How do I flag a function as being deprecated in an iOS Objective-C header file?

Instead of __attribute__((deprecated)), you can use use the macros defined in <cdefs.h>:

- (void)fooBar __deprecated;
// Or better:
- (void)fooBar __deprecated_msg("Use barFoo instead.");

Or you can use the macros defined in <AvailabilityMacros.h>:

- (void)fooBar DEPRECATED_ATTRIBUTE;
// Or better:
- (void)fooBar DEPRECATED_MSG_ATTRIBUTE("Use barFoo instead.");

If you use Objective-C, it makes no difference as you are going to use a modern compiler, so you can go for Apple short syntax __deprecated_msg(). But if you use C for cross-platform, then DEPRECATED_MSG_ATTRIBUTE() uses the optimal availability definitions (for instance, it supports GCC3.1).

Try appending an attribute to your method declaration:

- (void)fooBar __attribute__ ((deprecated));

Taken from here.

Tim's answer will actually produce a compiler warning; the other versions are merely comments which have no effect w.r.t. the compiler.

If you look at /usr/include/AvailabilityMacros.h, you'll see how Apple does this. That header uses __attribute__((deprecated)) and __attribute__((unavailable)) depending on whether the API is present but deprecated, or has actually been removed from the OS.