参考答案

class Object;
class SmartPointer;

class Counter
{
 friend class SmartPointer;
public:
 Counter()
 {
  ptr = NULL;
  cnt = 0;
 }
 Counter(Object* p)
 {
  ptr = p;
  cnt = 1;
 }
 ~Counter()
 {
  delete ptr;
 }
private:
 Object* ptr;
 int cnt;
};

class SmartPointer
{
public:
 SmartPointer(Object* p)
 {
  ptr_counter = new Counter(p);
 }
 SmartPointer(const SmartPointer &sp)
 {
  ptr_counter = sp.ptr_counter;
  ++ptr_count->cnt;
 }
 SmartPointer& operator=(const SmartPointer &sp)
 {
  ++sp.ptr_counter->cnt;
  --ptr_counter->cnt;
  if (ptr_counter->cnt == 0)
  {
   delete ptr_counter;
  }
  ptr_counter = sp.ptr_counter;
 }
 ~SmartPointer()
 {
  --ptr_counter->cnt;
  if (ptr_counter->cnt == 0)
  {
   delete ptr_counter;
  }
 }
private:
 Counter* ptr_counter;

};