Re: [ft-devel] MSVC build woes !

[Robin Watts]

On 11/06/2013 13:08, John Emmas wrote:
> #define FT_ASSERT( condition  )                               \
> do                                                            \
> {                                                             \
>     if ( !( condition ) )                                     \
>         FT_Panic( "assertion failed on line %d of file %s\n", \
>                   __LINE__, __FILE__ );                       \
> } while ( 0 )
>
> It usually gets called something like this:-
>
>            FT_ASSERT( some_var == some_other_var );
>
> and wherever it gets called, I get warning 4127.

Indeed.

> MSVC is warning that  there's no point putting this in a do/while loop,
> since nothing ever changes.  Rather than re-instate the pragma, a
> better solution would be to remove 'do' and 'while'.

No, no, no! Absolutely wrong!

There is a VERY good reason for the while being there. I will try to 
explain:

Imagine that you have the following code:

  if (x == 0)
      FT_ASSERT(some_condition);
  else
      return 42;
  return 23;

Without the while loop, the code would expand to:

   if (x == 0)
      if (!some_condition)
          FT_Panic( ... );
      else
          return 42;
   return 23;

(indentation changed for clarity)

i.e. the behaviour of the code is changed for x == 0; in the first case, 
the assert would be tested, then 23 would be returned. In the second the 
assert would be tested, then 42 would be returned.

By putting the do/while loop in, you avoid the possibility of the macro 
changing the surrounding control flow.

When writing macros in C it is considered good style to try to minimise 
unintended consequences of the macros use (a good example of "The 
Principle of Least Surprise").

> At first glance, it doesn't look like
> there'd be any adverse effects.  If you agree that removing 'do' and
> 'while' is the way forward, I'll find you the other occurrences.  Regards,

Those examples of do and while are put there very deliberately. Removing 
them would be a harmful step.

It is true that the MSVC warning is unhelpful, so finding a way to stop 
it (such as by putting the original set of lines back in) would be 
preferable.

Robin