libreoffice/include/salhelper/singletonref.hxx

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#ifndef INCLUDED_SALHELPER_SINGLETONREF_HXX
#define INCLUDED_SALHELPER_SINGLETONREF_HXX

#include <osl/mutex.hxx>
#include <rtl/instance.hxx>
#include <osl/diagnose.h>
#include <osl/getglobalmutex.hxx>


namespace salhelper{


/** @short  template for implementing singleton classes.

            Such classes can be instantiated every time they
            are needed. But the internal wrapped object will
            be created one times only. Of course its used
            resources are referenced one times only too.
            This template hold it alive till the last
            reference is gone. Further all operations
            on this reference are threadsafe. Only
            calls directly to the internal object (which modify
            its state) must be made threadsafe by the object itself
            or from outside.

    @attention  To prevent the code against race conditions, its not
                allowed to start operations inside the ctor
                of the internal wrapped object - especially operations
                which needs a reference to the same singleton too.

                The only chance to suppress such strange constellations
                is a lazy-init mechanism.

                <ul>
                    <li>a) The singleton class can provide a special init()
                           method, which must be called as first after creation.</li>
                    <li>b) The singleton class can call a special impl_init()
                           method implicit for every called interface method.</li>
                </ul>

                Note further that this singleton pattern can work only, if
                all user of such singleton are located inside the same library!
                Because static values can't be exported - e.g. from windows libraries.
 */
template< class SingletonClass >
class SingletonRef
{

    // member

    private :

        /** @short  pointer to the internal wrapped singleton. */
        static SingletonClass* m_pInstance;

        /** @short  ref count, which regulate creation and removing of m_pInstance. */
        static sal_Int32 m_nRef;


    // interface

    public :



        /** @short  standard ctor.

                    The internal wrapped object is created only,
                    if its ref count was 0. Otherwise this method
                    does nothing ... except increasing of the internal
                    ref count!
         */
        SingletonRef()
        {
            // GLOBAL SAFE ->
            ::osl::MutexGuard aLock(SingletonRef::ownStaticLock());

            // must be increased before(!) the check is done.
            // Otherwise this check can fail inside the same thread ...
            ++m_nRef;
            if (m_nRef == 1)
                m_pInstance = new SingletonClass();

            OSL_ENSURE(m_nRef>0 && m_pInstance, "Race? Ref count of singleton >0, but instance is NULL!");
            // <- GLOBAL SAFE
        }



        /** @short  standard dtor.

                    The internal wrapped object is removed only,
                    if its ref count wil be 0. Otherwise this method
                    does nothing ... except decreasing of the internal
                    ref count!
         */
        ~SingletonRef()
        {
            // GLOBAL SAFE ->
            ::osl::MutexGuard aLock(SingletonRef::ownStaticLock());

            // must be decreased before(!) the check is done.
            // Otherwise this check can fail inside the same thread ...
            --m_nRef;
            if (m_nRef == 0)
            {
                delete m_pInstance;
                m_pInstance = 0;
            }
            // <- GLOBAL SAFE
        }



        /** @short  Allows rSingle->someBodyOp().
         */
        SingletonClass* operator->() const
        {
            // GLOBAL SAFE ->
            ::osl::MutexGuard aLock(SingletonRef::ownStaticLock());
            return m_pInstance;
            // <- GLOBAL SAFE
        }



        /** @short  Allows (*rSingle).someBodyOp().
         */
        SingletonClass& operator*() const
        {
            // GLOBAL SAFE ->
            ::osl::MutexGuard aLock(SingletonRef::ownStaticLock());
            return *m_pInstance;
            // <- GLOBAL SAFE
        }


    // helper

    private :



        /** @short  creates an own mutex for guarding static contents.

                    The global mutex the osl library is used one times
                    only to create an own static mutex, which can be used
                    next time to guard own static member operations.
         */
        struct SingletonLockInit
        {
            ::osl::Mutex* operator()()
            {
                static ::osl::Mutex aInstance;
                return &aInstance;
            }
        };

        ::osl::Mutex& ownStaticLock() const
        {
            return *rtl_Instance< ::osl::Mutex,
                                  SingletonLockInit,
                                  ::osl::MutexGuard,
                                  ::osl::GetGlobalMutex >::create(SingletonLockInit(), ::osl::GetGlobalMutex());
        }
};

template< class SingletonClass >
SingletonClass* SingletonRef< SingletonClass >::m_pInstance = 0;

template< class SingletonClass >
sal_Int32 SingletonRef< SingletonClass >::m_nRef = 0;

} // namespace salhelper

#endif // INCLUDED_SALHELPER_SINGLETONREF_HXX

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