Loki/include/loki/LockingPtr.h

////////////////////////////////////////////////////////////////////////////////
// The Loki Library
// Copyright (c) 2001 by Andrei Alexandrescu
// This code is from the article:
//     "Generic<Programming>: volatile — Multithreaded Programmer’s Best Friend
//     Volatile-Correctness or How to Have Your Compiler Detect Race Conditions
//     for You" by Alexandrescu, Andrei.
//     Published in the February 2001 issue of the C/C++ Users Journal.
//     http://www.cuj.com/documents/s=7998/cujcexp1902alexandr/
// Prepared for Loki library by Richard Sposato
////////////////////////////////////////////////////////////////////////////////

// $Header$


#ifndef LOKI_LOCKING_PTR_INC_
#define LOKI_LOCKING_PTR_INC_

namespace Loki
{

    template<class T>
    struct NonConstObject
    {
        typedef T Type;
    };

    template<class T>
    struct ConstObject
    {
        typedef const T Type;
    };



    /** @class LockingPtr
     Locks a volatile object and casts away volatility so that the object
     can be safely used in a single-threaded region of code.
     Original version of LockingPtr had only one template - for the shared
     object, but not the mutex type.  This version allows users to specify a
     the mutex type as a LockingPolicy class.  The only requirements for a
     LockingPolicy class are to provide Lock and Unlock methods.
     */
    template < typename SharedObject, typename LockingPolicy = LOKI_DEFAULT_MUTEX, 
               template<class> class ConstPolicy = NonConstObject >
    class LockingPtr
    {
    public:

        typedef typename ConstPolicy<SharedObject>::Type ConstOrNotType;

        /** Constructor locks mutex associated with an object.
         @param obj Reference to object.
         @param mtx Mutex used to control thread access to object.
         */
        LockingPtr( volatile ConstOrNotType & object, LockingPolicy & mutex )
           : pObject_( const_cast< SharedObject * >( &object ) ),
            pMutex_( &mutex )
        {
            mutex.Lock();
        }

        /// Destructor unlocks the mutex.
        ~LockingPtr()
        {
            pMutex_->Unlock();
        }

        /// Star-operator dereferences pointer.
        ConstOrNotType & operator * ()
        {
            return *pObject_;
        }

        /// Point-operator returns pointer to object.
        ConstOrNotType * operator -> ()
        {
            return pObject_;
        }

    private:

        /// Default constructor is not implemented.
        LockingPtr();

        /// Copy-constructor is not implemented.
        LockingPtr( const LockingPtr & );

        /// Copy-assignment-operator is not implemented.
        LockingPtr & operator = ( const LockingPtr & );

        /// Pointer to the shared object.
        ConstOrNotType * pObject_;

        /// Pointer to the mutex.
        LockingPolicy * pMutex_;

    }; // end class LockingPtr


    template<typename SharedObject,    typename LockingPolicy = LOKI_DEFAULT_MUTEX>
    struct Locking
    {
        typedef LockingPtr<SharedObject, LockingPolicy, NonConstObject> Ptr;
        typedef LockingPtr<SharedObject, LockingPolicy, ConstObject>    ConstPtr;
    };


#if 0
    /** @class ConstLockingPtr
     Similar to LockingPtr, except that it returns pointers and references to
     a const SharedObject instead of a mutuable SharedObject.
     @see LockingPtr
     */
    template < typename SharedObject, typename LockingPolicy = LOKI_DEFAULT_MUTEX >
    class ConstLockingPtr
    {
    public:

        /** Constructor locks mutex associated with an object.
         @param obj Reference to const object.
         @param mtx Mutex used to control thread access to object.
         */
        ConstLockingPtr( volatile const SharedObject & object,
            LockingPolicy & mutex )
            : pObject_( const_cast< const SharedObject * >( &object ) ),
            pMutex_( &mutex )
        {
            mutex.Lock();
        }

        /// Destructor unlocks the mutex.
        ~ConstLockingPtr()
        {
            pMutex_->Unlock();
        }

        /// Star-operator dereferences pointer.
        const SharedObject & operator * ()
        {
            return *pObject_;
        }

        /// Point-operator returns pointer to object.
        const SharedObject * operator -> ()
        {
            return pObject_;
        }

    private:

        /// Default constructor is not implemented.
        ConstLockingPtr();

        /// Copy-constructor is not implemented.
        ConstLockingPtr( const ConstLockingPtr & );

        /// Copy-assignment-operator is not implemented.
        ConstLockingPtr & operator = ( const ConstLockingPtr & );

        /// Pointer to the shared object.
        const SharedObject * pObject_;

        /// Pointer to the mutex.
        LockingPolicy * pMutex_;

    }; // end class ConstLockingPtr
#endif


} // namespace Loki

#endif  // end file guardian



// $Log$
// Revision 1.7  2006/01/22 13:37:33  syntheticpp
// use macro LOKI_DEFAULT_MUTEX for Mutex default value, defined in Threads.h
//
// Revision 1.6  2006/01/21 14:09:09  syntheticpp
// replace LockPtr/ConstLockPtr implementation with a template policy based one
//
// Revision 1.5  2006/01/21 01:02:12  rich_sposato
// Added Mutex class to Loki.  Made it the default policy class for locking.
//
// Revision 1.4  2006/01/19 19:34:19  rich_sposato
// Added ConstLockingPtr class.
//
// Revision 1.3  2006/01/16 18:34:37  rich_sposato
// Changed return type from LockingPtr to SharedObject.
//
// Revision 1.2  2006/01/14 00:20:10  syntheticpp
// remove c&p error
//
// Revision 1.1  2005/11/19 22:00:23  rich_sposato
// Adding LockingPtr class to Loki project.
//