Up Top        Next SmallEiffel FAQ


Eiffel FAQ

Archive-name: eiffel-faq
Posting-Frequency: approximately monthly
Last-modified: 05 September 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 Franck Arnaud
(franck_arnaud@stratus.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 Franck Arnaud. Distribution is unrestricted.
It builds on the work of the previous maintainers: Rock Howard,
Roger Browne, Conrad Taylor in chronological order.

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:

  - Questions have been renumbered using 4-letter codes for easier
    reference.
  - FAQ section: all questions but QEIF, QORI and QBON changed.
  - Language section: LPAR, LEVC, LCAT, LTSK updated.

Frequently Asked Questions:

   QEIF  What is Eiffel?
   QORI  Where did Eiffel come from?
   QCOM  What Eiffel compilers are available?
   QFRE  Is Eiffel available for free or as shareware?
   QARC  Is there an archive of the comp.lang.eiffel newsgroup?
   QBOK  What books are available for learning about Eiffel?
   QWEB  Where can I find Eiffel on the World-Wide-Web?
   QEDI  Where can I get an Eiffel editor or emacs-mode?
   QBON  What is BON?
   QSAT  What is Sather? How does it compare to Eiffel?
   QSTD  Are there standards for the Eiffel language?
   QTGV  How fast do Eiffel applications run?
   QGRP  Are there any Eiffel user groups?
   QADR  Where can I get Eiffel products and services?
   QCNF  Are there any conferences for Eiffel users?
   QECC  Why do most Eiffel implementations compile to C?

Language Issues:

   LFEA  What features does Eiffel have?
   LCHN  What changes have been made to the Eiffel language definition?
   LLIB  What libraries come with Eiffel?
   LDBC  What's the big deal about preconditions and postconditions?
   LCON  Please explain and discuss covariance vs. contravariance.
   LCAT  Is it true that there are "holes" in the Eiffel type system?
   LTSK  Is there support for concurrency in Eiffel?
   LOVL  Why doesn't Eiffel allow function overloading?
   LPRC  Why are there no procedural types in Eiffel?
   LATR  Why are there no class attributes in Eiffel?
   LPAR  How can I call the parent-class version of a redefined
         routine?
   LEVC  Where can I find a comparison between Eiffel and C++?
   LDES  Are there any destructors in Eiffel?

----------

QEIF:   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.

----------

QORI:   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.

----------

QCOM:   What Eiffel compilers are available?

The following Eiffel compilers are currently available and supported
by their vendors or authors. The list is ordered by date of first
publication.

In the case of commercial products, the price is not mentioned because
there can be varying conditions depending on platforms, conditions of
use (personal vs. professional), etc. Please check with the vendors'
web-sites for up to date pricing information. As a rule of thumb,
limited or personal versions of compilers cost from US$ 50 to US$ 200
while a full-blown compiler for a single-user licence and the right to
royalty-free distribution varies from US$ 200 to US$ 1500, on
mainstream platforms.

In the list below, the 'target' entry indicates what code is produced
by the compiler. Most -- but not all -- compilers produce C code so a
supported C compiler is needed.

In the 'platform' entry, an indication of supported platforms is given.
"Win32" means Windows 95 and Windows NT on Intel x86. No compiler (but
indirectly Smalleiffel) is available under Windows NT on RISC platforms
to the best of our knowledge. "Unix" means various Unices, check with
vendor for the actual list of platforms. Most vendors supporting Unix
do support Linux on Intel x86.

The 'brief description' sections are abstracted from the vendors' web
pages.


 Vendor: Interactive Software Engineering Inc, USA
 Product: ISE Eiffel (current version: 4.1)
 Licensing conditions: Commercial; free time-limited evaluation version
 Target: C
 Platforms: Win32, Unix
 Web: http://www.eiffel.com/

 Brief description:
  The ISE Eiffel environment includes:
  - EiffelBench, a complete graphical development environment with
    unique facilities for fast compilation, power browsing,
    documentation, symbolic debugging and more
  - EiffelBase, a complete and professional set of classes covering
    containers, collections, I/O, iterators, object persistence, table
    searching etc.
  - Under Windows, the Windows Eiffel Library (WEL), combining the
    power of Eiffel with access to the Windows API.


