Realization

account

  class account {
  public:
  account() { _balance = 0; assert(invariant()); }
  
  virtual float balance() { return _balance; }
  
  
  void deposit(float x) {
  	require( x >= 0 );  
check precondition

hold();
to save the old state

_balance += x; promise( balance() == old_balance + x ); promise( invariant() ); } void withdraw(float x) { require( x <= _balance );
check precondition

hold();
to save the old state

_balance -= x; promise( balance() == old_balance - x ); promise( invariant() ); } virtual bool invariant() { return balance() >= 0; } protected: float _balance; float old_balance;
additional variable

virtual void hold() { old_balance = _balance; } };

slide: The realization of the $account$ contract