Re: Design Question Pt-1: Forcing const pointers and reference counting in graph structure

Nick Hounsome <>
Thu, 15 Oct 2009 02:11:04 CST
On 13 Oct, 19:43, Ismail Pazarbasi <> wrote:


I am trying to design a compact (scene) graph library. It has nothing
to do with actual rendering of a scene whatsoever but strictly dealing
with logical relationship between objects.

I didn't think about it thoroughly, and as a matter of fact there is
no real code at the moment. I didn't want to begin writing code before
thinking enough about it. If there is a fundamental flaw in the idea/
design, then there is no point to waste my time.

Traversing graph is the most important part and I consider using
parallel algorithms. To ease this operation, I thought about returning
a const pointer after instantiating objects. I thought this approach
would enforce a stricter policy for mutating objects, and increase
chance of dealing with read-only objects, make it easy to detect
races. To access mutator, caller should use MutableObject<T> for
"attaching" to the object to mutate.

The first problem I thought about was reference counting. All objects
are reference counted. Enough with long story, here is what I thought
(only theory, never compiled):

// reference counter, top in inheritance hierarchy
class RefCounted
   volatile long m_cref;
   long AddRef() { return atomic_increment(&m_cref); }
   long Release()
     long cref = atomic_decrement(&m_cref);
     if (0 == cref) delete this;
     return cref;
   virtual ~RefCounted() { }


// factory that instantiates object and return const pointer
template<typename T>
struct ObjectFactory
    static const T* Create()
       return new T;


// to prevent use of operator new directly
class ConstObject
   template<typename T> friend struct ObjectFactory<T>;
   void* operator new(size_t);
   void* operator new[](size_t);
   void operator delete(void*);
   // ... other overloads


// generic object class; holds basic information about an object
class Object : public RefCounted, public ConstObject
   template<typename T>
   friend class MutableObject<T>;
   void AttachMutator(); // some kind of mutex?
   void DetachMutator(); // unlocking the mutex?
   const std::string& GetName() const;
   void SetName(const std::string&);


// idea stolen from Microsoft's CComPtr
template<typename T>
class NoAddRefRelease : public T
   long AddRef();
   long Release();


// RefObj is intrusive smart pointer
template<typename T>
class RefObj
   T* m_p;
    explicit RefObj(const T* pT = NULL)
    : m_p(const_cast<T*>(pT)
      if (m_p) m_p->AddRef();
   NoAddRefRelease<T>* operator->() { return (NoAddRefRelease<T>*>)
m_p; } // see below note
   NoAddRefRelease<T>* get() const { return (NoAddRefRelease<T>*)m_p; }
   void reset(T* pT = NULL)
      if (m_p == pT) return;
      if (m_p) m_p->Release();
      if (pT) pT->AddRef();
      m_p = pT;


// a node in graph
class Node : public Object


To deal with RefObj<T> and reference counting, I thought may be
ObjectFactory<T>::Create returns a RefObj<T> directly, and, RefObj's
operator->() and get() returns a const NoAddRefRelease<T>*.

I would like to achieve following:
Node* pNode = new Node; // error
RefObj<Node> spNode = ObjectFactory<T>::Create();
spNode->SetName("myName"); // error
spNode.get()->SetName("myName"); // error
delete spNode.get(); // error
MutableObject<Node> mutNode(spNode);
mutNode->SetName("myName"); // ok

Therefore, MutableObject may look like:

template<typename T>
class MutableObject
   T* m_p;
   explicit MutableObject(RefObj<T>& rObj)
   : m_p(rObj.get())
   ~MutableObject() { m_p->DetachMutable(); }
   NoAddRefRelease<T>* operator->() { return m_p; }


This requires changes in RefObj<T> to allow access from
MutableObject<T> to its m_p or a private get() method which will
return a non-const qualified T*.

This is entirely on the paper, may be ridiculous, and I am not sure
how practical it is, and that's why I wanted to bring this up here for

I'd be very happy, if you share your valuable feedback.


It looks excessively complicated and confused to me.

For reference counting use shared_ptr (see boost or your compiler).

I don't understand what you are trying to do with MutableObject. It
seems to me that you are trying to violate const correctness.

The "normal" way to deal with graphs and const/mutable is to have a
graph class and work primarily through that rather than through the
nodes themselves - that way you don't have to convert a const node to
a mutable one.

The graph class also removes any need for a factory which is, in any
case, logically dubious - What does it mean to have a node that isn't
in a graph? and hence any desire for unnecessary clutter to prevent
allocating a node. The only downside is that you can't then have
polymorphic nodes but you don't seem to want them anyway.

If you do want to access a Node* from a const Node* then they way to
do it is to get the non const graph g to do it for you:

Graph g;
const Node* const_n;
Node* n = g.Find(const_n);

If the node contains a Graph* then

Node* Graph::Find(const Node* n) // graph is not const
     if( n->Graph() != this ) throw "a tantrum";
     return const_cast<Node*>(n);

You also seem to be concerned about concurrent access which is not
usually possible in the most general case for a graph so, again,
either it is best to work through a graph object as this can easily
provide thread safe operations or you probably want to go for some
sort of subtree locking (because it eliminates a lot of deadlock
problems) class using RAII:

subtree_lock l(node);
// subtree is locked
// subtree is unlocked

Hope this helps

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