Loki/include/loki/Singleton.h

////////////////////////////////////////////////////////////////////////////////
// The Loki Library
// Copyright (c) 2001 by Andrei Alexandrescu
// This code accompanies the book:
// Alexandrescu, Andrei. "Modern C++ Design: Generic Programming and Design
//     Patterns Applied". Copyright (c) 2001. Addison-Wesley.
// Permission to use, copy, modify, distribute and sell this software for any
//     purpose is hereby granted without fee, provided that the above copyright
//     notice appear in all copies and that both that copyright notice and this
//     permission notice appear in supporting documentation.
// The author or Addison-Wesley Longman make no representations about the
//     suitability of this software for any purpose. It is provided "as is"
//     without express or implied warranty.
////////////////////////////////////////////////////////////////////////////////
#ifndef LOKI_SINGLETON_INC_
#define LOKI_SINGLETON_INC_

// $Id$


#include <loki/LokiExport.h>
#include <loki/Threads.h>
#include <algorithm>
#include <stdexcept>
#include <cassert>
#include <cstdlib>
#include <new>
#include <vector>
#include <list>
#include <memory>

#ifdef _MSC_VER
#define LOKI_C_CALLING_CONVENTION_QUALIFIER __cdecl
#else
#define LOKI_C_CALLING_CONVENTION_QUALIFIER
#endif

///  \defgroup  SingletonGroup Singleton
///  \defgroup  CreationGroup Creation policies
///  \ingroup   SingletonGroup
///  \defgroup  LifetimeGroup Lifetime policies
///  \ingroup   SingletonGroup
///  The lifetimes of the singleton.
///  \par Special lifetime for SmallObjects
///  When the holded object is a Small(Value)Object or the holded object
///  uses objects which are or inherit from Small(Value)Object
///  then you can't use the default lifetime: you must use the lifetime
///  \code Loki::LongevityLifetime::DieAsSmallObjectChild \endcode
///  Be aware of this when you use Loki::Factory, Loki::Functor, or Loki::Function.



namespace Loki
{
    typedef void (LOKI_C_CALLING_CONVENTION_QUALIFIER *atexit_pfn_t)();

    namespace Private
    {

#ifndef LOKI_MAKE_DLL
        void LOKI_C_CALLING_CONVENTION_QUALIFIER AtExitFn(); // declaration needed below
#else
        void LOKI_EXPORT AtExitFn();
#endif

        class LifetimeTracker;

#define LOKI_ENABLE_NEW_SETLONGLIVITY_HELPER_DATA_IMPL
#ifdef LOKI_ENABLE_NEW_SETLONGLIVITY_HELPER_DATA_IMPL

        // Helper data
        // std::list because of the inserts
        typedef std::list<LifetimeTracker*> TrackerArray;
        extern LOKI_EXPORT TrackerArray* pTrackerArray;
#else
        // Helper data
        typedef LifetimeTracker** TrackerArray;
        extern TrackerArray pTrackerArray;
        extern unsigned int elements;
#endif

        ////////////////////////////////////////////////////////////////////////////////
        // class LifetimeTracker
        // Helper class for SetLongevity
        ////////////////////////////////////////////////////////////////////////////////

        class LifetimeTracker
        {
        public:
            LifetimeTracker(unsigned int x) : longevity_(x)
            {}

            virtual ~LifetimeTracker() = 0;

            static bool Compare(const LifetimeTracker* lhs,
                const LifetimeTracker* rhs)
            {
                return lhs->longevity_ > rhs->longevity_;
            }

        private:
            unsigned int longevity_;
        };

        // Definition required
        inline LifetimeTracker::~LifetimeTracker() {}

        // Helper destroyer function
        template <typename T>
        struct Deleter
        {
            typedef void (*Type)(T*);
            static void Delete(T* pObj)
            { delete pObj; }
        };

        // Concrete lifetime tracker for objects of type T
        template <typename T, typename Destroyer>
        class ConcreteLifetimeTracker : public LifetimeTracker
        {
        public:
            ConcreteLifetimeTracker(T* p,unsigned int longevity, Destroyer d)
                : LifetimeTracker(longevity)
                , pTracked_(p)
                , destroyer_(d)
            {}

            ~ConcreteLifetimeTracker()
            { destroyer_(pTracked_); }

        private:
            /// Copy constructor is not implemented.
            ConcreteLifetimeTracker( const ConcreteLifetimeTracker & );
            T* pTracked_;
            Destroyer destroyer_;
        };

