11.6 FishABC.cpp

#include <iostream>
using namespace std;

class Fish
{
public:
   // define a pure virtual function Swim
   virtual void Swim() = 0;
};

class Tuna:public Fish
{
public:
   void Swim()
   {
      cout << "Tuna swims fast in the sea!" << endl;
   }
};

class Carp:public Fish
{
   void Swim()
   {
      cout << "Carp swims slow in the lake!" << endl;
   }
};

void MakeFishSwim(Fish& inputFish)
{
   inputFish.Swim();
}

int main()
{
   // Fish myFish;   // Fails, cannot instantiate an ABC
   Carp myLunch;
   Tuna myDinner;

   MakeFishSwim(myLunch);
   MakeFishSwim(myDinner);

   return 0;
}