How do I know if an object can be weakly referenced?

Question

Foundation Release Notes for OS X v10.11 and iOS 9: NSNotificationCenter says: "If an object can be weakly referenced notifications will no longer be sent to the observer during deallocation." How can I tell "if an object can be weakly referenced"?

Answer 1

Basically, every object you know about can be weakly referenced. The previous sentence in the docs describes the exceptions to the rule: "i.e. it has a custom retain/release mechanism that would prevent the runtime from being able to store the object weakly". No object that you are likely to want to register fits that description.

Answer 2

There is some information on this in the Clang documentation:

It is explicitly permitted for Objective-C classes to not support __weak references. It is undefined behavior to perform an operation with weak assignment semantics with a pointer to an Objective-C object whose class does not support __weak references.

Rationale

Historically, it has been possible for a class to provide its own reference-count implementation by overriding retain, release, etc. However, weak references to an object require coordination with its class’s reference-count implementation because, among other things, weak loads and stores must be atomic with respect to the final release. Therefore, existing custom reference-count implementations will generally not support weak references without additional effort. This is unavoidable without breaking binary compatibility.

A class may indicate that it does not support weak references by providing the objc_arc_weak_reference_unavailable attribute on the class’s interface declaration. A retainable object pointer type is weak-unavailable if is a pointer to an (optionally protocol-qualified) Objective-C class T where T or one of its superclasses has the objc_arc_weak_reference_unavailable attribute. A program is ill-formed if it applies the __weak ownership qualifier to a weak-unavailable type or if the value operand of a weak assignment operation has a weak-unavailable type.

Additionally, assuming it's still current, Transitioning to ARC says:

Which classes don’t support weak references?

You cannot currently create weak references to instances of the following classes:

NSATSTypesetter, NSColorSpace, NSFont, NSMenuView, NSParagraphStyle, NSSimpleHorizontalTypesetter, and NSTextView.