__block attribute on property declarations

Question

I have to fix some existing code that builds just fine with LLVM (on iOS) so that it builds with llvm-gcc-4.2 too. I'm done with pretty much everything, except this pattern which is found at a few places:

@property (nonatomic, retain) __block id myProperty;

I suspect the intent here is to allow access to the property from inside a block without retaining self. How can I remove the __block attribute, which gcc doesn't support here, but still achieve the same effect?

Answer 1

I'll suggest you've found a compiler bug, the declaration:

@property (nonatomic, retain) __block id myProperty;

is meaningless. The __block qualifier is used on local (stack allocated) variables so they are passed by reference to blocks, so they can be updated, and are usually[*] stored on the heap rather than the stack.

Therefore the qualifier __block has no meaning on a property declaration which is concerned with object instances, which are heap allocated at all times in Obj-C.

Just drop the __block from the property declarations - for every compiler.

[*] If a block is never copied to the heap a compiler may optimize __block variables and not move those to the heap either.

Answer 2

just before you use the variable in a block, create a local pointer of type __block. Don't ever use __block in @property declarations.

EG: TypeOfVariable __block *bock_pointer = self.property;

^{ inside the block use bock_pointer }