singleton.h

#ifndef SINGLETON_H
#define SINGLETON_H

#include "appcore.h"
#include <QtGlobal>
#include <QDebug>

class ITEMFRAMEWORK_EXPORT AbstractSingleton
{
    Q_GADGET //Needed in order to use qobject_cast / interfaces stuff
public:
    AbstractSingleton() {} //In order to stay constructible (needed because of Q_DISABLE_COPY macro)
    virtual ~AbstractSingleton() {}
protected:
    friend class ObjectState; //Grant ObjectState access to the following members:
    virtual bool postInit() = 0;
    virtual bool preDestroy() = 0;

    Q_DISABLE_COPY(AbstractSingleton)
};
Q_DECLARE_INTERFACE(AbstractSingleton, "Itemframework.Interface.Singleton/1.0")


class ITEMFRAMEWORK_EXPORT SingletonStorage
{
    //Why do we need this class, and can't we just add a static data member of Type T* to the Singleton<T> class?
    // Well, the static member would need to be defined and initialized in a separate cpp file. This is not possible for template classes.
    // But you could return a reference to a local static variable from a static function and use that as storage!
    //  Yes, this works on Linux, but not on Windows :( (Reason unknown).
protected:
    static AbstractSingleton* getInstance(const char* name);
    static void storeInstance(const char* name, AbstractSingleton* inst);
};

template<typename T> class ITEMFRAMEWORK_EXPORT Singleton : public AbstractSingleton, private SingletonStorage
{
public:
    Singleton()   //Should only be called from ObjectState Class
    {
        storeInstance(T::staticMetaObject.className(), this); //Store this instance in the Hashmap
    }

    ~Singleton()  //Should only be called from ObjectState Class
    {
        storeInstance(T::staticMetaObject.className(), nullptr); //Remove this instance from the Hashmap
    }

    static T* instance()
    {
        return static_cast<T*>(getInstance(T::staticMetaObject.className())); //Retrieve Instance from Hashmap by classname
    }
};



#endif // SINGLETON_H