 Vendor: Tower Technology Corporation
 Product: TowerEiffel (current version: 2.0)
 Licensing condition: Commercial
 Target: C
 Platforms: Win32, Unix
 Web: http://www.twr.com/

 Brief description:
  TowerEiffel is a complete enviromenent that includes:
  - High Performance Eiffel 3 Compiler
  - Programming Environment
  - Automatic Documentation Generation
  - Hundreds of Reusable Classes
  - Multi-language Source Level Debugger
  - Browsing Tools
  - Project Management Tools


 Vendor: Dominique Colnet and others
 Product: SmallEiffel
 Licensing conditions: Freeware
 Target: ANSI C / Java Virtual Machine
 Platforms: Any ANSI C machine
 Web: No web site yet, master ftp site at
            ftp://ftp.loria.fr/pub/loria/genielog/SmallEiffel/

 Brief description:
  SmallEiffel is intended to be a complete, though small and very fast,
  free Eiffel compiler. SmallEiffel is already used by students of the
  University Henri Poincaré in Nancy, France.
  SmallEiffel has already been ported to several platforms. The current
  distribution includes an Eiffel to C compiler, an Eiffel to Java
  bytecode compiler, a pretty printer and various tools.


 Vendor: Halstenbach ACT GmbH, Germany
 Product: iss-base (current version: 1.6)
 Licensing conditions: Commercial
 Target: C
 Platforms: Win32, Unix
 Web: http://www.halstenbach.de/

 Brief description:
  iss-bench is the interactive and platform-independent tool for the
  development of Eiffel programs under MS Windows and UNIX. It features
  incremental compiling, automatic recognition of dependencies, a
  source-code debugger and browsing tools.
  Libraries include: iss-baselib (data structures), iss-vision
  (user interface-management-system providing Windows API support
  and a series of further abstractions to create complex dialogs),
  iss-store (relational databases interface).

  (Note: iss-bench and iss-baselib are based on but not the same
  as ISE Eiffel. iss-vision and iss-build are unrelated with ISE
  products.)


 Vendor: Object Tools GmbH, Germany
 Product: Visual Eiffel (current version: 2.0)
 Licensing conditions: Commercial; free feature-limited evaluation version
 Target: Native Intel x86
 Platforms: Win32 only
 Web: http://www.object-tools.com/

 Brief description:
  Using Visual Eiffel and DM will help you to develop complex Windows
  applications in a very short time. Visual Eiffel gives you
  - an integrated workbench with the Windows look and feel
  - a professional Eiffel compiler producing very efficient native
    code for Intel processors
  - DM - the most rapid RAD tool you have ever seen gives you
    everything to build applications for Windows fast.
  - many useful libraries for the production of commercial Windows
    applications - for ActiveX component integration, for ODBC access,
    for the creation of nice graphical packages and much more.


Other Eiffel compilers are worth mentioning although they may be
either not supported any more, or an older version, or at an early
stage of development so that their implementation of the language
is far from complete.


- SIG Eiffel/S, version 1.3: Eiffel/S was the first Eiffel 3
  compiler, and the first Eiffel compiler available on the PC
  platform. Version 1.3, is still available as shareware from
  Object Tools (formerly SIG) but it is a few years old. A
  much improved version has been announced for a while,
  but is not available at the time of writing. Eiffel/S 1.3 is
  a command line compiler producing C code, it is available
  for DOS32, Windows 95 and NT and many Unix platforms.
  Object Tools is at http://www.object-tools.com/.

- EON Eiffel: An Eiffel to C++ compiler, written in C++,
  not actively maintained.

- FEC is a native Eiffel compiler for SUN SPARC machines. An early
  beta version can be downloaded from Fridtjof Siebert's page at
  http://www.informatik.uni-stuttgart.de/ifi/ps/siebert/fridi_eiffel.html


----------

QFRE:   Is Eiffel available for free or as shareware?

SmallEiffel is a freeware compiler, provided as a highly
portable C package that can compile on most ANSI C platforms.
The actual source code of the compiler (in Eiffel, from which
the provided C code is compiled) is not available.

Many commercial vendors offer free evaluation versions, with
some limitations. Commercial vendors often also have cheap
entry-level versions for popular platforms like Win32 on
Intel-based PCs.

----------

QARC:   Is there an archive of the comp.lang.eiffel newsgroup?