    } // namespace Private

    ////////////////////////////////////////////////////////////////////////////////
    ///  \ingroup LifetimeGroup
    ///
    ///  Assigns an object a longevity; ensures ordered destructions of objects
    ///  registered thusly during the exit sequence of the application
    ////////////////////////////////////////////////////////////////////////////////

#ifdef LOKI_ENABLE_NEW_SETLONGLIVITY_HELPER_DATA_IMPL

    template <typename T, typename Destroyer>
    void SetLongevity(T* pDynObject, unsigned int longevity,
        Destroyer d)
    {
        using namespace Private;

        // manage lifetime of stack manually
        if(pTrackerArray==0)
            pTrackerArray = new TrackerArray;

        // automatically delete the ConcreteLifetimeTracker object when a exception is thrown
        std::auto_ptr<LifetimeTracker>
            p( new ConcreteLifetimeTracker<T, Destroyer>(pDynObject, longevity, d) );

        // Find correct position
        TrackerArray::iterator pos = std::upper_bound(
            pTrackerArray->begin(),
            pTrackerArray->end(),
            p.get(),
            LifetimeTracker::Compare);

        // Insert the pointer to the ConcreteLifetimeTracker object into the queue
        pTrackerArray->insert(pos, p.get());

        // nothing has thrown: don't delete the ConcreteLifetimeTracker object
        p.release();

        // Register a call to AtExitFn
        std::atexit(Private::AtExitFn);
    }

#else

    template <typename T, typename Destroyer>
    void SetLongevity(T* pDynObject, unsigned int longevity,
        Destroyer d)
    {
        using namespace Private;

        TrackerArray pNewArray = static_cast<TrackerArray>(
                std::realloc(pTrackerArray,
                    sizeof(*pTrackerArray) * (elements + 1)));
        if (!pNewArray) throw std::bad_alloc();

        // Delayed assignment for exception safety
        pTrackerArray = pNewArray;

        LifetimeTracker* p = new ConcreteLifetimeTracker<T, Destroyer>(
            pDynObject, longevity, d);

        // Insert a pointer to the object into the queue
        TrackerArray pos = std::upper_bound(
            pTrackerArray,
            pTrackerArray + elements,
            p,
            LifetimeTracker::Compare);
        std::copy_backward(
            pos,
            pTrackerArray + elements,
            pTrackerArray + elements + 1);
        *pos = p;
        ++elements;

        // Register a call to AtExitFn
        std::atexit(Private::AtExitFn);
    }

#endif

    template <typename T>
    void SetLongevity(T* pDynObject, unsigned int longevity,
        typename Private::Deleter<T>::Type d = Private::Deleter<T>::Delete)
    {
        SetLongevity<T, typename Private::Deleter<T>::Type>(pDynObject, longevity, d);
    }

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct CreateUsingNew
    ///
    ///  \ingroup CreationGroup
    ///  Implementation of the CreationPolicy used by SingletonHolder
    ///  Creates objects using a straight call to the new operator
    ////////////////////////////////////////////////////////////////////////////////
    template <class T> struct CreateUsingNew
    {
        static T* Create()
        { return new T; }

        static void Destroy(T* p)
        { delete p; }
    };

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct CreateUsing
    ///
    ///  \ingroup CreationGroup
    ///  Implementation of the CreationPolicy used by SingletonHolder
    ///  Creates objects using a custom allocater.
    ///  Usage: e.g. CreateUsing<std::allocator>::Allocator
    ////////////////////////////////////////////////////////////////////////////////
    template<template<class> class Alloc>
    struct CreateUsing
    {
        template <class T>
        struct Allocator
        {
            static Alloc<T> allocator;

            static T* Create()
            {
                return new (allocator.allocate(1)) T;
            }

            static void Destroy(T* p)
            {
                //allocator.destroy(p);
                p->~T();
                allocator.deallocate(p,1);
            }
        };
    };

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct CreateUsingMalloc
    ///
    ///  \ingroup CreationGroup
    ///  Implementation of the CreationPolicy used by SingletonHolder
    ///  Creates objects using a call to std::malloc, followed by a call to the
    ///  placement new operator
    ////////////////////////////////////////////////////////////////////////////////
    template <class T> struct CreateUsingMalloc
    {
        static T* Create()
        {
            void* p = std::malloc(sizeof(T));
            if (!p) return 0;
            return new(p) T;
        }

        static void Destroy(T* p)
        {
            p->~T();
            std::free(p);
        }
    };


