Re: Can't understand this problem with ref counting class?

From:
Francesco <entuland@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 14 Aug 2009 11:02:35 -0700 (PDT)
Message-ID:
<8c81f75f-8511-4fdc-b5e4-2554535284e3@z31g2000yqd.googlegroups.com>
On Aug 14, 7:06 pm, Angus <anguscom...@gmail.com> wrote:

Here is my code

///reference counting
struct ref_type
{
  ref_type() : cnt(0) {}

  int ref_add()
  {
        return ++cnt;
  }
  int ref_remove()
  {
   return --cnt;
  }

  unsigned ref_cnt() const { return cnt; }

private:
  unsigned cnt;

};

template <class T>
// T must derive from ref_type
// or supply methods ref_add and ref_remove
  struct ref_ptr
  {
    ref_ptr() : ptr(0) {}
    ref_ptr(T* p) : ptr(p) { if(p) p->ref_add(); }
    ref_ptr(const ref_ptr& that) : ptr(that.ptr) { if(ptr) ptr->ref_add
(); }

    ~ref_ptr() { dereference(); }

    T& operator*() const { return *ptr; }
    T* operator->() const { return ptr; }
    operator T*() const { return ptr; }

    void reset() { dereference(); ptr = 0; }

    void swap(ref_ptr& that) { std::swap(ptr, that.ptr); }

    T* release() //dereference without deleting
    {
      T* p = ptr;
      if(p) {
        p->ref_remove();
        ptr = 0;
      }
      return p;
    }

    ref_ptr& operator=(const ref_ptr& that)
    {
      if(&that!=this && that.ptr!=ptr) {
        dereference();
        if((ptr = that.ptr)!=0)
          ptr->ref_add();
      }
      return *this;
    }

    bool operator==(const ref_ptr& that) const { return ptr ==
that.ptr; }
    bool operator==(T* that) const { return ptr == that; }

    bool operator!=(const ref_ptr& that) const { return ptr !=
that.ptr; }
    bool operator!=(T* that) const { return ptr != that; }

    bool operator<(const ref_ptr& that) const { return ptr <
that.ptr; }
    bool operator<(T* that) const { return ptr < that; }

  private:
    T* ptr;

    void dereference()
    {
        if (ptr!=0 && ptr->ref_remove()==0)
            delete ptr;
    }
  };
//template <class T> END

template<class T>
void swap(ref_ptr<T>& a, ref_ptr<T>& b) { a.swap(b); }

class test : public ref_type
{
public:
    void set(int val){ m_value = val; }
    int get() const { return m_value; }

  typedef ref_ptr<test> ptr_type;
private:
    int m_value;

};

int main(){
    test::ptr_type pmytest = new test;
    pmytest->set(4);
    test::ptr_type pTest1(pmytest); //increments ref count

    //in above code line test::ptr_type pTest1(pmytest); correctly
increments ref counter to 2

    //however, if I use like this: test* pmytest = new test; then ref
counter is not incremented to 2. Why?

    return 0;

}

For convenience I want to be able to use more convenient test* pTest =
new test; - and for ref counter to be incremented. It works if use
test::ptr_type. But test inherits from ref_type so why does it not
work?

Angus


First of all, you should post complete code to facilitate people
testing it.
You missed to add the include and the actual test that demonstrates
your problem.

If I got it right, here is my response: I suppose that's because the
counter increment is done by the "template struct ref_ptr" - actually,
by the specific "ref_ptr<test> ptr_type" typedef. It's that template
that calls ref_add().

When you create a normal pointer with "test* pmytest = new test",
you're not actually using the template, hence those functions aren't
called at all.

Hope that helps,
Francesco.

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