Xref: utzoo comp.lang.c:15546 comp.lang.c++:2426
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From: mat@mole-end.UUCP (Mark A Terribile)
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Re: Would *you* get caught by this one?
Summary: What's the catch?
Keywords: Assignops
Message-ID: <144@mole-end.UUCP>
Date: 14 Jan 89 11:49:57 GMT
References: <139@mole-end.UUCP>
Organization: mole-end--private system.  admin: mole-end!newtnews
Lines: 52

> Here's a simple C question on a point that caught me recently.  Would *you*
> get caught?
> 
> 	When is     a *= b      not the same as     a = a * b   ?

I've seen a number of replies, and all but one respondant has concentrated
on things like the preprocessor or C++'s potential overloading of *= .  There
was also a mention of volatile.

It's much simpler than that, and much uglier.   To give credit where credit
is due I will take the liberty of publishing part of a private correspondence:

>From: Chris Torek <mimsy.umd.edu!chris@gatech>
>
>a *= b is always equivalent if both a and b are simple variables
>or expressions (i.e., no side effects, etc.).  A number of compilers
>do miscompile int_var *= float_expr, however.
>
>Chris

Yes, they do.  Given that the issue isn't directly addressed in either K&R,
I wasn't going to come out and say which way is right, but Chris did and,
since Microsoft (at least) agrees with him, I accept his judgement on the
matter.  (I remain open to a detailed examination of dpANS).

Some compilers interpret  i *= f  as

		i *= (int) f

This is obviously not the same as

		i = (int)( i * f )

and can produce wildly improbable results if the value of f is, let us say,
0.95 .

>Subject: i *= f
>
>is supposed to mean i = (int)(i * f).  PCC simply had a bug.  (The
>old PDP-11 compiler got it right...)
>
>Chris

Since PCC did define the language for many people for many years, this
``simply'' has widespread ramifications.

Chris gets a gold star and a bottle of Bactine for the burns suffered in
discovering this.
-- 

(This man's opinions are his own.)
From mole-end				Mark Terribile