SmartPointer Smar指针

#include<iostream>

using namespace std;

// private class for use by HasPtr only
class U_Ptr
{
    /**
    *因为在这个例子中所有的复制都是复制的“变量的常引用”，所以将U_Ptr这个变量的use增加，其他的HasPtr的use也会跟随着改变，
    *【目标变量一直就只有一个，所有的复制都只是改变指针而已】
    */
    friend class HasPtr;
    int *ip;
    size_t use;
    U_Ptr(int *p): ip(p), use(1) { }
    ~U_Ptr() { delete ip; }
};

/*** smart pointer class: takes ownership of the dynamically allocated
*          object to which it is bound
* User code must dynamically allocate an object to initialize a HasPtr
* and must not delete that object; the HasPtr class will delete it
***/
class HasPtr
{
     public:
         // HasPtr owns the pointer; pmust have been dynamically allocated
         HasPtr(int *p, int i): ptr(new U_Ptr(p)), val(i) { }
         // copy members and increment the use count
         HasPtr(const HasPtr &orig):
                ptr(orig.ptr), val(orig.val) { ++ptr->use; }
         HasPtr& operator=(const HasPtr&);
         // if use count goes to zero, delete the U_Ptr object
         ~HasPtr() { if (--ptr->use == 0) delete ptr; }
     public:
         // copy control and constructors as before
         // accessors must change to fetch value from U_Ptr object
         int *get_ptr() const { return ptr->ip; }
         int get_int() const { return val; }
         // change the appropriate data member
         void set_ptr(int *p) { ptr->ip = p; }
         void set_int(int i) { val = i; }
         // return or change the value pointed to, so ok for const objects
         // Note: *ptr->ip is equivalent to *(ptr->ip)
          int get_ptr_val() const { return *ptr->ip; }
          void set_ptr_val(int i) { *ptr->ip = i; }
          int get_usecount(){return ptr->use;}
     private:
         U_Ptr *ptr;
         // points to use-counted U_Ptr class
         int val;
};

HasPtr& HasPtr::operator=(const HasPtr &rhs)
{
          ++rhs.ptr->use;     // increment use count on rhs first
          if (--ptr->use == 0)
                delete ptr;    // if use count goes to 0 on this object, delete it
          ptr = rhs.ptr;      // copy the U_Ptr object
          val = rhs.val;      // copy the int member
          return *this;
}
int main()
{
    int obj;cout<<"obj="<<obj<<endl;;

HasPtr ptr1(&obj,0);cout<<"usecount1="<<ptr1.get_usecount()<<endl;
    HasPtr ptr2(ptr1);cout<<"usecount1="<<ptr1.get_usecount()<<endl;cout<<"usecount2="<<ptr2.get_usecount()<<endl;
    HasPtr ptr3(ptr1);cout<<"usecount1="<<ptr3.get_usecount()<<endl;cout<<"usecount2="<<ptr2.get_usecount()<<endl;

cout<<"int_A:"<<ptr1.get_int()<<"   int_B:"<<ptr2.get_int()<<"  int_C:"<<ptr3.get_int()<<endl;
    ptr1.set_int(9527); // changes val member only in ptr1
    cout<<"int_A:"<<ptr1.get_int()<<"   int_B:"<<ptr2.get_int()<<"  int_C:"<<ptr3.get_int()<<endl;

cout<<"ptr_val_A:"<<ptr1.get_ptr_val()<<"   ptr_val_B:"<<ptr2.get_ptr_val()<<"  ptr_val_C:"<<ptr3.get_ptr_val()<<endl;
    ptr1.set_ptr_val(123); // sets object to which both ptr1 and ptr2 point
    cout<<"ptr_val_A:"<<ptr1.get_ptr_val()<<"   ptr_val_B:"<<ptr2.get_ptr_val()<<"  ptr_val_C:"<<ptr3.get_ptr_val()<<endl;

cout<<"========================================";
}