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From: Ted Dennison <dennison@telepath.com>
Subject: 'Read for pointers
Date: 2000/07/26
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Date: 2000-07-26T00:00:00+00:00
List-Id: <comp.lang.ada>

I've seen it suggested in several places to handle Stream writing of
pointers by writing what the pointer points to instead. That's simple
enough.

But say I want to make this automatic. Lets say I have a pointer type
declared thusly:

package Example is
   type Handle is private;

   ...
private

   type Instance;

   type Handle is access all Instance;

end Example;

The user doesn't even really know or care that Handle is really a
pointer. But they may need to do stream I/O with it.

The obvious solution is to create my own 'Write and 'Read routines for
Handle. But there's a problem. Following is the profile for 'Read:

   procedure Read
     (Stream : access Ada.Streams.Root_Stream_Type'Class;
      Item   : out    Handle
     );
   for Handle'Read use Read;

The problem is that Item is an *out* parameter. That means I won't have
acces to the pointer's old value inside Read. There's no way I can put
the newly read data from the stream into the Instance that Handle
currently points to!

I see two possible solutions to this:
  1)  Redefine "Handle" to be a record with a single pointer field for
Instance in it.
  2)  Provide my own Read routine with an "in" or "in out" parameter for
Handle, then warn the user with copious comments not to use 'Read. This
may involve them writing their own 'Read for their compound objects to
prevent their users from transitivly calling Handle'Read.

I'm not too fond of either solution. It would have been nice if Read
used "in out" instead. Does anyone have any other solutions?

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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