Yes, on the WWW at:
  http://www.cm.cf.ac.uk/CLE/
or at the following FTP sites:
  ftp://wuarchive.wustl.edu/usenet/comp.lang.eiffel/

The newsgroup is also archived at the usual places on the web
(DejaNews, AltaVista, etc).

----------

QBOK:   What books are available for learning about Eiffel?

ESSENTIAL READING

 Title: Object-Oriented Software Construction, second edition
Author: Bertrand Meyer, ISE Inc.
  ISBN: ISBN 0-13-629155-4 -- Published 1997
 Short: This book 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.

        While not presented as an 'Eiffel book' (Eiffel is presented
        as the 'notation' used to illustrate the concept) it is
        essential reading for any Eiffelist and it actually includes
        a rather complete description of the 'notation' -- Eiffel.

        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.


 Title: Eiffel: The Language
Author: Bertrand Meyer
  ISBN: ISBN 0-13-247925-7 -- Published 1992
 Short: This book 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). This
        book is also available in French (ISBN 2-7296-0525-8).


OTHER BOOKS

 Title: An Object-Oriented Introduction to Computer Science Using Eiffel
Author: Richard Wiener -- ISBN: 0-13-838725 -- Published 1997
 Short: None

 Title: Object Technology for Scientific Computing Object-Oriented
            Numerical Software in Eiffel and C
Author: Paul Dubois -- ISBN: 0-13-267808-X -- Published 1996
 Short: Accompanying CD with the Free Eiffel for UNIX & Linux
        environments.

 Title: Object-Oriented Software Engineering with Eiffel
Author: Jean-Marc Jezequel -- ISBN: 0-201-63381-7 -- Published 1996
 Short: A comprehensive guide to Eiffel. In addition to describing
        Eiffel, the book contains descriptions and comparisons of
        compilers and libraries available on the market.

 Title: Object Structures: Building OO Software Components with Eiffel
Author: Jacob Gore -- ISBN: 0-201-63480-5 -- Published 1996
 Short: This is the first "data structures" book for Eiffel, bringing
        to the study of that language the first comprehensive
        treatment of one of the most important topics in any
        programming language.

 Title: Eiffel Object-Oriented Programming
Author: John Tyrrell -- ISBN: 0-333-64554-5 -- Published 1995
 Short: This is an inexpensive and very approachable book.

 Title: Software Development Using Eiffel: There can be life other than C++
Author: Richard Wiener -- ISBN: 0-13-100686-X -- Published 1995
 Short: This is a useful book with a lot of code examples for those
        with a grounding in another OO language.

 Title: Object Success
Author: Bertrand Meyer -- ISBN: 0-13-192833-3 -- Published 1995
 Short: This book is a  manager's guide to object orientation, its
        impact on the corporation and its use for re-engineering the
        software process.

 Title: Object Oriented Programming in Eiffel
Author: Pete Thomas and Ray Weedon -- ISBN: 0-201-59387-4 -- Published 1995
 Short: This book is a very comprehensive Eiffel tutorial and textbook,
        with a solid "Abstract Data Type" approach.

 Title: Object Oriented Programming in Eiffel
Author: R. Rist and R. Terwilliger -- ISBN: 0-13-205931-2 -- Published 1995
 Short: This is a textbook with an emphasis on design.

 Title: Seamless Object-Oriented Software Architecture: Analysis and
            Design of Reliable Systems
Author: Kim Walden and Jean-Marc Nerson -- ISBN: 0-13-031303-3 -- Publ. 1994
 Short: This book describes the Business Object Notation (BON) Method
        in detail.

 Title: Reusable Software: The Base Object-Oriented Component Libraries
Author: Bertrand Meyer -- ISBN: 0-13-245499-8 -- Published 1994
 Short: This book describes principles of library design and the
        taxonomy of fundamental computing structures. Serves as a
        manual for the EiffelBase libraries.

 Title: An Object-Oriented Environment: Principles and Application
Author: Bertrand Meyer -- ISBN: 0-13-245507-2 -- Published 1994
 Short: This book describes the ISE EiffelBench environment as well as
        the "Melting Ice" compilation technology and the EiffelBuild
        GUI application builder.

 Title: Object-Oriented Applications
Author: Meyer and Nerson editors -- ISBN: 0-13-013798-7 -- Published 1993
 Short: This book includes an introduction to Eiffel technology
        followed by seven in-depth descriptions of large applications
        written in Eiffel.

