A simple implementation of Smart Pointer Class

Question

In book C++ Primer 13.5.1, it implement a Smart Pointer Class using a Use-Count Class. Their implementation is as follows:

• Use-Count Class

  // private class for use by HasPtr only
  class U_Ptr {
      friend class HasPtr;
      int *ip;
      size_t use;
      U_Ptr(int *p): ip(p), use(1) { }
      ~U_Ptr() { delete ip; }
  };
  

• Smart Pointer Class

  /* 
     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; p must 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; } 
  
      friend ostream& operator<<(ostream&, const HasPtr&);
      // 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; }
  private:
      U_Ptr *ptr;        // points to use-counted U_Ptr class
      int val;
  };
  

---

Wonder: I am curious about why not simply using a int * to act like the Use-Count Class, just like the int* countPtr; used in the following new Smart Pointer Class:

class T
{
private:
    int* countPtr; //

    int* p;
    int val;

public:
    T(){
        p = new int();
        countPtr = new int();
        *countPtr = 1;
        val = 0;
    }
    T(T& t){
        p = t.p;
        countPtr = t.countPtr;
        val = t.val;
        *countPtr += 1;
    }
    T& operator = ( const T& rT){
        if(*countPtr>1){
            *countPtr -= 1;
        }
        else{
            delete p;
            delete countPtr;
        }
        p = rT.p;
        countPtr = rT.countPtr;
        val = rT.val;
        *countPtr += 1;
        return *this;
    }
    ~T(){
        if(*countPtr>1){
            *countPtr -= 1;
        }
        else{
            delete p;
            delete countPtr;
        }
    }

    int *get_ptr() const { return p; } 
    int get_int() const { return val; }

    // change the appropriate data member
    void set_ptr(int *ptr) { p = ptr; }
    void set_int(int i) { val = i; }
};

---

Test: I tested the above Smart Pointer Class using code like the following and it seems working well.

int main()
{
    T t1;
    T t2(t1);
    T t3(t1);
    T t4;
    t4 = t1;

    return 0;
}

---

Real question: Is this new Smart Pointer Class with simply a int *countPtr sufficient enough? If yes, why bother to use an extra Use-Count Class like in the book? If no, what do I miss?

Solution 1

One property of the original implementation is that the delete is performed, in the control block object, with the original pointer type. This is a partial type erasure. No matter how much the smart pointer objects are copied, with somewhat different types, the original control block remains the same, with delete via the original pointer type.

However, since the original code you show is not templated, one must assume that it is an early example, followed later by similar templated code.

Converting a pointer up in a base class hierarchy, as can happen with copying of a smart pointer, means that delete on the new pointer type is only valid if the statically known new type has a virtual destructor.

For example, std::shared_ptr also deletes (guaranteed) via the original pointer type, unless one explicitly supplies a deleter functor that does something else.

Solution 2

My guess is that the author - whether consciously or subconsciously - is aware that having a separate class is useful in real-world smart pointers, e.g.:

• a count of weak pointers (not sure if you'll have heard of them yet - they track an object without extending its lifetime, such that you can try to convert one into a (normal) shared pointer later, but it only works if there's at least one shared pointer to the object around to have kept it alive)

• a mutex to make the shared pointer thread safe (though atomic operations may be better when available),

• debugging information (e.g. boost::shared_ptr has a #ifdef to include an shared counter id)

• virtual dispatch table, used by e.g. boost shared pointers to dispatch to OS-appropriate code (see boost/smart_ptr/detail/sp_counted_base_*.hpp headers)

I don't know the book, but perhaps they'll go on to explain what else might go into U_Ptr....

Solution 3

Your code is equivalent to the standard code reported by the book. However it is worst in some respects:

1. you need two allocations/deallocations instead of one (two ints instead of a single object). This might be slower and a little bit more difficult to manage.

2. you have a copy of the pointer duplicated in every object. So: duplicated information which you should guarantee to keep valid.

3. your object is larger (two pointers instead of one)

You only have one positive note:

1. the access to the pointer is direct instead of having one indirection. This could mean that the access to the referred object is slightly faster with your implementation...

Comments

[Jeremy Friesner]

A mutex shouldn't be necessary in most cases; you can use an atomic int instead...

[Tony Delroy]

@JeremyFriesner: true, though atomic operations weren't in the Standard library until C++11 so an older book wanting to walk the reader through an implementation might avoid them. I'll add some other examples.