Can't call Member function in a non member function

I have a class and it has a member function and i want to call that member function in a non member function.

void mouseEvent(int evt, int x, int y, int flags, void* param);
class CMainClass
{

private:
// Xcoordinate of the click event
     int Xcoordinate;
public:
// Xcoordinate get method
         int CMainDialog::GetXcoordinate()
         {
          return Xcoordinate;
         }

// Xcoordinate set method
         void CMainDialog::SetXcoordinate(int newVal)
         {
           CMainDialog::Xcoordinate = newVal;
         }

};


void mouseEvent(int evt, int x, int y, int flags, void* param)
{
      CMainDialog::SetXcoordinate(x);

}

C++ a nonstatic member reference must be relative to a specific object