gistfile1.m

// 1.
// “Safe” but retain cycle.
[self setCompletionBlock:^{
    NSLog(@"1: %@", self->_foo);
}];


// 2.
// Unsafe. Could dereference nil.
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    NSLog(@"2: %@", weakSelf->_foo);
}];


// 3.
// Unsafe. Could be nilled out on another thread between check and dereference.
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    if (weakSelf)
        NSLog(@"3: %@", weakSelf->_foo);
}];


// 4.
// Unsafe. Could dereference nil.
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    __strong BCThing *strongSelf = weakSelf;
    NSLog(@"4: %@", strongSelf->_foo);
}];


// 5.
// Safe
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    __strong BCThing *strongSelf = weakSelf;
    if (strongSelf)
        NSLog(@"5: %@", strongSelf->_foo);
}];


// 6.
// Safe and clean
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    NSLog(@"5: %@", [weakSelf foo]);
}];

// ... or, if you need to check for nil
__weak BCThing *weakSelf = self;
[self setCompletionBlock:^{
    __strong BCThing *strongSelf = weakSelf;
    if (strongSelf)
        [someArray addObject:strongSelf];
}];