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Language: C++
Expertise: Beginner

Oct 5, 2004

The decision to use virtual functions is a simple matter. You just need to know when you'd want to override a base method. Take the following code as an example:

class Animal
{
public:
   void MakeSomeNoise()
   {
      cout << "nothing";
   }
};

class Bird : public Animal
{
public:
  void MakeSomeNoise()
  {
     cout << "Tweet";
  }
};

In this case, I'd want to override MakeSomeNoise(). But what if I didn't? This is what would happen:

Animal * pAnimal = new Bird;
pAnimal->MakeSomeNoise();

**Output**
nothing

The screen would say nothing because for all intents and purposes, C++ only sees an animal. However, if you declared it virtual, it knows to search for the lowest method in the class hierachy. Try it again:

class Bird : public Animal
{
public:
  virtual void MakeSomeNoise()
  {
     cout << "Tweet";
  }
};


Animal * pAnimal = new Bird;
pAnimal->MakeSomeNoise();

**Output**
Tweet