为什么__weak属性需要运行时支持

Question

In boost C++, a weak pointer is implemented as an observer to a shared (reference counted) pointer.

How are they implemented in objective-c, and why does this require runtime support? (ie besides having compiler support, iOS 5 or above is required to use weak references)

Answer 1

Objective-C weak references are set to nil when the object they point to is deallocated. This is very convenient (it lets you solve reference cycles and avoid sending messages to deallocated objects), but it does require that the runtime track all weak references to objects and, when the object is finally deallocated, the runtime must nil out those references.

Both of these things can only be done transparently by the Objective-C runtime. Objective-C does not have the same flexibility that C++ does for implementing this kind of magic yourself in your own code.

Answer 2

std::weak_ptr actually contains a pointer to a shared data-structure that holds book-keeping data and the referenced object. When the referenced object is destroyed this shared data is kept around so that weak_ptrs can see the book-keeping data that indicates the object is gone, and the weak_ptrs themselves don't have to be modified when the referenced object is deallocated.

In Objective-C weak references do not point to some intermediate object that holds book-keeping data. They are normal pointers that point either to the actual referenced object, or to nil if the referenced object is gone. Every __weak pointer has to be updated when some other part of the code releases the last non-weak pointer to an object. This requires runtime support.

In other words, Obj-C weak pointers are normal pointers except that there's compiler magic (which uses runtime support) working on them, whereas shared_ptrs and weak_ptrs are just wrappers that implement their own runtime support (in the smart pointers' constructors, assignment operators, destructors, etc.) around pointers.