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=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4 X-Google-Thread: a07f3367d7,c7ff90b15f15636c X-Google-Attributes: gida07f3367d7,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news4.google.com!feeder.news-service.com!newsfeed.straub-nv.de!uucp.gnuu.de!newsfeed.arcor.de!newsspool3.arcor-online.net!news.arcor.de.POSTED!not-for-mail From: "Dmitry A. Kazakov" Subject: Re: Runtime type selection Newsgroups: comp.lang.ada User-Agent: 40tude_Dialog/2.0.15.1 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Reply-To: mailbox@dmitry-kazakov.de Organization: cbb software GmbH References: <2c15c8a6-b555-4d08-8fe6-f77cb207e7a6@k33g2000yqa.googlegroups.com> <1fv6ippku9z2n$.rbm60r20961$.dlg@40tude.net> Date: Tue, 13 Oct 2009 12:17:51 +0200 Message-ID: <13962r9wa97u9$.6gtel38cl93s.dlg@40tude.net> NNTP-Posting-Date: 13 Oct 2009 12:17:51 CEST NNTP-Posting-Host: 491b8fbf.newsspool3.arcor-online.net X-Trace: DXC=aCOKL_hge`i9kIfcjg:0fdMcF=Q^Z^V3h4Fo<]lROoRa8kFKK4PYCSk[6LHn;2LCVn7enW;^6ZC`dIXm65S@:3>o_=AIN^cMK`j X-Complaints-To: usenet-abuse@arcor.de Xref: g2news2.google.com comp.lang.ada:8695 Date: 2009-10-13T12:17:51+02:00 List-Id: On Tue, 13 Oct 2009 10:45:41 +0000, Colin Paul Gloster wrote: > On Sun, 11 Oct 2009, Dmitry A. Kazakov wrote: > >|----------------------------------------------------------------------------| >|" | >|On Sun, 11 Oct 2009 09:41:54 -0700 (PDT), xorque wrote: | >| | >|> I'd like to develop a program that processes a graph. The contents of | >|> the graph are a random selection of objects of different types (all | >|> elements may be of the same type or may all be different - assuming | >|> there are enough types defined). By 'type' in this case, I am | >|> referring to Ada types. | >|> | >|> The basic problem is that I want to create an object of a type picked | >|> at random, at runtime. I'm not entirely sure what my options are here. | >| | >|The problem is that graph nodes have nothing in common. Thus there is | >|nothing except for the graph traversal operations you could implement on | >|the graph as a whole. | >| | >|[..]" | >|----------------------------------------------------------------------------| > > You made it sound as if a Visitor Pattern might work. In which case nodes would be descendants of some common root with the visitor's accept operation. A common interface would change everything, however. Interesting is that Ada's pool-specific pointers (to class-wide targets) are a kind of doubly dispatching (pool class x type class), which in the end allows visitor-like implementations without any common root. There is another case of hidden double dispatch in Ada. It is stream attributes T'Class'Input and T'Class'Output (stream class x type class). -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de