__block variables behavior with nested blocks

Question

I'm having a problem with __block variables in nested blocks. My question is, should a code like the following work?

__block NSString* s = nil;
[self methodWithBlock:^
 {
    s = [self methodThatReturnsAnAutoreleasedString];
    [self methodWithBlock:^
     {
         [NSLog @"%d", s.length];
     }];
 }];

I assumed the inner block retained s but that doesn't seem to be the case in my code. I get a "message sent to deallocated instance" when accessing s in the inner block. If I retain s when I assign it (s = [[self methodThatReturnsAnAutoreleasedString] retain];), it works fine.

Of course, that's not my actual code, which is more complex, but I just want to know if that minimal example should work fine. If so, I need to look for my problem elsewhere.

Answer 1

(I assume that you don't use ARC.) From the Transitioning to ARC Release Notes:

In manual reference counting mode, __block id x; has the effect of not retaining x.

So your assumption is wrong, the block does not retain s. You have to retain it if methodWithBlock works asynchronously.