Is readonly property always “atomic”?

Question

Sometimes we have a simple readOnly Property whose value may change

@property (readonly) NSFetchedResultsController * FetchController;
@property (readonly) NSFet         


        

Answer 1

I think you're misunderstanding what atomic (or more appropriately, nonatomic) means in this context. The simplest explanation can be found in objc-accessors.mm itself:

id objc_getProperty_non_gc(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
    // Retain release world
    id *slot = (id*) ((char*)self + offset);
    if (!atomic) return *slot;

    // Atomic retain release world
    spin_lock_t *slotlock = &PropertyLocks[GOODHASH(slot)];
    _spin_lock(slotlock);
    id value = objc_retain(*slot);
    _spin_unlock(slotlock);

    // for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
    return objc_autoreleaseReturnValue(value);
}

As you can see, atomicity here refers only to the validity of the returned object. It it retained and autoreleased in order to guarantee that it does not get deallocated while you are using it.

If this [[obj retain] autorelease] was not performed, another thread could set the property, which would cause the previous property (i.e. the one you're using) to be released and possibly deallocated.