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From: mab@dst17.wdl.loral.com (Mark A Biggar)
Subject: Re: Mut. Recurs. in Ada9X w/o Breaking Encaps.? (LONG)
Message-ID: <1994Sep27.184827.17813@wdl.loral.com>
Keywords: Ada 9X, C++, object-oriented
Sender: news@wdl.loral.com
Organization: Loral Western Development Labs
References: <1994Sep27.165203.9192@swlvx2.msd.ray.com>
Date: Tue, 27 Sep 1994 18:48:27 GMT
Xref: bga.com comp.lang.ada:6240 comp.object:6837 comp.lang.c++:30660
Date: 1994-09-27T18:48:27+00:00
List-Id: <comp.lang.ada>

[Discussion of how to set up mutualy recursive tagged types]

First of all you really only need to have a forward definition of one
of the two types, lets choose office.

So first define a abstract office type that doesn't knoe about employees:

	package Abs_Office is

	type Abstract_Office is abstract tagged null record;
	type Access_Office is access all Abstract_Office'class;

	end Abs_Office;


Now we can define the employee type in terms of the above abstract type:

	with Abs_Office; use Abs_Office;
	package Emp is

	type Employee is tagged record
		Office: Access_Office;
	end record;
	type Employee_Access is access all Employee'class;

	end Emp;

Now we can define the real office type:

	with Abs_Office; use Abs_Office;
	with Emp; use Emp;
	package Off is

	type Office is new Abstract_Office with record
		Employee: Employee_Access;
	end record;

	end Off;

This of course ignores the private aspects of the C++ example given in the 
original article, but these are easly added using private types and/or
private extensions in the above packages.

Moral: if you need mutualy recursive tagged types with out breaking
encapsulation, then use an abstract type for you forward reference.

--
Mark Biggar
mab@wdl.loral.com