Question

As I’m mastering my skills with multithreading with GCD, I’ve come across some question. Suppose you have the following method:

    - (void)method {
    NSString *string= [NSString string]; //will be autoreleased

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    //very very lengthy operation...

    NSLog(@"%@", string); //is it safe?
    });
}

I’m wondering if this is correct, because I think I should have retained string before the block execution: in fact I fear that the event loop finishes and sends string an autorelease message before using string in the block. That would crash the program.

Am I right? Should I send a retain and a release message to string or this is the correct implementation?
Thanks in advance!

Answer 1

I’m wondering if this is correct, because I think I should have retained string before the block execution: in fact I fear that the event loop finishes and sends string an autorelease message before using string in the block.

Fear not:
A block captures the scope of the surrounding method/function in that it automatically retains any object-variable that is used inside of the block. Be aware of that when you use self inside of a block, as this may greatly affect the lifetime of the object!

There is one exception to this rule, and that are variables declared as

__block SomeObjectPointerType variableName

Update

Because there’s a new comment on this answer, I should probably add that things changed a little with the introduction of ARC:

Under ARC all object variables default to __strong, and this holds for variables marked with __block as well. If you want to avoid strong capturing of a variable in a block, you should define a local variable that is __weak.

End Update

If you like to learn more about blocks, bbum gave an excellent session called Introducing Blocks and Grand Central Dispatch on iPhone at WWDC 2010.

The “Block Details” section starts at 11:30.