关联对象

关联对象会用被关联对象作为key，将关联对象存储到全局的哈希表里。

AssociationHashMap

AssociationsHashMap是用来存储关联对象的哈希表。

class AssociationsHashMap : public unordered_map
<disguised_ptr_t, ObjectAssociationMap *,
 DisguisedPointerHash, DisguisedPointerEqual, 
AssociationsHashMapAllocator> {
    public:
        void *operator new(size_t n) { return ::malloc(n); }
        void operator delete(void *ptr) { ::free(ptr); }
    }

AssociationsHashMap是继承的unordered_map, unordered_map是一个无序的键值对容器。unordered_map定义
键的类型是disguised_ptr_t, 值的类型是ObjectAssociationMap *。

disguised_ptr_t

typedef uintptr_t disguised_ptr_t;
typedef unsigned long  uintptr_t;

哈希表的key是disguised_ptr_t类型的，他其实是被关联的对象经过处理得到的：

//object是被关联的对象
disguised_ptr_t disguised_object = DISGUISE(object);

ObjectAssociationMap

哈希表的value是ObjectAssociationMap *类型的，ObjectAssociationMap也是一个map：

class ObjectAssociationMap : public std::map<void *, ObjcAssociation, 
ObjectPointerLess, ObjectAssociationMapAllocator> {
    public:
        void *operator new(size_t n) { return ::malloc(n); }
        void operator delete(void *ptr) { ::free(ptr); }
    };

这个哈希表的key就是关联对象的key，哈希表的value是 ObjcAssociation类型的对象

   ObjectAssociationMap *refs = new ObjectAssociationMap;
   (*refs)[key] = ObjcAssociation(policy, new_value);
   object->setHasAssociatedObjects();

ObjcAssociation

ObjcAssociation是用来存储关联对象的：

class ObjcAssociation {
        uintptr_t _policy;
        id _value;
    public:
        ObjcAssociation(uintptr_t policy, id value) : _policy(policy), _value(value) {}
        ObjcAssociation() : _policy(0), _value(nil) {}

        uintptr_t policy() const { return _policy; }
        id value() const { return _value; }
        
        bool hasValue() { return _value != nil; }
    };

总结：