item7.cpp

// timeKeeper.h
class TimeKeeper {
public:
    TimeKeeper();
    ~TimeKeeper();  // note the virtual keyword is missing
};

// wristWatch.h
class WristWatch : public TimeKeeper {
  
};

// main.cpp
int main(int argc, char * argv[]) {
    // suppose getTimeKeeper may return a WristWatch (legal syntax)
    TimeKeeper *ptk = getTimeKeeper();
    // without virtual destructor we would call the base class destructor
    delete ptk; // undefined behavior!
}