On Feb 18, 2012, at 9:23 AM, Fritz Anderson wrote:
> On 18 Feb 2012, at 7:41 AM, steven Hooley wrote:
>> Matt Neuburg wrote:
>>> The same issue came up again later the same day:
>>>
>>> __block UIBackgroundTaskIdentifier bti = [[UIApplication sharedApplication]
>>> beginBackgroundTaskWithExpirationHandler:
>>> ^{
>>> [[UIApplication sharedApplication] endBackgroundTask:bti];
>>> }];
>>>
>>> Without __block, bti is invalid, and you won't find that out easily because
>>> it's unlikely that you'll actually expire. :) m.
>>
>> Can this be right?
>
> If I understand why you're mystified, consider this, without the __block
> declaration.
>
> - bti starts as junk.
>
> - The block is instantiated, and captures the junk _value_ (because it's not
> a __block variable) of bti.
>
> - The beginBackgroundTask… method creates the background task identifier.
>
> - That identifier is then assigned to bti. bti is no longer junk, but that
> does the block no good, because it's already captured the junk value.
Precisely. clang now warns about this case (or will soon; I don't know which
release of Xcode has this).
test.m:9:33: warning: variable 'x' is uninitialized when captured by block
[-Wuninitialized]
id x = [Foo methodWithBlock:^{ return x; }];
--
Greg Parker gpar...@apple.com Runtime Wrangler
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