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From: Hyman Rosen <hymie@prolifics.com>
Subject: Re: Private Children
Date: 1999/06/24
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Date: 1999-06-24T16:27:38+00:00
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Richard D Riehle <laoXhai@ix.netcom.com> writes:
> In Mr. Freeman's Ada example, he assumes that the private part of
> the package must contain a complete view of the data. That is exactly 
> the opposite of the example I posted earlier.

which was --

     package P is
       type T is tagged private;
       -- operations on T
     private 
       type Item;
       type Item_Pointer is access all Item;
       type T is tagged record
          Data : Item_Pointer;  -- a reference to hidden data
       end record;
     end P;

> In Ada, we can declare an incomplete type, an access to that type,
> enclose within a tagged record a value of the access type, and defer
> the full description of the data until later. This is the essence of
> an opaque type. Opaque types are convenient in both Ada and Modula-3
> because of their clear separation of interface module (package
> specification) from implementation module (package body).  This is
> more difficult in Java, C++, and even Eiffel, as demonstrated in
> Mr. Freeman's article.

In C++ we can do the following. I'm not sure if I've duplicated the
semantics you're aiming for, but I think I did. Unless I'm not
understanding your intent, I don't see why my C++ is any more
difficult than your Ada.

class P
{
private:
	class Item;
	typedef Item *Item_Pointer;
public:
	class T
	{
	public:
		// operations on T
	private:
		Item_Pointer Data;
	};
};