Re: Feature request - a macro defined for GCC

[Chris Lattner <clattner at apple dot com>]

On Jul 3, 2008, at 2:12 PM, Josh Triplett wrote:
> I'd suggest defining exactly one new preprocessor symbol, to advertise
> the support for the feature-testing mechanism.  For instance,
> __HAVE_EXTENSION_SUPPORTED__, or __FEATURE_SUPPORTED_SUPPORTED__. :)
> The rest could use syntax like you suggest above.  For instance:


Sure.  The idea was that you'd do something like:

#ifndef __FEATURE_SUPPORTED_SUPPORTED__  /* or whatever :) */
#define has_extension(x) 0
#else
#define has_extension(x) __has_extension__(x)
#endif

which allows you to use "#if has_extension(typeof)" in your code  
portably.

In fact, it would be easy to make __has_extension__ actually be a  
macro itself, allowing one to say:

#ifdef __has_extension__
...

which is even more clear.

> The only problem then becomes maintaining the canonical list of
> extension names.  We could use the ugly approach of names like
> "org.gnu.gcc.extension.sentinel" and  
> "org.kernel.sparse.address_space",
> but that seems entirely too ugly.  The other alternative seems like a
> central registry of extension names; I'd happily help maintain such a
> registry.

I have no opinion on an approach, but I agree that it would be good to  
have a centralized list.  I would definitely add support for this to  
Clang.

-Chris