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From: Ted Dennison <dennison@telepath.com>
Subject: Re: I want 'Class'Input back!
Date: 1999/10/13
Message-ID: <7u2377$mvf$1@nnrp1.deja.com>#1/1
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Date: 1999-10-13T00:00:00+00:00
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In article <3803cc6c_1@news1.prserv.net>,
  "Matthew Heaney" <matthew_heaney@acm.org> wrote:
> In article <7u0as6$fmm$1@nnrp1.deja.com> , Ted Dennison
> <dennison@telepath.com>  wrote:

> > attributes. In fact, I need 'Input, 'Output, 'Class'Input, and
> > 'Class'Output.

Yikes! What was I thinking? I meant 'Read and 'Write. 'Input is clearly
nonsense on a limited type (as using it would require assignment).

> I think you only need to replace the stream attribute operations of
> the *specific* type T.
>
> The stream attributes T'Class'Input and T'Class'Output describe the
> attributes of the *class-wide* type, which the compiler supplies for
> you.

That's what I thought too after a cursory look at the literature.
However, according to both Gnat and my TFFE-based compiler, that is not
the case. Without a redefinition of "Instance'Class'Read", both
compilers yack at me with essentially the same error message. Here's the
Gnat example; a line reading:
            Instance'Class'Read (Ada.Streams.Stream_Io.Stream(File),
Table);

Gets flagged with the following error message:
interpolation_table.adb:92:18: limited type "Instance'Class" has no
stream attributes

Even though I *do* have the following:
   procedure Read (Stream : access Ada.Streams.Root_Stream_Type'Class;
                   Item   : out    Instance);
   type Instance is abstract tagged limited record
    ...
   for Instance'Read use Read;

If I also add the following lines, the error goes away (with both
compilers):
   procedure Read (Stream : access Ada.Streams.Root_Stream_Type'Class;
                   Item   : out    Instance'Class);
   ...
   for Instance'Class'Read use Read;

But the problem (as I mis-said before), is that I can't figure out how I
could possibly code my classwide "Read" function to do what 'Class'Read
does.

--
T.E.D.


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