C / C++ / MFC

class 
CControlPanelSheet{
...
  static CControlPanelSheet* GetActiveSheet()
  {
    return theOne; // could be NULL if no active sheet exists.
  }
  static BOOL CreateSheet(...){ // ... are the construction parameters
    if(theOne)return FALSE; // there can only be one sheet at a time
    new CControlPanelSheet();
    return TRUE;
  }
...
private: // having ctor private forces user to use CreateSheet
  static CControlPanelSheet theOne; // initialized to NULL in your .cpp
  CControlPanelSheet(...)
  {
    ... // call to Create here
    theOne=this;
  }
  ~CControlPanelSheet()
  {
    theOne=NULL;
  }
  void afx_msg OnDestroy() // add message handler for this, you know
  {
    CPropertySheet::OnDestroy();
    delete this;
  }
};

class CApp{
...
static long WINAPI SetFunction();
...);