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From: leake@cme-durer.ARPA (Stephe Leake)
Newsgroups: comp.lang.ada
Subject: Re: Ada language revision
Message-ID: <749@marvin.cme-durer.ARPA>
Date: 29 Nov 88 15:35:35 GMT
References: <733@marvin.cme-durer.ARPA> <3656@hubcap.UUCP>
Organization: National Bureau of Standards, Gaithersburg, MD
In-reply-to: billwolf@hubcap.clemson.edu's message of 23 Nov 88 22:22:13 GMT
List-Id: <comp.lang.ada>


In article <3656@hubcap.UUCP> billwolf@hubcap.clemson.edu (William Thomas Wolfe,2847,) writes:

	 Consider an ADT which contains pointers to substructures.
	 A user declares an access type pointing to an instance of your
	 ADT, and uses UNCHECKED_DEALLOCATION to destroy that instance
	 after finishing with it.  UNCHECKED_DEALLOCATION will not 
	 recursively chase down and free up what might constitute 
	 over 99% of the space occupied by the ADT.   Similarly, if your
	 ADT uses another ADT in its implememtation, your DESTROY procedure
	 will include a call to the DESTROY procedure of the sub-ADT,
	 but UNCHECKED_DEALLOCATION will remain oblivious to the method
	 which must be used to properly destroy the sub-ADT.

I see. Currently, the ADT author must provide a Destroy procedure, and
hope that users of the ADT call it as needed. So it would be nice if
the compiler would enforce this. This doesn't sound to bad; we just
need to add a generic parameter to UNCHECKED_DEALLOCATION (LRM 13.10.1):

generic
    type OBJECT is limited private;
    type NAME is access OBJECT;
    with procedure DESTROY (Y : in OBJECT) is <>;
procedure UNCHECKED_DEALLOCATION (X : in out NAME);

With a default definition of DESTROY declared in Standard, defining
UNCHECKED_DEALLOCATION this way would not break any current Ada
program that uses it. There is still a slight problem; if the
appropriate DESTROY procedure is not visible at the time of
instantiation, we lose.

Stephe Leake 	(301) 975-3431 		leake@cme.nbs.gov
National Institute of Standards and Technology
(formerly National Bureau of Standards)
Rm. B-124, Bldg. 220
Gaithersburg, MD  20899