[cfe-dev] Proposed change to __builtin_overload

[Ken Ferry]

On Tue, Feb 10, 2009 at 1:44 PM, Chris Lattner <clattner at apple.com> wrote:

>
> On Feb 9, 2009, at 8:33 AM, Douglas Gregor wrote:
> >>
> >
> > It would take a bit of work to support such a pragma in Clang,
> > because we currently have quite a few places where we make a lot of
> > assumptions about language-specific data structures based on the
> > current language we're parsing (e.g., we assume that all class types
> > are handled by CXXRecordDecl in C++ mode). However, it seems
> > reasonable that we would work toward making Clang #pragma c++'able.
> >
> > In the shorter term, I think some kind of __overload__ attribute
> > would be relatively easy to implement. Most of the pain would be in
> > making sure that the C++ implicit-conversion rules handle all of the
> > funky C99 types :)
>
> I spoke with Doug more about this offline, and I agree.  We should add
> an __attribute__((overloadable)) or something like that.  Doug
> volunteered to tackle this when he gets to a good intermediate point
> on his template work.  I filed PR 3542 to track this.
>
> Thanks Doug!  When this is done, I'll do the horrible header hacking
> stuff to hook it up for tgmath.h to use.
>
> -Chris
>

I'm curious if this would be useful for a case I've hit as a user of
compilers.

In Cocoa and CoreFoundation, there are a bunch of types that different as
far as the compiler is concerned, but the user knows that they are the same.
 NSString* and CFStringRef are interchangeable, for example.

I'd love to be able to inform the compiler that certain types are
compatible, but short of that, I have an NSToCF macro that does sort of a
type-safe cast.  If passed an argument of type of NSString*, for example,
the result is type CFStringRef.  It looks like this:

#define NSToCF(nsObject) ( \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSArray*), (CFArrayRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSAttributedString*), (CFAttributedStringRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSCalendar*), (CFCalendarRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSCharacterSet*), (CFCharacterSetRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSData*), (CFDataRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSDate*), (CFDateRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSDictionary*), (CFDictionaryRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSError*), (CFErrorRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSLocale*), (CFLocaleRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableArray*), (CFMutableArrayRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableAttributedString*), (CFMutableAttributedStringRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableCharacterSet*), (CFMutableCharacterSetRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableData*), (CFMutableDataRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableDictionary*), (CFMutableDictionaryRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableSet*), (CFMutableSetRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSMutableString*), (CFMutableStringRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSNumber*), (CFNumberRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSInputStream*), (CFReadStreamRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSTimer*), (CFRunLoopTimerRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSSet*), (CFSetRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSString*), (CFStringRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSTimeZone*), (CFTimeZoneRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSURL*), (CFURLRef)(nsObject), \
__builtin_choose_expr (__builtin_types_compatible_p (typeof (nsObject),
NSOutputStream*), (CFWriteStreamRef)(nsObject), \
1.0/*error on type not found*/ )))))))))))))))))))))))))

Would the changes you're talking about make this less disgusting?

-Ken
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