SGWeakReferenced.hxx

// Copyright (C) 2004-2009  Mathias Froehlich - Mathias.Froehlich@web.de
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Library General Public
// License as published by the Free Software Foundation; either
// version 2 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Library General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
//

#ifndef SGWeakReferenced_HXX
#define SGWeakReferenced_HXX

#include "SGReferenced.hxx"
#include "SGSharedPtr.hxx"

template<typename T>
class SGWeakPtr;

class SGWeakReferenced {
public:
  SGWeakReferenced(void) :
    mWeakData(new WeakData(this))
  {}
  SGWeakReferenced(const SGWeakReferenced& weakReferenced) :
    mWeakData(new WeakData(this))
  {}
  ~SGWeakReferenced(void)
  { mWeakData->mWeakReferenced = 0; }

  SGWeakReferenced& operator=(const SGWeakReferenced&)
  { return *this; }

  static unsigned get(const SGWeakReferenced* ref)
  { if (ref) return ++(ref->mWeakData->mRefcount); else return 0u; }
  static unsigned put(const SGWeakReferenced* ref)
  { if (ref) return --(ref->mWeakData->mRefcount); else return ~0u; }
  static unsigned count(const SGWeakReferenced* ref)
  { if (ref) return ref->mWeakData->mRefcount; else return 0u; }

private:
  class WeakData : public SGReferenced {
  public:
    WeakData(SGWeakReferenced* weakReferenced) :
      mRefcount(0u),
      mWeakReferenced(weakReferenced)
    { }

    template<typename T>
    T* getPointer()
    {
      // Try to increment the reference count if the count is greater
      // then zero. Since it should only be incremented iff it is nonzero, we
      // need to check that value and try to do an atomic test and set. If this
      // fails, try again. The usual lockless algorithm ...
      unsigned count;
      do {
        count = mRefcount;
        if (count == 0)
          return 0;
      } while (!mRefcount.compareAndExchange(count, count + 1));
      // We know that as long as the refcount is not zero, the pointer still
      // points to valid data. So it is safe to work on it.
      return static_cast<T*>(mWeakReferenced);
    }

    SGAtomic mRefcount;
    SGWeakReferenced* mWeakReferenced;

  private:
    WeakData(void);
    WeakData(const WeakData&);
    WeakData& operator=(const WeakData&);
  };

  SGSharedPtr<WeakData> mWeakData;

  template<typename T>
  friend class SGWeakPtr;
};

#endif

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