Strong Autoreleasing

Pass a pointer reference as argument

By default, we pass a pointer reference using Objective-C like this:

- (void) testReference:(NSObject **) object

When compiling, the compiler will add the default key word __autoreleasing for us to inform how to deal with this pointer reference like this:

- (void) testReference:(NSObject * __autoreleasing *) object

So we can also add key word __strong explictly by ourself:

- (void) testReference:(NSObject * __strong *) object

What’s the difference?

NSObject *object = [[NSObject alloc] init];
[self testReference:&object];

When using __autoreleasing to decorate the pointer reference of object, the code above will be transformed into:

NSObject *object = [[NSobject alloc] init];
NSObject * __autoreleasing tmp = object;
[self testReference:&object];
object = tmp;

Obviously, __autoreleasing will copy a referenc of the origin pointer reference than pass the copy one to function. When the function finished, the new value of the copy reference will be copied back to the origin one. But __strong will pass the origin reference to the function.

###Why use __autoreleasing as default choice?

It is because if using __strong as default one, some references of weak pointer may cause logic problem when using a __strong pointer to copy their value.

Links

What are the advantages of declaring method arguments autoreleasing

Handling pointer to pointer ownership issues in arc