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From: paulg@loxinfo.co.th (Paul Sjoerdsma)
Subject: Reading objects from a file
Date: 1997/04/18
Message-ID: <335837a9.11747231@news.loxinfo.co.th>#1/1
X-Deja-AN: 235580064
Organization: Straight Line Solutions
Reply-To: paulg@loxinfo.co.th
Newsgroups: comp.lang.ada
Date: 1997-04-18T00:00:00+00:00
List-Id: <comp.lang.ada>


Hi,

I'm just starting using Ada and I have a, hopefully simple, question.

=46irst I'll explain what I am trying to do:

I am rewriting a simple visualization program, which is used to output
geological (actually micro-paleontological) data. The program needs to
read frequence curves, process the data and output files.

The data structures used (for the graphics part) will look something
like:

Graphic_Elem (base class)
-- Line_Elem (child class)
-- Polyline_Elem (child class)
-- Solid_Elem (child class)
etc

The output would go tho=20
Graphic_Port (base class)
Dxf_port (child class)
Ascii_port (child class)

There will also be a class Figure
procedure Add(Figure, Graphic_Elem)
procedure Draw(Figure, Graphic_Port)


What I am looking for are methods for the Figure class
procedure Read
procedure Write

in such a way that invoking Read(Figure) will build the appropriate
structure. So when it encounters a saved Line_Elem in the file it will
invoke the read procedure associated with the Line_Elem.

I know that one way of doing it would be to write out a unique
identifier as part of each Graphic_Elem and in Read(Figure) use a case
statement. The problem is than that the knowledge about the different
Graphic_Elem children is hardcoded in the Figure class. Can this be
avoided?

regards
Paul