# include <iostream>
using namespace std ;

class Number
{
public :
	Number ( int i ) : i_ ( i ) {}
	int value ( void ) { return i_ ; }
	int previous ( void ) ;

private :
	int i_ ;
} ;

int Number :: previous ( void )
{
	static int previousValue ;
	static bool calculated = false ;

	if ( not calculated )
	{
		cout << "Not calculated.\nCalculating...\n" ;
		previousValue = i_ - 1 ;
		calculated = true ;
		cout << "Returning calculated value.\n" ;
		return previousValue ;
	}
	// else
	cout << "Already calculated.\nNot calculating again.\n" ;
	cout << "Returning previously calculated value.\n" ;
	return previousValue ;
}

int main ( void )
{
	Number a ( 100 ) ;

	cout << "a :\n" ;
	cout << a . value () ;

	cout << "\na.previous :\n" ;
	cout << a . previous () ;

	cout << "\nAgain a.previous :\n" ;
	cout << a . previous () ;

	Number b ( 200 ) ;

	cout << "\nb :\n" ;
	cout << b . value () ;

	cout << "\nb.previous :\n" ;
	cout << b . previous () ;

	cout << "\nAgain b.previous :\n" ;
	cout << b . previous () ;

	cout << "\n" ;
}