frohlich
0f7b65a921
Extend SGAtomic with atomic exchange and add. Import updates from the original implementation of that in OpenFDM. Modified Files: Makefile.am SGAtomic.cxx SGAtomic.hxx SGReferenced.hxx SGSharedPtr.hxx Added Files: SGWeakPtr.hxx SGWeakReferenced.hxx
104 lines
3.6 KiB
C++
104 lines
3.6 KiB
C++
// Copyright (C) 2004-2009 Mathias Froehlich - Mathias.Froehlich@web.de
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//
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// This library is free software; you can redistribute it and/or
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// modify it under the terms of the GNU Library General Public
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// License as published by the Free Software Foundation; either
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// version 2 of the License, or (at your option) any later version.
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//
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// Library General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program; if not, write to the Free Software
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// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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//
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#ifndef SGWeakReferenced_HXX
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#define SGWeakReferenced_HXX
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#include "SGReferenced.hxx"
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#include "SGSharedPtr.hxx"
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template<typename T>
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class SGWeakPtr;
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class SGWeakReferenced {
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public:
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/// The object backref and the reference count for this object need to be
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/// there in any case. Also these are per object and shall not be copied nor
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/// assigned.
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/// The reference count for this object is stored in a secondary object that
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/// is shared with all weak pointers to this current object. This way we
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/// have an atomic decision using the reference count of this current object
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/// if the backref is still valid. At the time where the atomic count is
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/// equal to zero the object is considered dead.
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SGWeakReferenced(void) :
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mWeakData(new WeakData(this))
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{}
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SGWeakReferenced(const SGWeakReferenced& weakReferenced) :
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mWeakData(new WeakData(this))
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{}
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~SGWeakReferenced(void)
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{ mWeakData->mWeakReferenced = 0; }
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/// Do not copy the weak backward references ...
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SGWeakReferenced& operator=(const SGWeakReferenced&)
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{ return *this; }
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/// The usual operations on weak pointers.
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/// The interface should stay the same then what we have in Referenced.
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static unsigned get(const SGWeakReferenced* ref)
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{ if (ref) return ++(ref->mWeakData->mRefcount); else return 0u; }
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static unsigned put(const SGWeakReferenced* ref)
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{ if (ref) return --(ref->mWeakData->mRefcount); else return ~0u; }
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static unsigned count(const SGWeakReferenced* ref)
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{ if (ref) return ref->mWeakData->mRefcount; else return 0u; }
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private:
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/// Support for weak references, not increasing the reference count
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/// that is done through that small helper class which holds an uncounted
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/// reference which is zeroed out on destruction of the current object
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class WeakData : public SGReferenced {
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public:
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WeakData(SGWeakReferenced* weakReferenced) :
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mRefcount(0u),
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mWeakReferenced(weakReferenced)
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{ }
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template<typename T>
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T* getPointer()
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{
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// Try to increment the reference count if the count is greater
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// then zero. Since it should only be incremented iff it is nonzero, we
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// need to check that value and try to do an atomic test and set. If this
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// fails, try again. The usual lockless algorithm ...
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unsigned count;
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do {
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count = mRefcount;
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if (count == 0)
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return 0;
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} while (!mRefcount.compareAndExchange(count, count + 1));
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// We know that as long as the refcount is not zero, the pointer still
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// points to valid data. So it is safe to work on it.
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return static_cast<T*>(mWeakReferenced);
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}
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SGAtomic mRefcount;
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SGWeakReferenced* mWeakReferenced;
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private:
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WeakData(void);
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WeakData(const WeakData&);
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WeakData& operator=(const WeakData&);
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};
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SGSharedPtr<WeakData> mWeakData;
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template<typename T>
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friend class SGWeakPtr;
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};
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#endif
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