1，动态创建对象
1.1 id class_createInstance ( Class cls, size_t extraBytes );
1.2 id objc_constructInstance ( Class cls, void *bytes );
1.3 void * objc_destructInstance ( id obj ); //销毁一个实例
2,把父类对象转换为子类对象
NSObject *a = [[NSObject alloc] init];
id newB = object_copy(a, class_getInstanceSize(MyClass.class));
object_setClass(newB, MyClass.class);
object_dispose(a);
//MyClass继承自NSObject。
3，读写对象的实例变量
3.1 修改实例变量的值
Ivar object_setInstanceVariable ( id obj, const char *name, void *value );
void object_setIvar ( id obj, Ivar ivar, id value );
3.2 获取实例变量的值
Ivar object_getInstanceVariable ( id obj, const char *name, void **outValue );
id object_getIvar ( id obj, Ivar ivar );
3.3 获取对象的类
Class object_getClass ( id obj );
3.4 设置对象的类
Class object_setClass ( id obj, Class cls );
3.5 返回对象的类名
const char * object_getClassName ( id obj );