    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct CreateStatic
    ///
    ///  \ingroup CreationGroup
    ///  Implementation of the CreationPolicy used by SingletonHolder
    ///  Creates an object in static memory
    ///  Implementation is slightly nonportable because it uses the MaxAlign trick
    ///  (an union of all types to ensure proper memory alignment). This trick is
    ///  nonportable in theory but highly portable in practice.
    ////////////////////////////////////////////////////////////////////////////////
    template <class T> struct CreateStatic
    {

#ifdef _MSC_VER
#pragma warning( push )
#pragma warning( disable : 4121 )
// alignment of a member was sensitive to packing
#endif // _MSC_VER

        union MaxAlign
        {
            char t_[sizeof(T)];
            short int shortInt_;
            int int_;
            long int longInt_;
            float float_;
            double double_;
            long double longDouble_;
            struct Test;
            int Test::* pMember_;
            int (Test::*pMemberFn_)(int);
        };

#ifdef _MSC_VER
#pragma warning( pop )
#endif // _MSC_VER

        static T* Create()
        {
            static MaxAlign staticMemory_;
            return new(&staticMemory_) T;
        }

        static void Destroy(T* p)
        {
            p->~T();
        }
    };

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct DefaultLifetime
    ///
    ///  \ingroup LifetimeGroup
    ///  Implementation of the LifetimePolicy used by SingletonHolder
    ///  Schedules an object's destruction as per C++ rules
    ///  Forwards to std::atexit
    ////////////////////////////////////////////////////////////////////////////////
    template <class T>
    struct DefaultLifetime
    {
        static void ScheduleDestruction(T*, atexit_pfn_t pFun)
        { std::atexit(pFun); }

        static void OnDeadReference()
        { throw std::logic_error("Dead Reference Detected"); }
    };

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct  PhoenixSingleton
    ///
    ///  \ingroup LifetimeGroup
    ///  Implementation of the LifetimePolicy used by SingletonHolder
    ///  Schedules an object's destruction as per C++ rules, and it allows object
    ///  recreation by not throwing an exception from OnDeadReference
    ////////////////////////////////////////////////////////////////////////////////
    template <class T>
    class PhoenixSingleton
    {
    public:
        static void ScheduleDestruction(T*, atexit_pfn_t pFun)
        {
#ifndef ATEXIT_FIXED
            if (!destroyedOnce_)
#endif
                std::atexit(pFun);
        }

        static void OnDeadReference()
        {
#ifndef ATEXIT_FIXED
            destroyedOnce_ = true;
#endif
        }

    private:
#ifndef ATEXIT_FIXED
        static bool destroyedOnce_;
#endif
    };

#ifndef ATEXIT_FIXED
    template <class T> bool PhoenixSingleton<T>::destroyedOnce_ = false;
#endif

    ////////////////////////////////////////////////////////////////////////////////
    // Copyright (c) 2004 by Curtis Krauskopf - curtis@decompile.com
    ///
    ///  \struct  DeletableSingleton
    ///
    ///  \ingroup LifetimeGroup
    ///
    ///  A DeletableSingleton allows the instantiated singleton to be
    ///  destroyed at any time. The singleton can be reinstantiated at
    ///  any time, even during program termination.
    ///  If the singleton exists when the program terminates, it will
    ///  be automatically deleted.
    ///
    ///  \par Usage:
    ///  The singleton can be deleted manually:
    ///
    ///  DeletableSingleton<MyClass>::GracefulDelete();
    ////////////////////////////////////////////////////////////////////////////////
    template <class T>
    class DeletableSingleton
    {
    public:

        static void ScheduleDestruction(T*, atexit_pfn_t pFun)
        {
            static bool firstPass = true;
            isDead = false;
            deleter = pFun;
            if (firstPass || needCallback)
            {
                std::atexit(atexitCallback);
                firstPass = false;
                needCallback = false;
            }
        }

        static void OnDeadReference()
        {
        }
        ///  delete singleton object manually
        static void GracefulDelete()
        {
            if (isDead)
                return;
            isDead = true;
            deleter();
        }

    protected:
        static atexit_pfn_t deleter;
        static bool isDead;
        static bool needCallback;

        static void atexitCallback()
        {
#ifdef ATEXIT_FIXED
            needCallback = true;
#else
            needCallback = false;
#endif
            GracefulDelete();
        }
    };

    template <class T>
    atexit_pfn_t DeletableSingleton<T>::deleter = 0;

    template <class T>
    bool DeletableSingleton<T>::isDead = true;

    template <class T>
    bool DeletableSingleton<T>::needCallback = true;

