From: "Brian F. Feldman" <green@...>
Date: 2001-07-23T01:48:40+09:00
Subject: [ruby-talk:18298] Re: NUM2LONG problem :)

Dave Thomas <Dave@PragmaticProgrammer.com> wrote:
> "Aleksei Guzev" <aleksei.guzev@bigfoot.com> writes:
> 
> > Just now I met a funny problem with NUM2LONG macro.
> > 
> > I've written a line like the following
> > 
> >  return NUM2LONG( fn( x ) );
> > 
> > Function fn has vivid side effect and NUM2LONG calls it twice ( see
> > its definition ) :) Do not do like above!
> 
> Patient: Doctor, it hurts when I type this.
> 
> Doctor: Then don't type that.
> 
> 
> Just call rb_num2long(fn(x)) directly.

The definition should be trivially fixed, nonetheless... the rules
I've learned for the implementation of macros have been:

* if they're meant to be a block of code not returning a value, they
  should be in "do { code } while (0)"
* parameters must be parenthesized in the implementation to allow for
  arbitrary expressions without operation order interference
* all parameters should be evaluated only once.

Generally, these are all easy to do and result in much more robust
macros that don't break in ways people don't expect.

-- 
 Brian Fundakowski Feldman           \  FreeBSD: The Power to Serve!  /
 green@FreeBSD.org                    `------------------------------'