented Software Construction, second edition, 1997, 1300 pp. Author: Bertrand Meyer, ISE Inc., Santa Barbara, CA ISBN: SBN 0-13-629155-4 Contacting ISE WWW Site: http://www.eiffel.com E-mail: info@eiffel.com Phone: (805) 685-1006 ---------- Q01) What is Eiffel? Eiffel is an advanced object-oriented programming language that emphasizes the design and construction of high-quality and reusable software. Eiffel is not a superset or extension of any other language. Eiffel strongly encourages OO programming and does not allow dangerous practices from previous generation languages although it does interface to other languages such as C and C++. Eiffel supports the concept of "Design by Contract" to improve software correctness. Beyond the language aspect Eiffel may be viewed as a method of software construction. Eiffel is an excellent vehicle for software education, including for a first programming course. ---------- Q02) Where did Eiffel come from? Eiffel was created by Bertrand Meyer and developed by his company, Interactive Software Engineering (ISE) of Goleta, CA. Dr. Meyer borrowed on his extensive experience with OOP, particularly with Simula. He also added in important concepts from his academic work on software verification and computer language definition. Eiffel's design addresses many practical concerns that software engineers face when creating complex software. Eiffel has evolved continually since its conception on September 14, 1985 and its first introduction in 1986. Eiffel is named after Gustave Eiffel, the engineer who designed the Eiffel Tower. ---------- Q03) What Eiffel products are available? Commercial Eiffel compilers, libraries and tools are available from the following vendors and their resellers: Interactive Software Engineering Inc (ISE Eiffel) Tower Technology Corporation (TowerEiffel) SIG Computer GmbH (Eiffel/S) Jan Willamowius (Eiffel adaption of SNiFF+ Programming Environment) The following platforms are supported by one or more of the above vendors: PC: DOS, OS/2, Windows 3.1, Windows 95, Windows NT, PC Unix (Interactive, SCO, and ESIX), Nextstep, Linux OTHER HARDWARE: Sparc (SunOS & Solaris), NeXTStep, HP9000, DEC 5xxx, Sony News, DG Aviion, DEC Alpha OSF-1, DEC OpenVMS, RS6000, Pyramid, QNX, Silicon Graphics, Macintosh (Motorola & PowerPC) Special offers are available on many of these products for personal or educational use. Further information about these products is available from the vendors by email and on the world-wide-web. See Q16 for contact details and websites. ---------- Q04) Is Eiffel available for free or as shareware? All of Eiffel - All graphical - All for free! FREE EIFFEL FOR WINDOWS is a free version of ISE's acclaimed Melting Ice Technology compiler and environment for Eiffel. No strings attached -- it's FREE! FREE EIFFEL FOR WINDOWS is now fully graphical, showing the power of the EiffelBench set of advanced development tools for fast compilation, documentation, browsing, debugging and much more! The earlier text-based environment is still available, but we think you'll want to try the graphical environment right away! Technical support for FREE EIFFEL FOR WINDOWS is available. Details are included in the delivery, or contact us. Also of interest Upgrade to Personal or Professional Eiffel for Windows for an unbeatable price and get printed documentation, more tools, more libraries, support and more. Also note our highly affordable ISE Eiffel for Linux , supporting the entire set of ISE Eiffel mechanisms for the fastest growing version of Unix. And of course ISE Eiffel is available for the major platforms in the industry, from Unix to VMS with more to come. Getting familiar with the environment On-line documentation is included in the delivery. But please do us a favor: if you are new to ISE Eiffel, please take a few minutes (now or later, but before you start using the environment) to consult the on-line introduction to EiffelBench. The powerful interaction techniques of ISE Eiffel are innovative and have been known to startle newcomers. We are sure you will love them, but first you must read about the essential ideas. Downloading FREE EIFFEL 4.0 FOR WINDOWS Note: if you want the graphical version (who doesn't?) do NOT download it from SimTel or any of the other sites. At the moment only ISE's site has FREE GRAPHICAL EIFFEL FOR WINDOWS. Be sure to choose the version for the appropriate variant of Windows, as listed below: Windows 95, Windows NT or Windows 3.1. The Windows 3.1 version will NOT work under 95 or NT. To download the FREE GRAPHICAL EIFFEL FOR WINDOWS, go to the ISE Web Site: From the WWW: http://www.eiffel.com Directly from FTP: ftp.eiffel.com SIG Computer's "Eiffel/S 1.3s" is a shareware version of their Eiffel compiler for DOS and Unix and is available by FTP from SimTel mirror sites in SimTel/msdos/eiffel, or from the UWCC archive at ftp//ftp.cm.cf.ac.uk/pub/Eiffel/SIG/Eiffel-S-1.3/ The EON Eiffel compiler is shareware under MS-DOS and Linux, and a beta-test version is available from ftp://ftp.demon.co.uk/pub/eiffel/eon-eiffel/ SmallEiffel is a free Eiffel compiler distributed under the terms of the GNU General Public License as published by the Free Software Foundation. You can download SmallEiffel at ftp://ftp.loria.fr/pub/loria/genielog/SmallEiffel The following Eiffel archive sites allow