Virtual Methods and Inheritance

When a base class declares a method as virtual, all implementations of that method in derived classes are automatically virtual. The destructor is treated in the same way. If the base class has a virtual destructor, so will all derived classes.
If a method is virtual in the base class, all derived implementations are also virtual even if not explicitly declared so in the derived class.

Example:
(Note that the destructor and hello() method in the derived classes are not explicitly declared to be virtual)

#include <iostream>
using namespace std;

class Vehicle {
   public:
      Vehicle() {}
      virtual ~Vehicle() {
         cout << "Vehicle destroyed." << endl;
      }
      virtual void hello() { 
         cout << "Hi, I'm a Vehicle" << endl; 
      }
};

class Car : public Vehicle {
   public:
      Car() {}
      ~Car() {
         cout << "Car destroyed." << endl;
      }
      void hello() { 
         cout << "Hi, I'm a Car" << endl; 
      }
};

class SportsCar : public Car {
   public:
      SportsCar() {}
      ~SportsCar() {
         cout << "SportsCar destroyed." << endl;
      }
      void hello() { 
         cout << "Hi, I'm a SportsCar" << endl; 
      }
};

int main() {
   cout << "Vehicle pointer to a SportsCar:" << endl;
   Vehicle *dbs = new SportsCar();
   dbs->hello();
   delete dbs;

   cout << endl << "Car pointer to a SportsCar:" << endl;
   Car *m5 = new SportsCar();
   m5->hello();
   delete m5;

   cout << endl << "Vehicle pointer to a Car:" << endl;
   Vehicle *civic = new Car();
   civic->hello();
   delete m5;
   return 0;

}

Output:

Vehicle pointer to a SportsCar:
Hi, I'm a SportsCar
SportsCar destroyed.
Car destroyed.
Vehicle destroyed.

Car pointer to a SportsCar:
Hi, I'm a SportsCar
SportsCar destroyed.
Car destroyed.
Vehicle destroyed.

Vehicle pointer to a Car:
Hi, I'm a Car
Car destroyed.
Vehicle destroyed.

Submitted by: Matt Ashbourne