Question

I’m using ARC.
Sometimes I wrote the following code to assert a object should be deallocated:

__weak weakVariableOrProperty = someObject;
....

someObject = nil;
// or someObject = anotherObject;
....

if (weakVariableOrProperty) {
    @throw [NSException exceptionWithName:NSInternalInconsistencyException reason:@"Object not deallocated" userInfo:nil];
}

For example, I use this code to check if a view controller is deallocated before creating a new view controller.

I believe that weak variable or weak property is set to nil immediately after last strong variable or property was set to nil or another object.

And this code is working as I expected until now.

Using weak variable or property to check if object is deallocated is a technique commonly used?
Is it possible that this code will cause problem in the future?

Answer 1

I believe that weak variable or weak property is set to nil immediately after last strong variable or property was set to nil or another object.

This is not exactly true, because an object could be autoreleased. In this case, the last strong reference may be gone, but the reference count of the instance would remain positive. In cases like that, the __weak reference would not be nil-ed out until the autorelease process takes place.

Is using weak variable or property to check if object is deallocated a technique commonly used?

I seriously doubt that this technique has gained much popularity, because ARC is a relatively new thing to Objective C. However, the technique appears valid.

Is it possible that this code will cause problem in the future?

This is very hard to guess, because the ARC Specification does not make any specific guarantees about the timing of nil-ing out the references, and because the spec allows compilers to optimize sequences of retain and release messages that they send to ARC objects.