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From: Marin David Condic <mcondic.nospam@acm.org>
Subject: S'Write and How To Count Bytes
Date: 2000/09/30
Message-ID: <39D6891A.7DC448B6@acm.org>#1/1
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Organization: Quadrus Corporation
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Newsgroups: comp.lang.ada
Date: 2000-10-01T00:44:47+00:00
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Suppose I have a type something like this:

    type My_Record is record
        A_String    : Unbounded_String := Null_Unbounded_String ;
    end record ;

In theory, I should be able to create an object that is derived from
Ada.Streams.Root_Stream_Type which has a Read and Write procedure that
will allow me to create a call such as:

    My_Record'Write (Ptr, A_Record) ;

where Ptr is a pointer to my derived Root_Stream_Type object. Now here's
where it gets fun:

The object derived from Root_Stream_Type is going to contain some
version of a Stream_Element_Array. Possibly dynamically allocated. The
Write procedure that I create for the Root_Stream_Type is going to get
automagically called with a Stream_Element_Array parameter, from which
you could determine the size, allocate what you need for a buffer and
attach it to the Root_Stream_Type. That buffer can then be sent
somewhere or used in some other manner.

The problem is this: What if you want the record to contain the number
of bytes that it converts to so that the number ends up in the stream?
You can't easily determine how many bytes there will be in the record
nor can you determine what the 'Write is really going to produce. You
could possibly retain the size information in your Root_Stream_Type
after the conversion, but by then its too late. You already converted it
and if you had a "Count" field in the record, its value is probably dead
wrong. You could convert it to the stream once, interrogate the
Root_Stream_Type you built to determine the number of bytes in it, then
include that byte count in your record and convert it all over again.
That seems a little inefficient.

Is there any good way of determining in advance how many bytes you are
going to get when converting an object to a stream?

MDC
--
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Marin David Condic - Quadrus Corporation - http://www.quadruscorp.com/
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