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From: robert_dewar@my-dejanews.com
Subject: Re: Can't export object of private type
Date: 1999/02/25
Message-ID: <7b3f6v$lc1$1@nnrp1.dejanews.com>#1/1
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References: <7b1k4h$13k6@news3.newsguy.com> <F7p7Dq.DE1@syd.csa.com.au>
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Date: 1999-02-25T00:00:00+00:00
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In article <F7p7Dq.DE1@syd.csa.com.au>,
  nospam@thanks.com.au wrote:
> :Ada doean't allow you to declare a private type and
> declare an object of that  :type in the visible part of
> the same spec. For example, the following is
> :illegal:
> :
> :  package Private_Stuff is
> :    type Private_Type is tagged private;
> :    procedure Do_Something (A: in Private_Type);
> :    P: Private_Type;
> :  private
> :    type Private_Type is tagged null record;
> :  end;

This is of course wrong at a very fundamental level. The
notion of linear elaboration is central to the integrity
of the Ada design, and is a notion that should be fully
understood and familiar to Ada programmers.

You mention "looking ahead", and it is of course true that
some trivial cases could be handled this way, but this
never works in the general case. Consider for example the
following semantic nonsense (which is of course illegal
as a special case of the general observation you are
making)

     type x is private;
     b : x;
     ...
   private
     type x is array (1 .. b'size) of Integer;

The point here is that this is not some kind of arbitrary
restriction, and it should be like parking in New York
City -- "don't even *think* of doing things which violate
the notion of linear elaboration" :-)

Now my own design preference would have been to avoid the
separate private part, and just put a private keyword on
selected declarations. I think the separation of private
parts, nice as it is in the simple cases, has caused SO
much confusion, and SO much wasted implementors time (it
is a source of lots of implementation difficulty) that it
does not begin to be worth it.


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