    ////////////////////////////////////////////////////////////////////////////////
    // class template Adapter
    // Helper for SingletonWithLongevity below
    ////////////////////////////////////////////////////////////////////////////////

    namespace Private
    {
        template <class T>
        struct Adapter
        {
            void operator()(T*) { return pFun_(); }
            atexit_pfn_t pFun_;
        };
    }

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct SingletonWithLongevity
    ///
    ///  \ingroup LifetimeGroup
    ///  Implementation of the LifetimePolicy used by SingletonHolder
    ///  Schedules an object's destruction in order of their longevities
    ///  Assumes a visible function GetLongevity(T*) that returns the longevity of the
    ///  object.
    ////////////////////////////////////////////////////////////////////////////////
    template <class T>
    class SingletonWithLongevity
    {
    public:
        static void ScheduleDestruction(T* pObj, atexit_pfn_t pFun)
        {
            Private::Adapter<T> adapter = { pFun };
            SetLongevity(pObj, GetLongevity(pObj), adapter);
        }

        static void OnDeadReference()
        { throw std::logic_error("Dead Reference Detected"); }
    };

    ////////////////////////////////////////////////////////////////////////////////
    ///  \struct NoDestroy
    ///
    ///  \ingroup LifetimeGroup
    ///  Implementation of the LifetimePolicy used by SingletonHolder
    ///  Never destroys the object
    ////////////////////////////////////////////////////////////////////////////////
    template <class T>
    struct NoDestroy
    {
        static void ScheduleDestruction(T*, atexit_pfn_t)
        {}

        static void OnDeadReference()
        {}
    };


    ////////////////////////////////////////////////////////////////////////////////
    ///  \defgroup LongevityLifetimeGroup LongevityLifetime
    ///  \ingroup LifetimeGroup
    ///
    ///  \namespace LongevityLifetime
    ///
    ///  \ingroup LongevityLifetimeGroup
    ///  \brief  In this namespace are special lifetime policies to manage lifetime
    ///  dependencies.
    ////////////////////////////////////////////////////////////////////////////////
    namespace LongevityLifetime
    {
        ////////////////////////////////////////////////////////////////////////////////
        ///  \struct  SingletonFixedLongevity
        ///
        ///  \ingroup LongevityLifetimeGroup
        ///  Add your own lifetimes into the namespace 'LongevityLifetime'
        ///  with your prefered lifetime by adding a struct like this:
        ///
        ///  template<class T>
        ///  struct MyLifetime  : SingletonFixedLongevity< MyLifetimeNumber ,T> {}
        ////////////////////////////////////////////////////////////////////////////////
        template <unsigned int Longevity, class T>
        class SingletonFixedLongevity
        {
        public:
            virtual ~SingletonFixedLongevity() {}

            static void ScheduleDestruction(T* pObj, atexit_pfn_t pFun)
            {
                Private::Adapter<T> adapter = { pFun };
                SetLongevity(pObj, Longevity , adapter);
            }

            static void OnDeadReference()
            { throw std::logic_error("Dead Reference Detected"); }
        };

        ///  \struct DieLast
        ///  \ingroup LongevityLifetimeGroup
        ///  \brief  Longest possible SingletonWithLongevity lifetime: 0xFFFFFFFF
        template <class T>
        struct DieLast  : SingletonFixedLongevity<0xFFFFFFFF ,T>
        {};

        ///  \struct DieDirectlyBeforeLast
        ///  \ingroup LongevityLifetimeGroup
        ///  \brief  Lifetime is a one less than DieLast: 0xFFFFFFFF-1
        template <class T>
        struct DieDirectlyBeforeLast  : SingletonFixedLongevity<0xFFFFFFFF-1 ,T>
        {};

        ///  \struct DieFirst
        ///  \ingroup LongevityLifetimeGroup
        ///  \brief  Shortest possible SingletonWithLongevity lifetime: 0
        template <class T>
        struct DieFirst : SingletonFixedLongevity<0,T>
        {};

