OW_COWIntrusiveCountableBase.hpp

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//
//  Copyright (c) 2001, 2002 Peter Dimov
//
//  Permission to copy, use, modify, sell and distribute this software
//  is granted provided this copyright notice appears in all copies.
//  This software is provided "as is" without express or implied
//  warranty, and with no claim as to its suitability for any purpose.
//

#ifndef OW_COW_INTRUSIVE_COUNTABLE_BASE_HPP_INCLUDE_GUARD_
#define OW_COW_INTRUSIVE_COUNTABLE_BASE_HPP_INCLUDE_GUARD_

#include "OW_config.h"
#include "OW_RefCount.hpp"

namespace OW_NAMESPACE
{

class COWIntrusiveCountableBase; 
void COWIntrusiveReferenceAddRef(COWIntrusiveCountableBase * p); 
void COWIntrusiveReferenceRelease(COWIntrusiveCountableBase * p); 
bool COWIntrusiveReferenceUnique(COWIntrusiveCountableBase* p); 
template <typename T> T* COWIntrusiveReferenceClone(T* p); 

class OW_COMMON_API COWIntrusiveCountableBase
{
private:
   RefCount m_usecount;

protected:
   COWIntrusiveCountableBase(COWIntrusiveCountableBase const &)
      : m_usecount(0)
   {}

   COWIntrusiveCountableBase & operator=(COWIntrusiveCountableBase const &x)
   {
      // don't assign or change the ref count, since it won't be different.
      return *this;
   }

   COWIntrusiveCountableBase(): m_usecount(0)
   {
   }

   virtual ~COWIntrusiveCountableBase();

   RefCount getRefCount() const
   {
      return m_usecount;
   }

public:
   inline friend void COWIntrusiveReferenceAddRef(COWIntrusiveCountableBase * p)
   {
      p->m_usecount.inc();
   }

   inline friend void COWIntrusiveReferenceRelease(COWIntrusiveCountableBase * p)
   {
      if (p->m_usecount.decAndTest())
         delete p;
   }
   
   inline friend bool COWIntrusiveReferenceUnique(COWIntrusiveCountableBase* p)
   {
      return p->m_usecount.get() == 1;
   }

   template <typename T>
   friend T* COWIntrusiveReferenceClone(T* p); 
};

template <typename T>
inline T* COWIntrusiveReferenceClone(T* p)
{
   // this needs to happen first to avoid a race condition between 
   // another thread deleting the object and this one making a copy.
   T* tmp = p->clone();
   if (p->m_usecount.decAndTest())
   {
      // only copy--don't need to clone, also not a race condition.
      // undo the decAndTest.
      p->m_usecount.inc();
      delete tmp; // we won't need this anymore.
      return p;
   }
   else
   {
      // need to become unique
      if (tmp) COWIntrusiveReferenceAddRef(tmp);

      return tmp;
   }
}

} // end namespace OW_NAMESPACE

#endif

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