RE: [boost] Re: shifted_ptr review

From: Powell, Gary (powellg_at_[hidden])
Date: 2003-09-29 17:29:52

Powell > I'm a little concerned that it appears you can create bad pointers for
Powell > this class by inadvertently not using placement new. I'd feel better
Powell > about it if there was some way to prevent this.

Abrahams >Generic smart pointer factories are the right way:

Abrahams >   shifted_ptr<U> p = make_shifted_ptr<T>(arg1, arg2, arg3, ref(arg4));

So you are saying remove the constructor,

   template<class U>
      shifted_ptr( U *);

and replace it with one that takes a temporary that is created only by make_shifted_ptr?

   template<class U>
     shifted_ptr( shifted_ptr::details::hold_ptr<U> &);

and add the following pseudo code:

namespace shifted_ptr {
  namespace details {

template <class T>
class hold_ptr 
{
private:
   T *m_ptr;

public:
   T * release() { // hand it off to a shifted_ptr
      T * result = m_ptr;
      m_ptr = 0;
      return result;
   }
  
   hold_ptr(T *rhs)
   : m_ptr(rhs) {}   

   ~hold_ptr() { // fail to hand it off to a shifted_ptr.
       if (m_ptr) {
         ~(*m_ptr)(); // destruct it.
          m_ptr = 0;
       }
    }

};

} // details

template<class T, class MemoryModel>
hold_ptr<T> make_shifted_ptr(MemoryModel mm);

template<class T, class Arg1, class MemoryModel>
hold_ptr<T> make_shifted_ptr(class Arg1, mm);

.....

} // shifted_ptr

 You may have to pass the memory model into the hold_ptr class to know what the right thing to do destruct a lost ptr, I haven't fully thought this through. Anyway it seems a solvable problem with limited overhead and better validation that the pointers held by shifted_ptr are of the right type.

        -Gary-