[vox-tech] Nifty macro expansion bug

[Peter Jay Salzman]

On Thu 18 Jan 07,  4:14 PM, Bill Kendrick <nbs at sonic.net> said:
> 
> So at work, a coworker noticed an issue with the following code:
> 
>   for (i = 0; i < n; i++)
>     myFREE(foo[i]);
> 
> which went away when he wrapped it in braces:
> 
>   for (i = 0; i < n; i++)
>   {
>     myFREE(foo[i]);
>   }
> 
> Figured maybe it was a weird compiler bug.  Being author of the myFREE()
> code -- which happens to be a macro -- I realized immediately what I did
> wrong.  (And continue to be annoyed with C syntax ;) )
> 
> See, myFREE() is actually a macro that looks like this:
> 
>   #define myFREE(x) if ((x) != NULL) FREE(x); x = NULL;
> 
> Two statements... hence the need for {} around the call to the macro.
> (Or, more sensibly, including {}s around what the macro expands to.)
> 
> That was a neat catch. :)
 

I ran into something similar, but more complicated, last week.  An
expression that should've executed didn't.

I looked at the output of "cl.exe -E" (similar to gcc -E) which displays
what the compiler "sees" after the preprocessor does its thing.  From this
view, it was easy to see that what looked like:


   if ( some condition )
      die( some error message )
   else
      do_something();


was arriving at the compiler as:


   if ( some condition )
      if ( lots of stuff )
         print error message with lots of stuff.
      else
         do_something();

So the code needs to look like:

   if ( some condition ) {
      die( some error message )
   } else {
      do_something();
   }


Pete

-- 
How VBA rounds a number depends on the number's internal representation.
You cannot always predict how it will round when the rounding digit is 5.
If you want a rounding function that rounds according to predictable rules,
you should write your own.
              -- MSDN, on Microsoft VBA's "stochastic" rounding function

Peter Jay Salzman, email: p at dirac.org web: http://www.dirac.org/p    
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