On Apr 14, 2011, at 1:19 PM, Jonathan Taylor wrote:
> I tested this out before replying as I wasn't 100% certain. It may be that we
> have misunderstood each other somehow, but the following code (in a clean new
> project) was what I used to confirm to myself that two concurrently-executing
> threads have independent instances of the variable:
> http://www.dur.ac.uk/j.m.taylor/block_test_code.m
More likely, I've misunderstood the documentation. It's happened before, many
times. :)
I looked at your code only briefly now but will return to study it in detail
later (kinda busy right now). I must say that I'm confused now more than
before, because I really thought that the __block qualifier meant that there is
only one instance of that variable, shared by all scopes that declare it and
shared by all blocks that capture it.
>>> However I don't think you should need to do this anyway. I would change
>>> your code to something like this:
>>>
>>> - (SomeObjType) foo
>>> {
>>> dispatch_sync(queue,
>>> ^{
>>> // Code in this block ensures bah is valid
>>> if (nil == bah)
>>> {
>>> // Code to compute and store bah goes here
>>> }
>>> });
>>>
>>> return bah;
>>> }
>>
>> I don't see how that could possibly work as is because bah is out of scope
>> by the time the return statement is executed. Perhaps you meant that bah is
>> an ivar or property declared in the same class as the accessor. I'm suddenly
>> drawing a blank here because I considered this before and came to the
>> conclusion that it wouldn't work but now I can't remember why I thought that
>> way. I need to think this through a bit more.
> OK, well since you do not declare bah locally, and from what you are doing it
> is presumably meant to be persistent, I assumed that it was indeed a property
> (or a global?). If you are doing something else with it (though I'm not quite
> sure what...) then obviously what I wrote may be wrong...
No, actually you are totally correct here. As it happens, I do use the pattern
you suggest, in several places where the conditions are precisely those you
laid out: the ivars are meant to be instantiated only once and never released.
It's in other cases where those conditions break that I'm struggling with.
Clearly I need to refine my understanding of blocks and discussions like this
greatly help.
Thanks again.
WT_______________________________________________
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