anonymous file transfer: ftp://ftp.tu-clausthal.de/pub/atari/languages/eiffel/vici_102.lzh The Atari ST interpreter referred to above. ftp://ftp.cm.cf.ac.uk/pub/eiffel University of Wales. Contains the latest version of this FAQ, plus the front-end parser (ep) and various public domain classes. To contribute, contact Ted Lawson (ted@cm.cf.ac.uk). ftp://ftp.fu-berlin.de/pub/unix/languages/heron This is an Eiffel front-end parser (HERON) in the public domain, created by Burghardt Groeber and Olaf Langmack of the Freie Universitaet in Berlin. ftp://ftp.informatik.uni-stuttgart.de/pub/eiffel Falkultaet Informatik der Universitaet Stuttgart, Germany. Contains a compiler generator, several encapsulations, a pretty-printer for Eiffel/S, and some utility classes. To contribute, contact Joerg Schulz (schulz@adam.informatik.uni-stuttgart.de). ftp://utarlg.uta.edu/CSE/EIFFEL UT-Arlington, USA. Contains some code from Eiffel Outlook back issues. SIG Computer produces a CD-ROM containing most of the available freeware, shareware and public domain Eiffel material. ---------- Q05) Is there an archive of the comp.lang.eiffel newsgroup? Yes, at the following FTP sites: ftp://wuarchive.wustl.edu/usenet/comp.lang.eiffel/ and on the WWW at http://www.cm.cf.ac.uk/CLE/ ---------- Q06) What is Sather? How does it compare to Eiffel? Sather is an OO language, originally patterned after Eiffel but now very different, created at ICSI of Berkeley, CA. Sather does not support Design by Contract, but has some other interesting features. See the usenet newsgroup comp.lang.sather. ---------- Q07) What books are available for learning about Eiffel? The classic text for learning about Eiffel (and OO programming in general) is Dr. Meyer's "Object Oriented Software Construction". Although the language has evolved significantly since the book was published, the presentation of the basic problems and solutions which motivate the OO mind set are still quite compelling. This is the book to get if you are new to the object-oriented world (Prentice Hall, ISBN 13-629031-0). Available in French as "Conception et Programmation par Objets" (90/10 InterEditions, ISBN 2-7296-0272-0) and in German as "Objektorientierte Softwareentwicklung (Hanser, ISBN 3-446-15773-5). Also by Dr. Meyer, "Eiffel: The Language", combines an introduction to Eiffel, the language reference, and a good deal of philosophy into its 600 pages. This is a rigorous and comprehensive book which some readers may find heavy going despite Dr. Meyer's clarity of expression. It is the definitive language reference, and essential reading for all serious Eiffel users. Get the second or later printing (same ISBN), which includes many corrections and changes (there is not a second edition, and none is currently underway) (Prentice Hall, ISBN 13-247925-7). Available in French as "Eiffel, le langage" (94/10 InterEditions, ISBN 2-7296-0525-8). Dr. Meyer and Jean-Marc Nerson have edited "Object-Oriented Applications". It includes an introduction to Eiffel technology followed by seven in-depth descriptions of large applications written in Eiffel. (Prentice Hall, ISBN 13-013798-7) Robert Switzer has written "Eiffel: An Introduction". This is a very clear and concise Eiffel primer, with many code fragments and two substantial Eiffel applications. (Prentice Hall, ISBN 13-105909-2). Available in French as "Introduction a Eiffel" (Masson, ISBN 2-225-84-656-1) ISE distributes a set of 6 video lectures entitled "Object-Oriented Software Construction", taught by Bertrand Meyer. These provide an overall introduction to the method and use ISE Eiffel 3 to illustrate the concepts. Frieder Monninger's book "Eiffel: Objektorientiertes Programmieren in der Praxis" is a very down-to-earth Eiffel handbook in German. (Heise, ISBN 3-88229-028-5). Bertrand Meyer's "Reusable Software: The Base Object-Oriented Component Libraries" (Prentice Hall, ISBN 0-13-245499-8, about 530 pages) describes principles of library design and the taxonomy of fundamental computing structures. Serves as a manual for the EiffelBase libraries. Bertrand Meyer's "An Object-Oriented Environment: Principles and Application" (Prentice Hall, ISBN 0-13-245507-2, 240 pages) describes the ISE EiffelBench environment as well as the "Melting Ice" compilation technology and the EiffelBuild GUI application builder. Richard Wiener's "Software Development Using Eiffel: There can be life other than C++" (Prentice-Hall, ISBN 0-13-100686-X) is a useful book (full of serious code samples) for those with a grounding in another OO language. A book by Kim Walden and Jean-Marc Nerson, "Seamless Object-Oriented Software Architecture: Analysis and Design of Reliable Systems", describes the BON method in detail (Prentice Hall, ISBN 0-13-031303-3). Pete Thomas and Ray Weedon's "OO Programming in Eiffel" is a very comprehensive Eiffel tutorial and textbook, with a solid "Abstract Data Type" approach (Addison-Wesley, ISBN 0-201-59387-4). Another book called "OO Programming in Eiffel" is by Robert Rist and Robert Terwilliger, and is a textbook with an emphasis on design. (Prentice-Hall, ISBN 0-13-205931-2). Bertrand Meyer's "Object Success" is a manager's guide to object orientation, its impact on the corporation and its use for re-engineering the software process (Prentice-Hall, ISBN 0-13-192833-3). Macmillan publishes