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From: sjw <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Generalized serialization for enumeration types
Date: Wed, 26 Aug 2009 22:05:24 -0700 (PDT)
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Date: 2009-08-26T22:05:24-07:00
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On Aug 26, 11:22=A0am, xorque <xorquew...@googlemail.com> wrote:
> Hello.
>
> I'm designing a package that uses a lot of similar but distinct
> enumeration types.
>
> At some point, those types need to be encoded to be sent over
> the wire. The encoding rules are simple:
>
> =A0 The enumeration values are converted to unsigned 32 bit
> =A0 integers with the first value as 0 and increasing sequentially
> =A0 with each new value. The 32 bit value is packed into big-endian
> =A0 byte order.
>
> The problem is: With so many enumeration types, I now have about 300
> lines of almost identical procedures (to be used as stream attributes)
> that just call a procedure that packs Unsigned_32 values into
> Storage_Element arrays.
>
> Is there some clever way I can just write ONE 'Write attribute
> procedure
> and ONE 'Read attribute procedure and have all the defined enumeration
> types use those?
>
> Freezing rules prevented me from writing a generic
> Packed_Enumeration_IO
> package ("representation item appears too late").

This _compiles_:

with Ada.Streams;
generic
   type E is (<>);
procedure Xorque_Writer_G (Stream : access
Ada.Streams.Root_Stream_Type'Class;
                    V : E);
procedure Xorque_Writer_G (Stream : access
Ada.Streams.Root_Stream_Type'Class;
                    V : E)
is
begin
   null;
end Xorque_Writer_G;
with Xorque_Writer_G;
package Xorque is
   type E_Base is (A, B, C);
   procedure E_Base_Writer is new Xorque_Writer_G (E_Base);
   type E is new E_Base;
   for E'Write use E_Base_Writer;
end Xorque;