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-Thread: a07f3367d7,c7ff90b15f15636c
X-Google-Attributes: gida07f3367d7,public,usenet
X-Google-NewGroupId: yes
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news3.google.com!feeder.news-service.com!newsfeed101.telia.com!nf02.dk.telia.net!news.tele.dk!news.tele.dk!small.news.tele.dk!fi.sn.net!newsfeed2.fi.sn.net!news.song.fi!not-for-mail
Date: Sun, 11 Oct 2009 20:02:48 +0300
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Organization: Tidorum Ltd
User-Agent: Mozilla-Thunderbird 2.0.0.22 (X11/20090706)
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: Runtime type selection
References: 
 <2c15c8a6-b555-4d08-8fe6-f77cb207e7a6@k33g2000yqa.googlegroups.com>
In-Reply-To: 
 <2c15c8a6-b555-4d08-8fe6-f77cb207e7a6@k33g2000yqa.googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <4ad20fd5$0$26308$4f793bc4@news.tdc.fi>
NNTP-Posting-Host: 81.17.205.61
X-Trace: 1255280597 news.tdc.fi 26308 81.17.205.61:37128
X-Complaints-To: abuse@tdcnet.fi
Xref: g2news2.google.com comp.lang.ada:8668
Date: 2009-10-11T20:02:48+03:00
List-Id: <comp.lang.ada>

xorque wrote:
> Hello.
> 
> 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.
> 
> I could create a base tagged type and derive a set of types from that
> base, selecting at random. I could use variant records. It seems that
> in
> either case, I'd need to create an enumeration type with a 1:1 mapping
> between enumeration values and derived types.
> 
> Both of these solutions seem a little unwieldly. They also seem like
> they'd end up making the program difficult to extend (I'd ideally just
> like to derive a new type and have it automatically available to be
> selected at random for the graph).

Keep a list or array of (accesses to) a factory function for each 
possible (derived) type. When you derive a new type from the base 
graph-element type, add its factory function to this list (this is often 
called "registering" the new type/function). When you want to make a 
graph element of a "random" type, pick a random index into the list or 
array of factory functions and call the chosen function, which creates 
the new element and returns an access to it (type access all 
Base_Graph_Element_Type'Class).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .