Global Variables in Qt

I want to make a global variable in Qt. So I wrote a singleton class. But I am getting the following errors

:: error: symbol(s) not found,

:: error: collect2: ld returned 1 exit status

I am using the following code :

CityBookGlobalVariables.h:

class CityBookGlobalVariables
{  
private:
  CityBookGlobalVariables();
  CityBookGlobalVariables( const CityBookGlobalVariables& _instance );

  static CityBookGlobalVariables* m_pInstance;

public:   
     QStringList m_cityList;

     static CityBookGlobalVariables* instance()
     {
       if ( !m_pInstance )
         m_pInstance = new CityBookGlobalVariables;

       return m_p开发者_开发技巧Instance;
     }

     void  setCityList(QStringList cityList);
     QStringList getCityList();    
};

CityBookGlobalVariables.cpp:

//CityBookGlobalVariables* CityBookGlobalVariables::m_pInstance = 0;
CityBookGlobalVariables* m_pInstance = 0;
CityBookGlobalVariables::CityBookGlobalVariables()
{
}

CityBookGlobalVariables::CityBookGlobalVariables(const CityBookGlobalVariables& _instance ) 
{    
}

void  CityBookGlobalVariables::setCityList(QStringList cityList)
{
    m_cityList = cityList;   
}

QStringList CityBookGlobalVariables::getCityList()
{
  return  m_cityList;
}

setCityList calling from A1.cpp

CityBookGlobalVariables *pointer = CityBookGlobalVariables::instance();
pointer->setCityList(cityitems);

getCityList calling from B1.cpp

CityBookGlobalVariables *pointer = CityBookGlobalVariables::instance();
cityModel->setStringList(pointer->getCityList());