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From: BRYAN@SU-SIERRA.ARPA (Doug Bryan)
Newsgroups: net.lang.ada
Subject: design problem/?
Message-ID: <8509130608.AA02281@UCB-VAX.ARPA>
Date: Fri, 13-Sep-85 02:08:44 EDT
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Posted: Fri Sep 13 02:08:44 1985
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Hello expert Ada designers,

I could use your suggestions.  This is the design problem:

	Consider a heavily layered system.  One of the lower level packages
	exports a limited type.  The higher levels are all abstractions
	of the operations avaiable at all lower levels.  Thus higher level
	packages must reference the limited type and will export operations
	in this reference.  The reference will be used in all parameter
	modes.

	The problem is, what type do the higher level packages use to
	reference the lower level limited type?

For example:

		with text_io;
		package file_stuff is
		   type a_file is limited private;
		   procedure x (file : in out a_file);
		   ...
		private
		  type a_file somehow maps to text_io.file_type;
		end file_stuff;

I have been using access types:
		type a_file is access text_io.file_type;
This method has its limitations, for instance if the implementation of
the limited type is an unconstrained record (all objects created using
allocation are contrained).

Another method involves arrays:

		private
		   max_files : constant := 100;
		   type a_file is range 1 .. max_files;
		...
		package body file_stuff is
		   the_files : array (a_file) of text_io.file_type;
		   ...

The limitations to the array solution are obvious: fixed max number of
objects, lots of memory used when it may not need be ( of the user only
needs 2 files)....

what do you think??

doug
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