John Tyrrell's inexpensive and very approachable textbook "Eiffel Object-Oriented Programming" (ISBN 0-333-64554-5). "Object-Oriented Software Engineering with Eiffel" By Jean-Marc Jezequel, Addison-Wesley Eiffel in Practice Series ISBN 0-201-63381-7 * Paperback * 368 pages * ) 1996 "Object Technology for Scientific Computing Object-Oriented Numerical Software in Eiffel and C" by Paul Dubois, (Prentice Hall and its editor for the Object Oriented Series, (ISBN 0-13-267808-X * paper * 350 pages * Accompanying CD with the Free Eiffel for UNIX & Linux environments) There is also a white paper titled 'Eiffel: A Manager's Perspective' which provides a quick introduction to Eiffel and the benefits it brings to the software development process. For a free copy, send your name and postal address to tower@twr.com ---------- Q08) Are any magazines or newsletters available concerning Eiffel? Eiffel Outlook is a bi-monthly journal devoted to Eiffel, published since 1991. Topics cover all areas of interest to the Eiffel community. Subscriptions, trial subscriptions and back issues are available from: SIG Computer in Germany Everything Eiffel in the UK & France Simon Parker in Ireland IMSL in Japan Enea Data in Sweden Tower Technology in the USA and all other countries Details are available at <http://www.cm.cf.ac.uk/Tower/Outlook.html> or by sending email to <outlook@twr.com>. ISE produces a newsletter "Eiffel World". Subscriptions are free (email your request to info@eiffel.com). Individual copies are available from: Cybertech in Argentina Class Technology in Australia Jay-Kell Technologies in Canada SOL in France SIG Computer in Germany Eiffel Ireland in Ireland Etnoteam in Italy Information & Math Science Lab Software Research Associates in Japan Chromasoft in Mexico Science OO Products & Systems in the Netherlands Objective Methods in New Zealand Ruperez & Associates in Spain Enea Data in Sweden Abstraction in Switzerland Everything Eiffel in the UK Also, EiffelWorld is now available on-line! This is your first quarterly reminder to visit the EiffelWorld Online magazine at http://www.eiffel.com/doc/eiffelworld/5.2 If you're interested in Software Design Patterns you may like to subscribe to the Eiffel Patterns mailing list. Send email to: e-patterns-request@cbr.dit.csiro.au ---------- Q09) Where can I find Eiffel on the World-Wide-Web? An Eiffel home page is held on the University of Wales College of Cardiff's server at http://www.cm.cf.ac.uk/CLE/. Vendor websites are listed in Q16. ---------- Q10) Where can I get an Eiffel editor or emacs-mode? Franck Arnaud's Eiffel extension to the Windows/WindowsNT programmers editor Codewright from Premia allows you to see Eiffel code in colour, has smart indenting and a few templates. Available by anonymous FTP from ftp://ftp.cm.cf.ac.uk/pub/eiffel/tools/cweiffel.zip The WINEDIT shareware programmer's editor offers colour syntax highlighting, works with Eiffel/S under MS-Windows, and is available from: ftp://src.doc.ic.ac.uk/computing/systems/ibmpc/windows3/programr/we-30d.zip and ftp://gatekeeper.dec.com/.f/micro/msdos/win3/programr/we-30d.zip Alan Philips' free Programmers File Editor also works with Eiffel/S under MS-Windows, has templates but not syntax highlighting, available from ftp://ftp.demon.co.uk/pub/ibmpc/windows/editors/pfe0507.zip Tower Technology Corporation supplies an Eiffel 3 emacs mode that supports syntax-directed highlighting, auto-indentation and is easily customized for font use, color and indentation amounts. It comes as part of the TowerEiffel system, but is also available free for anyone who requests it. Send email to elisp@atlanta.twr.com to get the latest version. ---------- Q11) What is BON? BON ("Business Object Notation") is a method for high-level analysis and design, offering a seamless reversible transition to an Eiffel implementation. The method emphasizes Design by Contract and systematic development. ISE supports BON with its EiffelCASE tool. ---------- Q12) How large are typical Eiffel executables? (How large are typical C executables?) In general, the size of an executable depends on the compiler used. Thus, a good Eiffel compiler would produce good C code by removing dead code such that a C executable is not smaller than an Eiffel executable. This is true of SmallEiffel and ISE Eiffel compilers. Also, I think this is similar for others like Tower Eiffel and SIG Eiffel compilers. Interestingly, Eiffel applications seem to grow less rapidly as new capabilities are added. Reuse does help out tremendously in this regard. A good Eiffel compiler allows large applications to be smaller than equally functional applications written in C. Note that leaving assertion checking in the code increases the size of applications a lot. Despite this, many of us prefer that they remain throughout development. Some even deliver a PRECONDITIONS-only version of their applications to their early customers. ---------- Q13) Are there standards for the Eiffel language? The definition of the Eiffel language is in the public domain. This definition is controlled by NICE, the Non-profit International Consortium for Eiffel. This means that anyone or any company may create a compiler, interpreter, or whatever having to do with Eiffel. NICE reserves the right to validate that any such tool conforms to the current definition