 Title: Eiffel: Objektorientiertes Programmieren in der Praxis
Author: Frieder Monninger -- ISBN: ISBN 3-88229-028-5 -- Published 1993
 Short: This book is a very down-to-earth Eiffel handbook in German.

 Title: Eiffel: An Introduction
Author: Robert Switzer -- ISBN: 0-13-105909-2 -- Published 1993
 Short: This book is a very clear and concise Eiffel primer, with many
        code fragments and two substantial Eiffel applications. Also
        available in French (ISBN 2-225-84-656-1).

 Title: Object Oriented Software Construction, first edition
Author: Bertrand Meyer -- ISBN: 0-13-629049-3 -- Published 1988
 Short: An earlier edition of the second edition mentioned above, based
        on a previous version of the language.
        Also available in French, German, Italian, Dutch, etc.

----------

QWEB:   Where can I find Eiffel on the World-Wide-Web?

http://www.cm.cf.ac.uk/CLE/
  An Eiffel home page that is held on the University of Wales College
  of Cardiff's server.

http://www.progsoc.uts.edu.au/~geldridg/eiffel/
  Geoff Eldridge's Eiffel pages including GUERL, a preview of Eiffel
  Liberty, an online journal, the online C++ Critique, and other
  useful information.

http://www.totalweb.co.uk/gustave/
  Gustave is a repository of Eiffel resources maintained by Roger
  Browne of Everything Eiffel.

The main vendors websites are:

 Halstenbach ACT   http://www.halstenbach.de/
 ISE               http://www.eiffel.com/
 Object Tools      http://www.object-tools.com/
 Tower Technology  http://www.twr.com/

----------

QEDI:   Where can I get an Eiffel editor or emacs-mode?

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.

The WINEDIT shareware programmer's editor offers colour syntax
highlighting, works with Eiffel/S under MS-Windows, and is available
from all main Windows shareware archives.

Alan Philips' free Programmers File Editor also works with Eiffel/S
under MS-Windows, has templates but not syntax highlighting, available
from http://www.lancs.ac.uk/people/cpaap/pfe/ .

Franck Arnaud's Eiffel extension to the Windows/WindowsNT programmers
editor Codewright from Premia implements chromacoding of Eiffel code,
has smart indenting and some templates. Available from
http://www.altsoft.demon.co.uk/free/ .

----------

QBON:   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.

----------

QSAT:   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
or the Sather home page at http://www.icsi.berkeley.edu/~sather/.

----------

QSTD:   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.

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 1997 consists of Simon Parker,
Bertrand Meyer, Rock Howard, James McKim.

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

----------

QTGV:  How fast do Eiffel applications run?

Early implementations 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.

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. The effect of garbage collection is an oft-debated point,
generally the overhead is quite low (10%) and in principle a style of
programming assuming a GC could be more efficient than typical manual
memory management. This also depends on the kind of application, the
implementation of the garbage collector, etc.

----------

QGRP:  Are there any Eiffel user groups?

Compiler vendors usually run user groups for their user base, often
in the form of a mailing-list or meetings during conferences. Contact
the individual vendors for more information.

UK & Ireland Eiffel Interest Group (currently inactive)


----------

QADR:   Where can I get Eiffel products and services?

These vendors, resellers and suppliers of Eiffel training and
consultancy are listed in alphabetical order:

(The vendor names followed by an asterisk are those whose
address could not be verified. If the people concerned read
the FAQ or someone knows an email address to contact them,
please contact the FAQ maintainer.)


ARGENTINA

Cybertech*
 POST Systens Integration for CIM, Suarez 1281, Third Floor, Apt.A
       CP-1288 Buenos Aires, Argentina
 TEL +54 1 28 1950             FAX +54 1 322 1071 or 963 0070


AUSTRALIA

Class Technology Pty. Ltd.
 POST PO Box 6274, North Sydney NSW 2060, Australia
 TEL +61 2 9922 7222           FAX +61 2 9922 7703
 EMAIL eiffel@class.com.au     WEB http://www.class.com.au/


CANADA

Jay-Kell Technologies, Inc.*
 POST 48 Lakeshore Road, Suite #1,
      Pointe Claire, Quebec, Canada H9S 4H4
 TEL +1 514 630 1005           FAX +1 514 630 1456


