7. Declare destructors virtual in polymorphic base classes.

polymorphic base class

class TimeKeeper {public:    TimeKeepr();    virtual ~TimeKeeper();    ...};class AtomicClock: public TimeKeeper { ... };class WaterClock: public TimeKeeper { ... };class WristWatch: public TimeKeeper { ... };

abstract class

class AWOV {public:    virtual ~AWOV() = 0;};

polymorphic base class should declare virtual destructors. If a class has any virtual functions, it should have a virtual destrucor.