of the Eiffel language before it can be distributed with the Eiffel trademark. (i.e. advertised as an "Eiffel" compiler.) The Eiffel trademark is owned and controlled by NICE. NICE is using Bertrand Meyer's book, "Eiffel: The Language" (2nd Printing), as the initial definition of the language. The NICE board of directors for 1995 consists of Christine Mingins (chair), Bertrand Meyer (treasurer), Simon Parker (secretary) and Paul Johnson. In June 1995 NICE published the first version (called "Vintage 95") of the Eiffel Library Standard. Those parts of an Eiffel application that use only the standard classes and features should run with minimal change on any compiler supporting ELS-95. NICE (Nonprofit International Consortium for Eiffel) 45 Hazelwood Shankill Co Dublin Republic of Ireland TEL: +353 1 282 3487 email: nice@atlanta.twr.com ---------- Q14) How fast do Eiffel applications run? Early versions of Eiffel were slow. Recent implementations have improved dramatically. However, to achieve maximum performance under any Eiffel implementation, run-time assertion monitoring must be switched off. (But see Q12 on when to switch assertions on or off.) It's hard to generalise, but compared to C++, simple computation-intensive applications will run perhaps 15% slower. Large applications are often dominated by memory management rather than computation. ISE recently demonstrated that by simply adding a call to the garbage collector's "full-collect" routine at a time when there were known to be few live objects, performance became dramatically faster than a corresponding C++ version. ---------- Q15) Are there any Eiffel user groups? International Eiffel User Group Darcy Harrison - Attention: IEUG ISE Inc. 270 Storke Road, Suite 7 Goleta, CA 93117, USA TEL (805) 685-1006 FAX (805) 685-6869 darcyh@eiffel.com UK & Ireland Eiffel Interest Group (currently inactive) GUE, Groupe des Utilisateurs Eiffel (France) Jean-Marc Nerson 104 rue Castagnary, 75015 Paris TEL +33 1 45 32 58 80 FAX +33 1 44 32 58 81 marc@eiffel.fr (meets every 2 months or so) TowerEiffel User's Group Private cyberspace mailing list for supported Tower customers with meetings coinciding with major OO Conferences and Events. ---------- Q16) Where can I get Eiffel products and services? These vendors, resellers and suppliers of Eiffel training and consultancy are listed in alphabetical order: Argentina Cybertech Systens Integration for CIM Suarez 1281, Third Floor,Apt.A CP-1288 Buenos Aires TEL +54 1 28 1950 FAX +54 1 322 1071 or 963 0070 Australia Class Technology Pty. Ltd. PO Box 6274 North Sydney NSW 2060 TEL +61 2 9922 7222 FAX +61 2 9922 7703 email eiffel@class.com.au http://www.class.com.au/ Canada Jay-Kell Technologies, Inc. 48 Lakeshore Road, Suite #1 Pointe Claire, Quebec Canada H9S 4H4 TEL +51 4 630 1005 FAX +51 4 630 1456 France SOL 104 rue Castagnary 75015 Paris TEL +33 1 45 32 58 80 FAX +33 1 45 32 58 81 email eiffel@eiffel.fr Germany Feinarbeit Altenbraker Strasse 4 D-12053 Berlin TEL +49 30 6215827 FAX +49 30 6215863 email langmack@netmbx.netmbx.de SIG Computer GmbH Zu den Bettern 4 D 35619 Braunfels, Germany TEL +49 6472 2096 FAX +49 6472 7213 email eiffel@eiffel.de www http://www.sigco.com/ Jan Willamowius Semperstr. 1 D-22303 Hamburg Hamburg, Germany Tel: +49 40-2806209 Email: jan@janhh.shnet.org WWW: http://swt1.informatik.uni-hamburg.de/~1willamo/dl.html Halstenbach ACT GmbH Dr. Joachim Wolffram Breidenbrucher Strasse 2 D-51674 Wiehl-Bomig Tel.: +49 2261 9902-0 email: info@halstenbach.de WWW: http://www.halstenbach.de Ireland Eiffel Ireland 45 Hazelwood Shankill Co Dublin TEL +353 1 282 3487 email sparker@eiffel.ie www http://www.eiffel.ie/Eiffel/ India Sritech Information Technology 744/51 2nd Floor 10 Mian Road, 4th Block Jayanagar, Bangalore, India 560011 TEL +91 812 640661 FAX +91 812 643608 Italy EtnoTeam Via Adelaide Bono Cairoli 34 20217 Milano TEL +39 2 261621 FAX +39 2 26110755 email sales@etnomi.it Japan Information and Math Science Lab Inc. 2-43-1, Ikebukuro, Toshima-ku Tokyo 171 email fushimi@idas.imslab.co.jp TEL +81 3 3590 5211 FAX +81 3 3590 5353 Software Research Associates 1-1-1 Hirakawo-Cho Chiyoda-ku Tokyo 102 TEL +81 3 3234 8789 FAX +81 3 3262 9719 email sugita@sra.co.jp Mexico Cromasoft SA de CV Mazatlan 161 Col Condesa, 06140 Mexico TEL +52 5 286 82 13 FAX +52 5 286 80 57 email claudio@croma.sunmexico.sun.com The Netherlands SOOPS Sarphatistraat 133 NL-1018 GC Amsterdam, The Netherlands TEL +31 20 525 6644 FAX +31 20 624 6392 email A731CISK@HASARA11.BITNET New Zealand Objective Methods Ltd PO Box 17356 (77 Chamberlain Rd) Karori, Wellington, New Zealand TEL +64 4 476 9499 FAX +64 4 476 9237 or 8772 email dkenny@swell.actrix.gen.nz Russia SIG Computer, Germany has a branch office in Moscow. email eiffel@sigcomp.msk.su Spain Eiffel Iberica Aptdo 1646 20080 San Sebastian TEL +34 943 471906 email ean@sc.ehu.es Ruperez & Associates c/San Bartolome 21, 5F 20001 San Sebastian TEL +34 43 461801 email jipferur@si.ehu.es Sweden Enea Data Box 232, Nytorpsvagen 5 S-183 23 Taby TEL +46 8 792 25 00 FAX +46 8 768 43 88 email eiffel@enea.se Switzerland Abstraction 18 Faubourg de l'Hopital 2000 Neuchatel TEL +41.38.25.04.93 FAX +41.38.259.857 email silberstein@clients.switch.ch Objectif Concept Passage Cour-Robert 5 CH 1700 Fribourg TEL +41 37 232977 FAX +41 37 464889 United