    }//namespace LongevityLifetime

    ////////////////////////////////////////////////////////////////////////////////
    ///  \class FollowIntoDeath
    ///
    ///  \ingroup LifetimeGroup
    ///
    ///  Lifetime policyfor the SingletonHolder tempalte.
    ///  Followers will die after the master dies Followers will not die, if
    ///    - master never dies (NoDestroy policy)
    ///    - master never created
    ///    - master dies not in the function registered with atexit
    ///    - master dies not by a call of a the atexit registerd function (DeletableSingleton::GracefulDelete)
    ///
    ///  \par Usage:
    ///
    ///  Lifetimes of the master and the follower singletons,  e.g. with a M and a F class:
    ///   \code SingletonHolder< M , FollowIntoDeath::With<DefaultLifetime>::AsMasterLifetime > MasterSingleton; \endcode
    ///   \code SingletonHolder< F , CreateUsingNew, FollowIntoDeath::AfterMaster< MasterSingleton >::IsDestroyed > FollowerSingleton \endcode
    ////////////////////////////////////////////////////////////////////////////////
    class FollowIntoDeath
    {
        template<class T>
        class Followers
        {
            typedef std::vector<atexit_pfn_t> Container;
            typedef typename Container::iterator iterator;
            static Container* followers_;

        public:
            static void Init()
            {
                static bool done = false;
                if(!done)
                {
                    followers_ = new Container;
                    done = true;
                }
            }

            static void AddFollower(atexit_pfn_t ae)
            {
                Init();
                followers_->push_back(ae);
            }

            static void DestroyFollowers()
            {
                Init();
                for(iterator it = followers_->begin();it != followers_->end();++it)
                    (*it)();
                delete followers_;
            }
        };

    public:

        ///  \struct With
        ///  Template for the master
        ///  \param Lifetime Lifetime policy for the master
        template<template <class> class Lifetime>
        struct With
        {
            ///  \struct AsMasterLifetime
            ///  Policy for master
            template<class Master>
            struct AsMasterLifetime
            {
                static void ScheduleDestruction(Master* pObj, atexit_pfn_t pFun)
                {
                    Followers<Master>::Init();
                    Lifetime<Master>::ScheduleDestruction(pObj, pFun);

                    // use same policy for the followers and force a new
                    // template instantiation,  this adds a additional atexit entry
                    // does not work with SetLonlevity, but there you can control
                    // the lifetime with the GetLongevity function.
                    Lifetime<Followers<Master> >::ScheduleDestruction(0,Followers<Master>::DestroyFollowers);
                }

                static void OnDeadReference()
                {
                    throw std::logic_error("Dead Reference Detected");
                }
            };
        };

        ///  \struct AfterMaster
        ///  Template for the follower
        ///  \param Master Master to follow into death
        template<class Master>
        struct AfterMaster
        {
            ///  \struct IsDestroyed
            ///  Policy for followers
            template<class F>
            struct IsDestroyed
            {
                static void ScheduleDestruction(F*, atexit_pfn_t pFun)
                {
                    Followers<Master>::AddFollower(pFun);
                }

                static void OnDeadReference()
                {
                    throw std::logic_error("Dead Reference Detected");
                }
            };
        };
    };

    template<class T>
    typename FollowIntoDeath::Followers<T>::Container*
    FollowIntoDeath::Followers<T>::followers_ = 0;



    ////////////////////////////////////////////////////////////////////////////////
    ///  \class  SingletonHolder
    ///
    ///  \ingroup SingletonGroup
    ///
    ///  Provides Singleton amenities for a type T
    ///  To protect that type from spurious instantiations,
    ///  you have to protect it yourself.
    ///
    ///  \param CreationPolicy Creation policy, default: CreateUsingNew
    ///  \param LifetimePolicy Lifetime policy, default: DefaultLifetime,
    ///  \param ThreadingModel Threading policy,
    ///                         default: LOKI_DEFAULT_THREADING_NO_OBJ_LEVEL
    ////////////////////////////////////////////////////////////////////////////////
    template
    <
        typename T,
        template <class> class CreationPolicy = CreateUsingNew,
        template <class> class LifetimePolicy = DefaultLifetime,
        template <class, class> class ThreadingModel = LOKI_DEFAULT_THREADING_NO_OBJ_LEVEL,
        class MutexPolicy = LOKI_DEFAULT_MUTEX
    >
    class SingletonHolder
    {
    public:

