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From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
Subject: Re: 'Read for pointers
Date: 2000/07/28
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Date: 2000-07-28T22:06:50+00:00
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Ted Dennison <dennison@telepath.com> writes:

> 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!

But if you are reading from a stream, Handle doesn't yet point to
anything! So 'Read needs to allocate an Instance, and set Handle to that.

I guess Handle could point to an Instance that the user no longer
needs; it should be free'd before the new Instance pointer is
assigned. So Handle must be a controlled type. My, this is getting fun
:). 

-- 
-- Stephe