From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,436e4ce138981b82 X-Google-Attributes: gid103376,public X-Google-ArrivalTime: 2004-03-11 06:48:46 PST Path: archiver1.google.com!news2.google.com!fu-berlin.de!uni-berlin.de!tar-meneldur.cbb-automation.DE!not-for-mail From: Dmitry A. Kazakov Newsgroups: comp.lang.ada Subject: Re: abstract sub programs overriding Date: Thu, 11 Mar 2004 15:59:24 +0100 Message-ID: <67u0505uu3gfmlt8p28e9jkaco0nljquut@4ax.com> References: <1078776213.376775@master.nyc.kbcfp.com> <1078839257.157439@master.nyc.kbcfp.com> <5cmr40t76va200betf07b7bd6er05ltto9@4ax.com> <1078845298.702789@master.nyc.kbcfp.com> <0ipr40thtf86b520a0qdi003aj87gtuhd4@4ax.com> <1078849973.701176@master.nyc.kbcfp.com> <1078924150.268074@master.nyc.kbcfp.com> <1079014276.527406@master.nyc.kbcfp.com> NNTP-Posting-Host: tar-meneldur.cbb-automation.de (212.79.194.119) Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: news.uni-berlin.de 1079016523 67673442 I 212.79.194.119 ([77047]) X-Newsreader: Forte Agent 1.8/32.548 Xref: archiver1.google.com comp.lang.ada:6238 Date: 2004-03-11T15:59:24+01:00 List-Id: On Thu, 11 Mar 2004 09:10:36 -0500, Hyman Rosen wrote: >Dmitry A. Kazakov wrote: >> To dispatch vs. not has to be defined by solely the object type. This >> is the only right way, IMO, and Ada conforms to it. > >"Has to be"? "The only right way"? That's simply your opinion. No, this is just a contract model. >> Returning to your example, if Type is an abstract type, and >> Derived_Type has to have File, accessed by Type via some dispatching >> method, then File (a handle to) has to be initialized and opened at >> the stage one, and the class-wide constructor of Type may safely use >> it at the stage two. > >Right, but what if you didn't realize that, and didn't code this >correctly? These are different things - to have a bad design or to have a good one, but poorly coded one. >Presumably with File, you would get an exception, but >there could be other types which would cause subtle errors that >could go undetected until much later in the program. The question >isn't how to do it right, but what happens when you do it wrong. You cannot do it right if the language does not support object construction properly. >C++ doesn't let you dispatch to the child in such a case, so the >problem is noticed right away. Noticed but not solved. >If the parent actually needs some >part of the child, that has to be coded explicitly, and that >visibility makes an error much less likely. I tried to show that the source of the problem is that to construct T /= to construct T'Class. It is not obvious, especially if you are using C++ pretending that T=T'Class. So a desire to dispatch from a constructor is not a whim, it is the necessty to construct a *class-wide* object. -- Regards, Dmitry Kazakov www.dmitry-kazakov.de