RE: [boost] Review: concept checks.

From: Moore, Paul (paul.moore_at_[hidden])
Date: 2000-11-20 11:07:42

From: Jeremy Siek [mailto:jsiek_at_[hidden]]
> > You have used a do{}while loop to scope your dummy variable, is this
> > actually required?  Could you not just use:
> > 
> > #define REQUIRE(__type_var, __concept) \
> > { \
> >   void (__concept##_concept <__type_var>::*__x)() = \
> >     BOOST_FPTR __concept##_concept <__type_var>::constraints; \
> >  (void) __x; }
> 
> I used do{}while out of habit... just {} should be fine.

The usual reason for do...while is that {...} is not "quite" a statement.
Example

#define test1 do { int n = 2; (void)n } while (0)
#define test2    { int n = 2; (void)n }

if (0)
    test1;
else
    /* whatever */;

if (0)
    test2;
else
    /* whatever */;

The second fails (dangling else) as test2; expands to {...}; and the extra ;
is a null statement which stops the else from coming straight after the
if...

I'm depressed that I know this...
Paul.