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From: eepjm@cc.newcastle.edu.au
Newsgroups: comp.lang.modula2
Subject: Re: How to write NEW?
Message-ID: <1991Apr22.155056.10010@cc.newcastle.edu.au>
Date: 22 Apr 91 05:50:56 GMT
References: <5122672@janhh.hanse.de>
Organization: University of Newcastle, AUSTRALIA
Lines: 45

In article <5122672@janhh.hanse.de>, jan@janhh.hanse.de (Jan Willamowius)
writes:
> It looks like Wirth is using different ways to allocate memory
> in different versions of his books. Sometimes he uses
> 'Allocate(Var,SIZE(Type)' and sometimes 'NEW(Var)'.
> 
> I like the second way better, but my compiler only supports Allocate.
> So, is there a way to write a NEW (and DISPOSE, of course) ?
> 
I wasn't going to respond to this at first, on the grounds that it's a
common problem for beginners and that surely somebody else would quickly
sort out Jan's problem ... then I watched in horror as a whole string of
responses seemed to confuse the issue even more.  Either a lot of people
have misunderstood the question, or I have.

The answer is actually quite simple, it's only the documentation that's
bad.  EVERY Modula-2 compiler supports NEW and DISPOSE (if you find one
that doesn't, you've been cheated) in the following way.  When NEW(Var)
appears in your source, it is automatically translated by the compiler
into Allocate(Var,SIZE(Var^)).  The catch is - and this is where almost all
first-time users of Modula-2 are trapped - that you have to remember to
import Allocate from module Storage (or elsewhere).  That is, you have to
write something like:

      FROM Storage IMPORT Allocate, Deallocate;

        .
        .
        NEW(p);

i.e. you have to call Allocate even though there is no explicit call to
Allocate in your program.  (By the way, some compilers spell it Allocate,
and others use the upper-case ALLOCATE).  NEW is not a procedure, even though
the syntax makes it look like one.  It's just a shorthand way of setting up
a call to Allocate.

By the way, you are perfectly free to re-write your own version of
Allocate if you don't like the one supplied with your compiler.  (I have
done this myself, since the version of module Storage supplied with the
FTL compiler doesn't have critical section protection, and crashes in
multitasking applications).

Hope this helps.

Peter Moylan                   eepjm@cc.newcastle.edu.au