EUROPEAN UNION

Advanced Media Technology Ltd.
 POST Box 16, Hatolantie 140, SF-34 301 Kuru, Finland
 TEL +358 400 620 236          FAX +358 3 4737 117
 EMAIL jukka.haukijarvi@eiffel.fi
 WEB http://www.eiffel.fi/

Cap Gemini France, ITMI APTOR, Eiffel Group
 POST 86-90 rue Thiers, F-92513 Boulogne-Billancourt Cedex, France
 TEL +33 1 4910 5300           FAX +33 1 4910 5102
 EMAIL eiffel@capgemini.fr

Eiffel Software Iberica
 POST Isabel II, 4, 1D; 20011 San Sebastián, Spain
 TEL/FAX +34 943 472108
 EMAIL jipferur@si.ehu.es

Eiffel Ireland
 POST 45 Hazelwood, Shankill, Co Dublin, Ireland
 TEL +353 1 282 3487
 EMAIL sparker@eiffel.ie       WEB http://www.eiffel.ie/Eiffel/

Enea Data
 POST Box 232, Nytorpsvagen 5, S-183 23 Taby, Sweden
 TEL +46 8 638 5000            FAX +46 8 638 50 50
 EMAIL eiffel@enea.se          WEB http://www.enea.se/

EtnoTeam
 POST Via Adelaide Bono Cairoli 34, I-20127 Milano, Italy
 TEL +39 2 261621              FAX +39 2 26110755
 EMAIL sales@etnomi.it         WEB http://www.etnomi.it/

Everything Eiffel
 POST 6 Bambers Walk, Wesham PR4 3DG, England
 TEL & FAX +44 1772 687525     WEB http://www.eiffel.demon.co.uk/
 EMAIL roger@eiffel.demon.co.uk

Halstenbach ACT GmbH
 POST Breidenbrucher Strasse 2, D-51674 Wiehl, Germany
 TEL + 49 2261 9902 0          FAX +49 2261 9902 99
 EMAIL info@halstenbach.de     WEB http://www.halstenbach.de/

Langmack & Partner, Feinarbeit
 POST Gitshinner Strasse 91 - 2. Hof, D-10969 Berlin, Germany
 TEL +49 30 616794 61          FAX +49 30 616794 67
 EMAIL langmack@feinarbeit.de

Object Tools GmbH
 POST Zu den Bettern 4, D-35619 Braunfels, Germany
 TEL +49 6472 911030           FAX +49 6472 911031
 EMAIL eiffel@eiffel.de        WEB http://www.eiffel.de/

Jan Willamowius
 POST Semperstr. 1, D-22303 Hamburg, Germany
 TEL +49 40-2806209
 EMAIL jan@janhh.shnet.org
 WEB http://swt-www.informatik.uni-hamburg.de/~1willamo/dl.html


INDIA

Sritech Information Technology*
 POST 744/51 2nd Floor, 10 Mian Road, 4th Block
      Jayanagar, Bangalore, India 560011
 TEL +91 812 640661            FAX +91 812 643608


JAPAN

Information and Math Science Lab Inc.
 POST 2-43-1, Ikebukuro, Toshima-ku, Tokyo 171
 TEL +81 3 3590 5211           FAX +81 3 3590 5353
 EMAIL fushimi@imslab.co.jp    WEB http://www.imslab.co.jp/


NEW ZEALAND

Objective Methods Ltd
 POST PO Box 17356 (77 Chamberlain Rd)
       Karori, Wellington, New Zealand
 TEL +64 4 476 9499            FAX +64 4 476 9237
 EMAIL dkenny@actrix.gen.nz


SWITZERLAND

Abstraction
 POST Faubourg de l'Hopital, CH-2000 Neuchatel, Switzerland
 TEL +41 32 7250493            FAX +41 32 7259857
 EMAIL abstraction@access.ch

Objectif Concept*
 POST Passage Cour-Robert 5, CH-1700 Fribourg, Switzerland
 TEL +41 37 232977             FAX +41 37 464889


UNITED STATES OF AMERICA

Halstenbach ACT, Inc.
 POST 827 State Street, Santa Barbara, CA 93101, USA
 TEL +1 805 568 0023           FAX +1 805 884 0806
 EMAIL info@halstenbach.de     WEB http://www.halstenbach.de/