Kingdom Eon Software 19 Stapleton Road Heddington, Oxford OX3 7LX TEL +44 865 741452 email ian@eonsw.demon.co.uk Everything Eiffel 6 Bambers Walk Wesham PR4 3DG England TEL & FAX +44 1772 687525 email rogerb@eiffel.demon.co.uk United States of America Interactive Software Engineering, Inc 270 Storke Road, Suite 7 Goleta, CA 93117 TEL 805-685-1006 FAX 805-685-6869 email info@eiffel.com www http://www.eiffel.com/ Tower Technology Corporation 1501 Koenig Lane Austin, TX 78756 USA TEL 512-452-9455 FAX 512-452-1721 email: tower@twr.com www http://www.twr.com/ ZumaSoft 6235 Paseo Canyon Drive Malibu, California 90265, USA TEL & FAX +1 310 457-6263 email tstevens@netcom.com ---------- Q17) Are there any conferences for Eiffel users? The conferences listed here are not just for Eiffel. Eiffel shares the spotlight with other OO languages including C++ and Smalltalk. Feb 26 - 29 1996 TOOLS Europe, Paris France Jul 29 - Aug 2 1996 TOOLS USA, Santa Barbara California TOOLS is the major international conference devoted to the applications of OO technology. Other events, such as Eiffel User Group meetings or NICE meetings are often held in conjunction with TOOLS. TOOLS Conferences PO Box 6863, Santa Barbara, CA 93160, USA TEL (805) 685 1006, FAX (805) 685 6869 email tools@tools.com ---------- Q18) Why do most Eiffel implementations compile to C? By using C as a target language, an Eiffel implementor can: - bring Eiffel to the marketplace faster and at lower cost - port their implementation more easily to other platforms - take advantage of optimisation provided by the C compiler Much of the technology that makes Eiffel relatively simple to use also makes it more difficult to implement (an Eiffel-to-C compiler is perhaps 4 to 5 times more difficult to create than a native Pascal compiler). Compiling Eiffel to C seems to work well under Unix. C is sometimes thought of as the native code of Unix. On the other hand, C is not universal on other platforms, and the Eiffel purchaser may need to buy a C compiler as well, and possibly replace it if the supported C compilers change with new versions of the Eiffel compiler. With a native-code compiler, you'd get somewhat better throughput and the potential for smaller executables and slightly better performance. You'd also get a higher price and an even longer wait for Eiffel to show up on other than the leading market share machines. ---------- L01) What features does Eiffel have? Eiffel is a pure object-oriented language. Its modularity is based on classes. It stresses reliability, and facilitates design by contract. It brings design and programming closer together. It encourages the re-use of software components. Eiffel offers classes, multiple inheritance, polymorphism, static typing and dynamic binding, genericity (constrained and unconstrained), a disciplined exception mechanism, systematic use of assertions to promote programming by contract, and deferred classes for high-level design and analysis. Eiffel has an elegant design and programming style, and is easy to learn. An overview is available at http://www.eiffel.com/doc/manuals/language/intro/ ---------- L02) What changes have been made to the Eiffel language definition? Eiffel is still a relatively new language, and there have been a number of changes to its definition. Here is a summary of the major changes: 1. Changes between the publication of "Object-Oriented Software Construction" in 1988, and the release of Eiffel 2.3: - Constrained genericity enables a generic class to restrict its generic parameters to the descendants of a given class - The "indexing" clause allows information about a class to be recorded for extraction by archival, browsing and query tools - The assignment attempt operator "?=" provides a way to make type-safe assignments going against the inheritance hierarchy - User-defined infix and prefix operator features - Expanded types support composite objects without dynamic allocation, and with value semantics - The "obsolete" clause for smooth library evolution - The "unique" keyword for implicitly-assigned integer codes - The multi-branch instruction (similar to a case statement) - The boolean operator for implication ("implies") 2. Changes with the introduction of Eiffel Version 3: - The feature adaptation subclause must now be terminated with "end" - Semicolons as instruction separators are optional - Groups of features are bracketed by a feature clause. All features are exported unless the feature clause specifies a restriction. The repeat subclause is no longer needed, because inherited features keep the original export status they had in the parent unless they are redefined, or are the subject of an export subclause in the feature adaptation clause. - Preconditions can only be replaced by weaker ones, postconditions can only be replaced by stronger ones. This is now enforced by the language through the use of "require else" in preconditions and "ensure then" in postconditions. - Ambiguities in repeated inheritance are resolved by a "select" clause. - A feature can no longer be replicated and redefined in the same feature adaptation clause, however the same effect can be achieved through repeated inheritance - Two or more features may be defined at the same time (e.g. "f1, f2 is..."). - The keyword "frozen" before a feature name prohibits redefinition