Revisiting generalized lvalues

[Andrew Pinski]

> 
> I don't know if it's too late, but there is yet another reason
> for the use of generalized lvalues besides just ignorance:
> using them in macros that verify pointer and struct usage
> in a large framework. A comma expression makes it very
> convenient, for example (just a general idea):
> 
> #ifdef  PRODUCTION
> #define X_ABC(x)       ( check( x ), x->abc )
> #else
> #define X_ABC(x)        x->abc
> #endif
> 
> which expands
> 
>      X_ABC(x) = y;
> 
> to:
> 
>      ( check( x ), x->abc ) = y;
> 
> It is something that is used during development only and
> is switched off for the production builds. C++ compatibility
> is not an issue either.


GCC does this through another extension, like:
#ifdef CHECKING
#define CHECK_X(x)   	({const tree __t = x; check(__t); __t; })
#else
#define CHECK_X(x)	x
#endif
#define X_ABC(x) 	CHECK_X(x)->abc


-- Pinski