Re: Smart pointers and inclomplete types

Kai-Uwe Bux <>
Sun, 27 Apr 2008 08:11:37 -0400
Marcel M?ller wrote:

I have a problem with a cyclic dependency of two classes:

class Iref_count // Interface for the intrusive_ptr
{ friend class int_ptr_base; // for access to Count
   volatile unsigned Count;
   Iref_count() : Count(0) {}
   // You must not call the non-virtual destructor directly.
   ~Iref_count() {}

class Slice;

class Iterator
{ intrusive_ptr<Slice> root;
   // some more stuff

class Slice : public Iref_count
{ scopec_ptr<Iterator> start;
   scopec_ptr<Iterator> stop;
   // some more stuff

To get this to compile either scoped_ptr or intrusive_ptr have to accept
an incomplete type.

Unfortunately I did not have luck with this so far. Neither Slice nor
Iterator are PODs. And the forward declaration of Slice generates wrong
code in intrusive_ptr at the conversion from Iref_count* to Slice*. The
pointer value is not translated by the offset of Iref_count.

Note that intrusive_ptr is not boost::intrusive_ptr, because that won't
compile on my platform. I wrote something similar (see below). However,
I am unsure whether boost::intrusive_ptr would have defined behavior in
this case, and if it does so, why?

It would not.

However, std::tr1::shared_ptr (and boost::shared_ptr) probably would.

Any Ideas?
The cyclic dependency is really needed from the designs point of view.

You might want to have a look at the implementation of boost::shared_ptr.
The key idea is to store a deleter within the shared_ptr and initialize
that one upon construction (with an appropriate default). This way, the
problem can be postponed until shared_ptr objects need to be initialized.
Only at that point, the type has to be complete.

/* Abstract non-template base class of int_ptr */
class int_ptr_base
   Iref_Count* Ptr;
   // Store a new object under reference count control
   // or initialize a NULL pointer.
   int_ptr_base(Iref_Count* ptr);
   // Copy constructor
   int_ptr_base(const int_ptr_base& r);
   // Destructor core
   Iref_Count* unassign();
   // some more functions...

template <class T>
class int_ptr : protected int_ptr_base
   // Store a new object under reference count control
   // or initialize a NULL pointer.
   int_ptr(T* ptr = NULL) : int_ptr_base(ptr) {}
   // Destructor, frees the stored object if this is the last reference.
   ~int_ptr() { delete (T*)unassign(); }

The line above is either wrong or too smart: nothing (except the comment)
indicates that the reference has to be last.

   // Basic operators
   T* get() const { return (T*)Ptr; }
   T& operator*() const { assert(Ptr); return *(T*)Ptr; }
   T* operator->() const { assert(Ptr); return (T*)Ptr; }
   // some more functions...

int_ptr_base::int_ptr_base(Iref_Count* ptr)
: Ptr(ptr)
{ if (Ptr)
     ++Ptr->Count; // normally InterlockedInc(Ptr->Count);
int_ptr_base::int_ptr_base(const int_ptr_base& r)
: Ptr(r.Ptr)
{ if (Ptr)
     ++Ptr->Count; // normally InterlockedInc(Ptr->Count);

Iref_Count* int_ptr_base::unassign()
{ return Ptr && --Ptr->Count == 0 ? Ptr : NULL; // normally


Kai-Uwe Bux

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