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From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.lang.modula2
Subject: Re: Standard Modula2
Message-ID: <21589@neptune.inf.ethz.ch>
Date: 16 Jan 91 08:10:42 GMT
References:  
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 27

In article  Modula2 List  writes:
>>(according to Whirth)? Also if a pointer is unset will SIZE(ptr) return the
>>size of what ptr points to?
>If a pointer is "unset" (by that I assume you mean DEALLOCATEd), it
>doesn't point to anything, therefore whatever SIZE might return would
>have no meaning.  You can of course call SIZE on the type of the
>pointer and get the size that it would require.  As far as I could
>tell (and I didn't look that close) SIZE is not defined in the case
>of a pointer being NIL or undefined, although it might be reasonable
>to assume it would return 0 in the case of the former; what it
>returns in the case of the former is entirely up to the compiler
>writer and how the heap was implemented.

SIZE is most often (in fact, in each compiler that I know of) implemented as 
the size of the type. That is, the compiler evaluates the type of the 
expression that stands inside the parentheses and then replaces the 
SIZE(expression) by the size of this type, which is a constant. This means
especially that SIZE(expression) is never dependent on the actual value of
the expression. In the above case, it means that it does not make any 
difference whether the pointer is NIL or not, the result is always the size
of the type to which the pointer type is declared to point to.


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch