Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uwm.edu!spool.mu.edu!munnari.oz.au!comp.vuw.ac.nz!cc-server4.massey.ac.nz!S.J.Lovatt
From: S.J.Lovatt@massey.ac.nz (S.J. Lovatt)
Newsgroups: comp.lang.modula2
Subject: Re: How to write NEW?
Keywords: NEW, DISPOSE
Message-ID: <1991Apr21.005506.24764@massey.ac.nz>
Date: 21 Apr 91 00:55:06 GMT
References: <5122672@janhh.hanse.de>
Organization: Massey University, Palmerston North, New Zealand
Lines: 54
X-Reader: NETNEWS/PC Version 2.2

From: jan@janhh.hanse.de (Jan Willamowius)
>
> 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) ?

< Code omitted >

> PROCEDURE NEW(VAR p : ADDRESS);
> BEGIN
>   Allocate(p,SIZE(p^));
> END NEW;
>
> The problem is that p 'looses' its type in the conversion to ADDRESS and
> only a WORD is Allocated (ADDRESS is POINTER TO WORD).
>
> Any ideas ?

Unfortunately, it is impossible to write NEW in Modula-2. One of the
reasons I prefer Modula-2 over Pascal (for instance) is that almost all of
the Modula-2's library procedures can be written in Modula-2. The
exceptions which now remain are things like VAL, INC etc which translate
directly into inline code on many compilers anyway.

If I desperately wanted to do NEW, DISPOSE, etc, I would tack a macro
processor on the front of my Modula-2 compiler (eg a C preprocessor) and
define NEW as a macro which expands to 'Allocate (p,SIZE(p^))', thus not
losing the size of p^. Modula-2 purists may feel free to cringe. Actually,
I don't usually do this when programming in Modula-2 as I'd have to give
up my integrated environment, although I do it a lot when programming in
Fortran on Unix systems. What I sometimes do if I am doing a lot of
allocating and deallocating is something like this:

TYPE     WidgetPtr = POINTER TO Widget;
         Widget = RECORD ...... END;

PROCEDURE NEWWidget (VAR p : WidgetPtr);
BEGIN
   Allocate (p,SIZE(p^));
END NEWWidget;

If your compiler supports inline procedures, this is equivalent to the
macro solution in space and time efficiency, while still being type-safe.

- Simon


-- 
Simon Lovatt            | S.J.Lovatt@massey.ac.nz |
Dept of Biotechnology   |                         |
Massey University, N.Z. |                         |