objective c - Can I declare dispatch_once_t predicate as a member variable instead of static?

Question

I want to run a block of code only once per instance.

Can I declare dispatch_once_t predicate as a member variable instead of static variable?

From GCD Reference, it is not clear to me.

The predicate must point to a variable stored in global or static scope. The result of using a predicate with automatic or dynamic storage is undefined.

I know I can use dispatch_semaphore_t and a boolean flag to do the same thing. I'm just curious.

Answer 1

dispatch_once_t must not be an instance variable.

The implementation of dispatch_once() requires that the dispatch_once_t is zero, and has never been non-zero. The previously-not-zero case would need additional memory barriers to work correctly, but dispatch_once() omits those barriers for performance reasons.

Instance variables are initialized to zero, but their memory may have previously stored another value. This makes them unsafe for dispatch_once() use.