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From: dewar@cs.nyu.edu (Robert Dewar)
Newsgroups: comp.lang.ada
Subject: Re: Tagged type: how to know which specific type
Date: 19 Mar 1995 17:27:23 -0500
Organization: Courant Institute of Mathematical Sciences
Message-ID: <3kib4b$igt@gnat.cs.nyu.edu>
References: <WOODRUFF.95Mar18114334@tanana.llnl.gov>
NNTP-Posting-Host: gnat.cs.nyu.edu
Date: 1995-03-19T17:27:23-05:00
List-Id: <comp.lang.ada>

You can of course pass Tag values around, but you can't dispatch on them.
It is indeed true that if you want to dispatch you must send along at least
a dummy instance of the value. I have run into this in writing the file
stuff for GNAT (which is incidentally quite a nice example of the use of
tagged types -- each of the I/O modules, text_io, sequential_io etc uses
a control block derived by extension from an abstract common control block,
and then operations like open are class-wide operations, and there is a
dispatching operation for close called when files are closed automatically.

Well when I wrote open, the common code needs to call the dispatching
Allocate routine to allocate an instance of the control block. I found
that I had to declare a dummy instance of the control block to pass to the
open procedure so it knew which Allocate to call. It's a little annoying,
since at some level it would be fine to pass just the Tag, since in practice
that is a pointer to the dispatching table - oh well. It is not so terrible
to write:

    Dummy_Control_Block : Text_File_Control_Block;

    Open ( ......, Dummy_Control_Block, ....)

and I guess you have to do the same with your Gasoline and Printer example

    Fuel_Type : Gasolene;

    Print ( ...., Fuel_Type, ....)

again, not so terrible!