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From: "Jonas Nygren" <ehsjony@ehs.ericsson.se>
Subject: [Q] : Unchecked conversion
Date: 1996/11/09
Message-ID: <01bbce46$0f818680$829d6482@joy.ericsson.se>#1/1
X-Deja-AN: 195493074
organization: Ericsson
newsgroups: comp.lang.ada
Date: 1996-11-09T00:00:00+00:00
List-Id: <comp.lang.ada>



I have defined an IO buffer type which I intend to use for my own IO
packages.
The buffer type is defined as follows:

   subtype Byte is Integer range 0..255;
   
   type Buffer is array (Positive range <>) of Byte;
   Pragma Pack(Buffer);
   for Buffer'Component_Size use 8;

I also want to be able to convert between Buffer and String types. I use
Gnat and
I know that the underlying data representation of Buffer and String are the
same
so I thought I could do an Unchecked_Conversion:

   function To_Buffer (S : String) return Buffer is
      function Convert is
         new Ada.Unchecked_Conversion(String, Buffer);
   begin
      return Convert(S);
   end To_Buffer;

but Gnat gives me an error:

   unconstrained type "Standard.String" not allowed in unchecked conversion

According to the RM95 Unchecked_Conversion should accept unconstrained
types
but implementations are allowed to impose limitations.

My questions are:

1)  Is this a Gnat(3.04a) imposed limitation on Unchecked_Conversion
     or is my code illegal irregardless of compiler?

2)  Is there a  way of doing this type conversion, save copying?

/jonas