class myClass;       // Class declaration

extern myClass x;    // Declaration of a myClass variable
                     // This will NOT reserve memory space, 
                     // so compiler do NOT need to know 
                     // the structure of myClass



class yourClass
{
   int a;

   friend myClass f1( myClass x );   // An address is passed to f1
                                     // Compiler does not need to
                                     // know the structure
                                     // Declaration is sufficient information
};