Question

There is a conceptual overview of Blocks objects in objective-c within this Apple reference:

http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Blocks/Blocks.pdf

However, it does not really explain two topics that concern me and may concern other people. The first question is like that: can I assign a nil to a Block reference? Or should I use NULL? Or can I use neither of them?

The second problem lies in the sphere of memory management. Say, I have declared such a method creating a block object on the stack.

-(void)makeTheClass
{
    TheClass *object = [[TheClass alloc] init];

    object.blockReference = ^(void) { return nil; } 
}

This object, being created within some scope, is going to be destroyed after it goes out of it. But TheClass object is actually going to store a reference to this (nearly destroyed) Block:

typedef id (^WeirdBlockType)(void);

@interface TheClass {
    WeirdBlockType blockReference;   
}

How do I declare a class property for such a block?
What’s the difference between these two:

@property (nonatomic, retain) WeirdBlockType blockReference;
@property (nonatomic, copy)   WeirdBlockType blockReference; 

?

It is clearly said in the Apple documentation that block copying moves the block to the heap. But what if I just retain it? Is it going to be destroyed after it goes out of makeTheClass method scope?

Answer 1

Well I’ve found the solution.
Thanks to Gojan for his answer, but he was actually wrong in one place:

Wevah was right. Retain on a block has no effect until it is completely moved to the heap, and only Block_copy accomplishes such a task.

Perhaps blocks are not the only objects that cannot be retained while they are on the stack; but whilst you create (alloc and init) any NSObject subclass instances on the heap by default, you don’t care about it – retain works as usual. Block objects are created on the stack by default, that’s why the work a little unexpectedly.

Thanks everybody!