Availability.h -like macro

Question

Is it possible to have a custom availability macro like the __OSX_AVAILABLE_STARTING for instance. I need it to perform in the same way, I just need to change its name and the versions and number of parameters?

Answer 1

Yes, certainly. Objective-C is a strict superset of C, so C macros are very much at your disposal, and that facility is simply a set of C macros that eventually expand to gcc's __attribute__ keyword to declare special attributes of a function.

The relevant declarations are all in

• Availability.h
• AvailabilityInternal.h

To refresh, you use the __OSX_AVAILABLE_STARTING macro to tag a function declaration as being supported for a particular version, like this:

extern void mymacfunc() __OSX_AVAILABLE_STARTING(__MAC_10_5,__IPHONE_NA);

So what do we need to implement this ourselves? If you strip their support for two different OS (mac, iphone), the availability facility boils down to:

1. A macro that takes a version argument like __MY_AVAILABLE_STARTING(<version>):

   #define __MY_AVAILABLE_STARTING(_myversion) __MY_AVAILABILITY_INTERNAL##_myversion
   

2. Set of version arguments, like those in Availability.h, that are valid arguments for the above:

   #define __MYVER_2_0     20000  
   #define __MYVER_2_1     20100  
   #define __MYVER_2_2     20200  
   #define __MYVER_3_0     30000  
   

3. Another set of macros, like thos in AvailabilityInternal.h that specifies what should happen for each version (regular support, deprecated, unavailable, weak, etc). Again, this is a function of the compiler, see gcc docs (there are lots of other interesting options):

   #define __MY_AVAILABILITY_INTERNAL__MYVER_2_0 __AVAILABILITY_INTERNAL_UNAVAILABLE
   #define __MY_AVAILABILITY_INTERNAL__MYVER_2_1 __AVAILABILITY_INTERNAL_WEAK_IMPORT
   #define __MY_AVAILABILITY_INTERNAL__MYVER_2_1 __AVAILABILITY_INTERNAL_REGULAR
   

4. And finally, where the buck ends, the macros that expand to the __attribute__ facility.

   For the ones I have above, you can just keep using Apple's macros:

   #define __AVAILABILITY_INTERNAL_DEPRECATED         __attribute__((deprecated,visibility("default")))
   #define __AVAILABILITY_INTERNAL_UNAVAILABLE        __attribute__((unavailable,visibility("default")))
   #define __AVAILABILITY_INTERNAL_WEAK_IMPORT        __attribute__((weak_import,visibility("default")))
   #define __AVAILABILITY_INTERNAL_REGULAR            __attribute__((visibility("default")))
   

   Or, of course, you can define your own craziness.

C Macros are powerful stuff, often overlooked. Good luck!