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Path: utzoo!watmath!watdaisy!ndiamond
From: ndiamond@watdaisy.UUCP (Norman Diamond)
Newsgroups: net.lang
Subject: Re: levelheight: automatic "nil pointer" handling
Message-ID: <6881@watdaisy.UUCP>
Date: Fri, 25-Jan-85 10:04:48 EST
Article-I.D.: watdaisy.6881
Posted: Fri Jan 25 10:04:48 1985
Date-Received: Sat, 26-Jan-85 04:49:59 EST
References: <2340@hplabsc.UUCP> <4948@utzoo.UUCP> <6292@boring.UUCP> <536@mako.UUCP>
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 42

> > For instance, it is possible to statically guarantee that nil pointers are
> > never dereferenced (imagine the bugs that that would prevent!) but I know of
> > no language that supports it.
> 
> Ada supports this:  [quoted from Booch]
> 
> |Not only can we explicitly create objects designated by access values, but we
> |can also destroy them.  Such objects will no longer be accessible if all
> |references to them are removed, such as:
> |
> |	MY_PACKET := new BUFFER;	-- a BUFFER object is created
> |	MY_PACKET := null;		-- the original object is unreachable
> |
> |Furthermore, these dynamic objects will be destroyed once we leave the scope
> |of the access objects.  For example:
> |
> |	declare
> |		POINTER : BUFFER_POINTER;
> |	begin
> |		...
> |		POINTER := new BUFFER;	-- create a BUFFER object
> |		...
> |	end;				-- BUFFER object is destroyed

The "answer" has nothing to do with the problem.  Of course objects that are
no longer referenced can be de-allocated.  Furthermore, it is done dynamically.

The first netter's suggested problem was to diagnose programs that dereference
nil pointers.  This too can be done dynamically.  He suggested that it can be
done statically.  I find that very hard to believe.

Can a compiler do static checking and generate code to de-allocate objects, as
in the second netter's quotation?  I find that very hard to believe, too.
Aside from this disbelief, the two issues have little in common.

-- Norman Diamond

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"Opinions are those of the keyboard, and do not reflect on me or higher-ups."