Interactive Software Engineering, Inc
 POST ISE Building, 2nd floor, 270 Storke Road, Goleta, CA 93117, USA
 TEL +1 805 685 1006           FAX +1 805 685 6869
 EMAIL info@eiffel.com         WEB http://www.eiffel.com/

Object Tools, Inc.
 POST 13267 Summit Sq. Center, Route 413 & Doublewoods Rd,
      Langhorne, PA 19047, USA
 TEL/FAX +1 215 504 0854
 EMAIL info@object-tools.com   WEB http://www.object-tools.com/

Tower Technology Corporation
 POST 1501 Koenig Lane, Austin, TX 78756, USA
 TEL +1 512 452 9455           FAX +1 512 452 1721
 EMAIL tower@twr.com           WEB ttp://www.twr.com/

----------

QCNF:   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.

TOOLS is one of the major international conferences 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.

The TOOLS home page is at http://www.tools.com/, email tools@tools.com

The ACM SIGPLAN Conference On Object-Oriented Programming Systems,
Languages and Applications (OOPSLA) is another well-known conference
about OO Technology. OOPSLA home page is at
http://www.acm.org/sigplan/oopsla/

ECOOP is the annual European Conference for Object-Oriented Programming.
http://iamwww.unibe.ch/ECOOP/

----------

QECC:   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 may get somewhat better throughput
and the potential for smaller executables and slightly better
performance.

----------

LFEA:   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/

----------

LCHN:   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.

There were significant changes between the publication of
"Object-Oriented Software Construction", first edition in 1988,
and the release of Eiffel 2.3.

More significant changes came with the introduction of Eiffel 3, the
current and only version of the language in use today. These changes
are summarised in Eiffel: The Language.

There were some less significant changes between the 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://ftp.cm.cf.ac.uk/pub/eiffel/Docs in both Microsoft
Rich Text Format and ASCII.

----------

LLIB:   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.

----------

LDBC:   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.

----------

LCON:   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).

----------

LCAT:   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.

The two main type of errors that cannot be checked easily are:
 (a) covariant redefinition of routines parameters as in question LDBC.
 (b) restriction of exports in a descendant class.

There is a proposal underway that, if accepted, will allow compilers
to incrementally check this class of errors by looking at classes
and not at the whole system every time.

Because system-wide compile-time validity checking can be complex,
no compiler available today implements full static type checking. Some
insert run-time checks. On the other hand, the cases where system
level validity problems can occur are not too frequent so this is not
a major annoyance in practice.

----------

LTSK:   Is there support for concurrency in Eiffel?

Eiffel does support concurrency in the latest specification for the
language as defined by Interactive Software Engineering (ISE).  A
more complete description on concurrency -- the SCOOP model
-- can be found in "Object Oriented Software Construction 2ed" by
Bertrand Meyer. Papers are also available from ISE web site at
http://www.eiffel.com/doc/manuals/technology/concurrency/ .

Some compilers also support various forms of multithreading
independently of SCOOP.

----------

LOVL:   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.

----------

LPRC:   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".

----------

LATR:   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 per system (not per instance of the class), 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.

----------

LPAR:   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

3) While usable both these constructs have their limitations and a
   proposal is under way to replace them with a more direct solution:
   the new reserved word Precursor that allows to call the parent
   version of a redefined routine in its body. In the rare cases when
   a routine has more than one precursor, the call can be qualified to
   specify which precursor is called.

   Under this proposal, the example becomes:

    class CHILD
    inherit
       PARENT
          redefine foo
       end
    feature
       foo is
          do
             Precursor -- call previous version
             ...
          end

----------

LEVC:   Where can I find a comparison between Eiffel and other languages?

In Richard Wiener's book "Software Development Using Eiffel: There can
be life after C++" (Prentice-Hall, ISBN 0-13-100686-X).

Ian Joyner's "C++ critique" includes a comparison between C++, Eiffel
and other languages. It is at the following URL:
http://www.progsoc.uts.edu.au/~geldridg/cpp/cppcv3.html

You can also find a comparison of Eiffel, C++, Java, and Smalltalk at
ISE's Web site, http://www.eiffel.com/doc/manuals/technology/oo_comparison/

----------

LDES:   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 "destroy"
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.




Up Top        Next SmallEiffel FAQ