of the feature in descendants - In an anchored declaration, the anchor may now also be a formal argument of the enclosing routine - A class may have zero, one or more creation procedures, designated with the "creation" keyword. A new creation syntax using the "!!" symbol allows the appropriate creation procedure to be specified. It is also possible to directly create an object of any type which conforms to the entity to which it is being attached. - The meaning of dot notation has been made more uniform, and alternative constructs have been provided for the special language-defined features that previously used dot notation: x.Create is now !! x y.Clone(x) is now y := clone(x) x.Forget is now x := Void x.Void is now x = Void x.Equal(y) is now equal(x, y) - Manifest arrays can be specified, for example <<"Jan", "Feb", "Mar">> which also provides a way to pass a variable number of arguments to a routine. - The command-line parameters are made available to the creation procedure of the root class as an array of strings. - A default rescue procedure called default_rescue may be defined and inherited. - A class may be declared to be an expanded class, in which case any type based on that class will be expanded. - An object may no longer contain a reference to an expanded object that is a sub-object of another object. Instead, upon assignment of an expanded object to a non-expanded object, the expanded object will be cloned, and a reference to the newly-cloned object will be stored in the non-expanded object. - The operator "div" has been replaced by "//", and the operator "mod" has been replaced by "\\". 3. Changes between first and second printings of "Eiffel: The Language" - New basic types INTEGER_REF, REAL_REF, CHARACTER_REF and BOOLEAN_REF etc have been introduced to provide non-expanded basic types. - Introduction of the POINTER type to enable external references to be passed around in Eiffel programs. - Calls from Eiffel to external routines no longer implicitly pass the current object as the first parameter. There are many other (more minor) changes, which Neil Wilson has summarized in ftp.cm.cf.ac.uk:/pub/eiffel/Docs in both Microsoft Rich Text Format and ASCII. ---------- L03) What libraries come with Eiffel? All vendors aim to support the Eiffel Library Standard kernel classes. In addition, extensive library classes are supplied with the compilers including data structures, graphics, lexical analysis and parsing, IO, persistence, formatting and more. Contact the vendors for further details. ---------- L04) What's the big deal about preconditions and postconditions? The big deal is that it supports programming by contract. For example, preconditions (require clauses) are simple boolean statements that are used to check that the input arguments are valid and that the object is in a reasonable state to do the requested operation. If not, an exception is generated. Similarly, postconditions (ensure clauses) make sure that a method has successfully performed its duties, thus "fulfilling its contract" with the caller. Invariants are boolean expressions that are checked every time an object method returns back to a separate object. You can use these ideas in any OO programming language, but usually must supply your own assertion mechanisms or rely on programmer discipline. In Eiffel, the ideas are integrated into the whole fabric of the environment. We find them used by: -- the exception handling mechanism. (Tracebacks almost always identify the correct culprit code since preconditions almost always denote an error in the calling method, while postconditions denote an error in the called method.) -- the automatic compilation system. (Assertions can be disabled entirely or selectively by type on a per class basis.) -- the Eiffel compiler (Invariants, preconditions and postconditions are all inherited in a manner that makes logical sense.) (Assertion expressions are not allowed to produce side effects so they can be omitted without effect.) -- the automatic documentation tools (Preconditions and postconditions are important statements about what a method does, often effectively describing the "contract" between the caller and callee. Invariants can yield information about legal states an object can have.) In the future we expect to see formal methods technology work its way into the assertion capability. This will allow progressively more powerful constraints to be put into place. In addition, if a conjecture by Dr. Meyer bears fruit, the notion of preconditions may be extended into an important mechanism for the development of parallel programming. ---------- L05) Please explain and discuss covariance vs. contravariance. Consider the following situation: we have two classes PARENT and CHILD. CHILD inherits from PARENT, and redefines PARENT's feature 'foo'. class PARENT feature foo (arg: A) is ... end class CHILD inherit PARENT redefine foo end feature foo (arg: B) is ... end The question is: what restrictions are placed on the type of argument to 'foo', that is 'A' and 'B'? (If they are the same, there is no problem.) Here are two possibilities: (1) B must be a child of A (the covariant rule - so named because in the child class the types of arguments in redefined routines are children of types in the parent's routine, so the inheritance "varies" for