1) Why does this retain its __block var:
{
void (^blockWithOutPointer)(NSObject * __autoreleasing *) = ^(NSObject * __autoreleasing * outPointer) {
*outPointer = [NSObject new];
};
NSObject * __block blockVar1;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(1 * NSEC_PER_SEC)),
dispatch_get_main_queue(),
^{
NSLog(@"blockVar1: %@",
blockVar1);
// prints non-nil. WHY????
});
blockWithOutPointer(&blockVar1);
}
2) But this doesn't?
void (^blockWithOutPointerThatDispatchesLater)(NSObject * __autoreleasing *,
dispatch_block_t) = ^(NSObject * __autoreleasing * outPointer,
dispatch_block_t block) {
dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(1 * NSEC_PER_SEC)),
dispatch_get_main_queue(),
block);
*outPointer = [NSObject new];
};
{
NSObject * __block blockVar2;
blockWithOutPointerThatDispatchesLater(&blockVar2,
^{
NSLog(@"blockVar2: %@",
blockVar2);
});
// prints nil, which is expected.
}
3) If I instead use an __autoreleasing variable as my out pointer destination, and then assign that variable to my __block pointer, everything works fine.
{
NSObject * __autoreleasing autoreleasingVar;
NSObject * __block blockVar3;
blockWithOutPointerThatDispatchesLater(&autoreleasingVar,
^{
NSLog(@"blockVar3: %@",
blockVar3);
});
blockVar3 = autoreleasingVar;
// prints non-nil, which is expected.
}
I've read CRD's answer about ARC pointer-to-pointer issues, it makes sense that #2 would print nil because ARC assumes that blockVar2 is __autoreleasing, and doesn't retain its value. Thus, in #3, when we assign autoreleasingVar to blockVar3, ARC properly retains the value. However, there is no such assignment for #1. Why does #1 retain its value?
Even more amazingly, #1 is unaffected if I wrap the out-pointer assignment in an @autoreleasepool:
{
void (^blockWithOutPointer)(NSObject * __autoreleasing *) = ^(NSObject * __autoreleasing * outPointer) {
@autoreleasepool {
*outPointer = [NSObject new];
}
};
NSObject * __block blockVar1;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(1 * NSEC_PER_SEC)),
dispatch_get_main_queue(),
^{
NSLog(@"blockVar1: %@",
blockVar1);
// still prints non-nil. WHY???
});
blockWithOutPointer(&blockVar1);
}
Whereas #3 crashes, as expected since the @autoreleasepool released the out-pointer's object, and I guess ARC doesn't set __autoreleasing variables to nil.
void (^blockWithOutPointerThatDispatchesLater)(NSObject * __autoreleasing *,
dispatch_block_t) = ^(NSObject * __autoreleasing * outPointer,
dispatch_block_t block) {
dispatch_after(dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(1 * NSEC_PER_SEC)),
dispatch_get_main_queue(),
block);
@autoreleasepool {
*outPointer = [NSObject new];
}
};
{
NSObject * __autoreleasing autoreleasingVar;
NSObject * __block blockVar3;
blockWithOutPointerThatDispatchesLater(&autoreleasingVar,
^{
NSLog(@"blockVar3: %@",
blockVar3);
// crashes on the NSLog!
});
blockVar3 = autoreleasingVar;
}
I filed a radar about this.
