In Clang's Objective-C Automatic Reference Counting we see the following
For __weak objects, the lvalue is updated to point to the new pointee, unless the new pointee is an object currently undergoing deallocation, in which case the lvalue is updated to a null pointer. This must execute atomically with respect to other assignments to the object, to reads from the object, and to the final release of the new pointee.
In objc-weak.mm wee see the following chunk of code in weak_register_no_lock():
if (deallocating) {
if (crashIfDeallocating) {
_objc_fatal("Cannot form weak reference to instance (%p) of "
"class %s. It is possible that this object was "
"over-released, or is in the process of deallocation.",
(void*)referent, object_getClassName((id)referent));
} else {
return nil;
}
}
I set a breakpoint in my UIViewController subclass dealloc method and tried invoking [self allowsWeakReference] in lldb which resulted in NO value.
If we try to set self to weak property of another object the app will crash in accordance with the objc-weak.mm code.
The question is – why does this happen? Is the clang's specification wrong? Is is this a bug in objc implementation?
Here is a simple piece of code that will reproduce the crash:
//cc -fmodules -fobjc-arc -g crash.m -o crash
@import Foundation;
@interface Foo : NSObject
@end
@implementation Foo
- (void)dealloc {
Foo * __weak weakSelf = self; // crashes on this line
}
@end
int main() {
(void)[[Foo alloc] init];
return 0;
}