both in the same direction) (2) B must be a parent of A (the contravariant rule) Eiffel uses the covariant rule. At first, the contravariant rule seems theoretically appealing. Recall that polymorphism means that an attribute can hold not only objects of its declared type, but also of any descendant (child) type. Dynamic binding means that a feature call on an attribute will trigger the corresponding feature call for the *actual* type of the object, which may be a descendant of the declared type of the attribute. With contravariance, we can assign an object of descendant type to an attribute, and all feature calls will still work because the descendant can cope with feature arguments at least as general as those of the ancestor. In fact, the descendant object is in every way also a fully-valid instance of the ancestor object: we are using inheritance to implement subtyping. However, in programming real-world applications we frequently need to specialize related classes jointly. Here is an example, where PLOT_3D inherits from PLOT, and DATA_SAMPLE_3D inherits from DATA_SAMPLE. class PLOT feature add(arg: DATA_SAMPLE) is ... class PLOT_3D inherit PLOT redefine add end feature add(arg: DATA_SAMPLE_3D) is ... This requires the covariant rule, and works well in Eiffel. It would fail if we were to put a PLOT_3D object into a PLOT attribute and try to add a DATA_SAMPLE to it. It fails because we have used inheritance to implement code re-use rather than subtyping, but have called a feature of the ancestor class on an object of the descendant class as if the descendant object were a true subtype. It is the compiler's job to detect and reject this error, to avoid the possibility of a run-time type error. Here's another example where a real-world situation suggests a covariant solution. Herbivores eat plants. Cows are herbivores. Grass is a plant. Cows eat grass but not other plants. class HERBIVORE class PLANT feature eat(food: PLANT) is ... diet: LIST[PLANT] class COW class GRASS inherit inherit HERBIVORE PLANT redefine eat end feature eat(food: GRASS) is ... This does what we want. The compiler must stop us from putting a COW object into a HERBIVORE attribute and trying to feed it a PLANT, but we shouldn't be trying to do this anyway. Also consider the container 'diet'. We are not forced to redefine this feature in descendant classes, because with covariant redefinition of the argument to 'eat', the feature 'diet' can always contain any object that can be eaten (e.g. grass for a cow). (With contravariant redefinition of the argument to 'eat', it would be necessary to re-open the parent class to make the type of the container 'diet' more general). To summarise: Real-world problems often lend themselves to covariant solutions. Eiffel handles these well. Incorrect programs in the presence of covariant argument redefinition can cause run-time type errors unless the compiler catches these. Sather uses the contravariant rule, but uses separate mechanisms for subtyping and code reuse and only allows dynamic binding on true subtypes. This seems to make contravariance work well, but it can force the Sather programmer to use concrete types when modelling covariant problems. Concrete types cannot be further subtyped in Sather, so this can reduce the potential for re-use (in Eiffel, any type can be further subtyped, but the compiler must check that it is used validly). ---------- L06) Is it true that there are "holes" in the Eiffel type system? No. The design of Eiffel makes it possible to catch all type errors at compile time, so that an Eiffel program cannot abort with a run time type error. However, to catch a class of certain more obscure type errors at compile time, the compiler must analyse the way that classes interact within the entire system, rather than just looking at each class one by one. There is a proposal underway that, if accepted, will allow compilers to check this class of errors by looking at classes and not at the whole system. Because system-wide compile-time validity checking can be complex, some compilers insert run-time traps for these errors instead, and some may fail to correctly trap these errors. Ask your Eiffel compiler vendor how they handle these type problems. ---------- L07) Is there support for concurrency in Eiffel? Eiffel does not yet support concurrency; neither do current commercial compilers. However, work on concurrency for Eiffel is a hot research topic. For four articles on concurrency facilities for Eiffel, including Bertrand Meyer's article "Systematic Concurrent Object-Oriented Programming", see the September 1993 "Communications of the ACM" (Vol. 36, Number 9). At least one of these articles can also be obtained from ISE's WWW site (http://www.eiffel.com). ---------- L08) Why doesn't Eiffel allow function overloading? In Eiffel, no two features of a class may have the same identifier, regardless of their respective signatures. This prevents the use of function overloading ("multiple polymorphism"), a common programming technique in languages like C++. Eiffel is designed to be minimal: it includes exactly the features that its designer considered necessary, and nothing else. Because Eiffel already supports (single) polymorphism through its inheritance system, the only positive thing that function overloading buys you is reducing the