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; } };