        ///  Type of the singleton object
        typedef T ObjectType;

        ///  Returns a reference to singleton object
        static T& Instance();

    private:
        // Helpers
        static void MakeInstance();
        static void LOKI_C_CALLING_CONVENTION_QUALIFIER DestroySingleton();

        // Protection
        SingletonHolder();

        // Data
        typedef typename ThreadingModel<T*,MutexPolicy>::VolatileType PtrInstanceType;
        static PtrInstanceType pInstance_;
        static bool destroyed_;
    };

    ////////////////////////////////////////////////////////////////////////////////
    // SingletonHolder's data
    ////////////////////////////////////////////////////////////////////////////////

    template
    <
        class T,
        template <class> class C,
        template <class> class L,
        template <class, class> class M,
        class X
    >
    typename SingletonHolder<T, C, L, M, X>::PtrInstanceType
        SingletonHolder<T, C, L, M, X>::pInstance_ = 0;

    template
    <
        class T,
        template <class> class C,
        template <class> class L,
        template <class, class> class M,
        class X
    >
    bool SingletonHolder<T, C, L, M, X>::destroyed_ = false;

    ////////////////////////////////////////////////////////////////////////////////
    // SingletonHolder::Instance
    ////////////////////////////////////////////////////////////////////////////////

    template
    <
        class T,
        template <class> class CreationPolicy,
        template <class> class LifetimePolicy,
        template <class, class> class ThreadingModel,
        class MutexPolicy
    >
    inline T& SingletonHolder<T, CreationPolicy,
        LifetimePolicy, ThreadingModel, MutexPolicy>::Instance()
    {
        if (!pInstance_)
        {
            MakeInstance();
        }
        return *pInstance_;
    }

    ////////////////////////////////////////////////////////////////////////////////
    // SingletonHolder::MakeInstance (helper for Instance)
    ////////////////////////////////////////////////////////////////////////////////

    template
    <
        class T,
        template <class> class CreationPolicy,
        template <class> class LifetimePolicy,
        template <class, class> class ThreadingModel,
        class MutexPolicy
    >
    void SingletonHolder<T, CreationPolicy,
        LifetimePolicy, ThreadingModel, MutexPolicy>::MakeInstance()
    {
        typename ThreadingModel<SingletonHolder,MutexPolicy>::Lock guard;
        (void)guard;

        if (!pInstance_)
        {
            if (destroyed_)
            {
                LifetimePolicy<T>::OnDeadReference();
                destroyed_ = false;
            }
            pInstance_ = CreationPolicy<T>::Create();
            LifetimePolicy<T>::ScheduleDestruction(pInstance_,
                &DestroySingleton);
        }
    }

    template
    <
        class T,
        template <class> class CreationPolicy,
        template <class> class L,
        template <class, class> class M,
        class X
    >
    void LOKI_C_CALLING_CONVENTION_QUALIFIER
    SingletonHolder<T, CreationPolicy, L, M, X>::DestroySingleton()
    {
        assert(!destroyed_);
        CreationPolicy<T>::Destroy(pInstance_);
        pInstance_ = 0;
        destroyed_ = true;
    }


    ////////////////////////////////////////////////////////////////////////////////
    ///  \class  Singleton
    ///
    ///  \ingroup SingletonGroup
    ///
    ///  Convenience template to implement a getter function for a singleton object.
    ///  Often needed in a shared library which hosts singletons.
    ///
    ///  \par Usage
    ///
    ///  see test/SingletonDll
    ///
    ////////////////////////////////////////////////////////////////////////////////

#ifndef LOKI_SINGLETON_EXPORT
#define LOKI_SINGLETON_EXPORT
#endif

    template<class T>
    class LOKI_SINGLETON_EXPORT Singleton
    {
    public:
        static T& Instance();
    };

} // namespace Loki


/// \def LOKI_SINGLETON_INSTANCE_DEFINITION(SHOLDER)
/// Convenience macro for the definition of the static Instance member function
/// Put this macro called with a SingletonHolder typedef into your cpp file.

#define LOKI_SINGLETON_INSTANCE_DEFINITION(SHOLDER)                     \
namespace Loki                                                          \
{                                                                        \
    template<>                                                          \
    SHOLDER::ObjectType&  Singleton<SHOLDER::ObjectType>::Instance()    \
    {                                                                   \
        return SHOLDER::Instance();                                     \
    }                                                                    \
}


#endif // end file guardian