number of feature names you have to learn. This is at the expense of reducing the ability of the compiler to trap mistakes (often type errors). Readability is also enhanced when overloading is not possible. With overloading you would need to consider the type of the arguments as well as the type of the target before you can work out which feature is called. With multiple inheritance and dynamic binding this is awkward for a compiler and error-prone for a human. There is no intuitive rule which could be used to disambiguate routine calls where there is no "nearest" routine. However, in Eiffel it's easy to write one routine with arguments of the most general applicable type, then use the assignment attempt operator to carry out the appropriate operation according to the run-time type of the arguments (thereby explicitly programming the disambiguation "rules"). Having said that, the lack of multiple polymorphism does force us to write some common mathematical operations (e.g. matrix math) in an awkward way, and forces arithmetic expressions to be treated specially (the "arithmetic balancing rule", ETL p385). But no-one has come up with a solution which is so simple, elegant and useful that it improves the quality of Eiffel as a whole. ---------- L09) Why are there no procedural types in Eiffel? The notion of allowing a routine to be passed as an argument to a routine is in many people's view incompatible with the OO method. The definition of object-orientation implies that every operation belongs to an object type, so one does not manipulate routines just by themselves. A possible technique when one feels the need to use a routine argument is to write a class and include the routine in it. Then (rather than passing a routine argument) pass an object - an instance of this class - to which the routine can then be applied. This is a more flexible approach in the long term. For example, you may later add an "undo" routine to your routine - containing class, or an attribute such as "time of last execution". ---------- L10) Why are there no class attributes in Eiffel? In Eiffel, the "once" function provides greater functionality in a more disciplined way. The body of a "once" function is executed once only, when it is first called. Thereafter, the "once" function returns the same Result without re-executing its body. The "once" function can therefore be used to implement a shared attribute of reference type (initialized on its first use). A "once" function can be included in a mixin class. The shared attribute returned by that once function is then available to all instances of classes which inherit from the mixin class. ---------- L11) How can I call the parent-class version of a redefined routine? When an inherited routine is redefined in a child class, is there a way for the redefined routine to call the version in the parent class? 1) If you are responsible for the design of the parent class, you may anticipate such a need. You may provide multiple versions of the same routine body, with some versions frozen (not redefinable): class PARENT feature foo, frozen parent_foo is do ... end end class CHILD inherit PARENT redefine foo end feature foo is do parent_foo ... end end 2) Otherwise, you use repeated inheritance to get two versions of 'foo', and redefine one of them: class PARENT feature foo is do ... end end class CHILD inherit PARENT rename foo as parent_foo end PARENT redefine foo select foo -- (in case of dynamic binding) end feature foo is do parent_foo ... end end ---------- L12) Where can I find a comparison between Eiffel and C++? In Richard Wiener's book "Software Development Using Eiffel: There can be life after C++" (Prentice-Hall, ISBN 0-13-100686-X). ---------- L13) Are there any destructors in Eiffel? Eiffel objects are garbage-collected, so that there is no need for the software developer to worry about whether, how and when to "destruct" or "free" them in the software text. Some implementations offer a "free" procedure for programmers who absolutely want to remove an object manually. Such a procedure is "use at your own risk" and is not needed in normal Eiffel development. Coming back to normal usage, the need may arise to ensure that certain operations will automatically take place whenever the garbage collector reclaims an object. For example if an Eiffel object describing a file becomes unreachable and hence is eventually garbage-collected, you may want to ensure that the physical file will be closed at that time. Some implementations of Eiffel provide a mechanism for that purpose: procedure 'dispose' from the Kernel Library class MEMORY. Whenever the garbage collector collects an object, it calls 'dispose' on that object. The procedure does nothing by default (so that a smart GC will of course avoid executing any actual call). But any class may inherit from MEMORY and redefine 'dispose' to perform appropriate actions, such as closing a file. Such actions are sometimes called "finalization". This technique achieves it conveniently. Because there is no guarantee as to the order in which the garbage collector will reclaim objects that have become unreachable, safe redefinitions of 'dispose' should only act on external resources such as file descriptors, database elements, window system resources etc, not on Eiffel object structures themselves.