In Hollywood, lots of actors and actresses take on many different roles when filming movies. They can be the hero of a story, a villain, or anything else as they inhabit a role. No matter what role they've gotten, when they are being filmed they are acting even if what they do specifically can be quite different. This kind of behavior acts similarly to the idea of polymorphism.

Polymorphism is one of the three pillars of an object-oriented language (along with encapsulation and inheritance). It comes from the words poly meaning many and morph meaning change.

Polymorphism is a way to call different specific class functions in an inheritance hierarchy, even though our code only uses a single type. That single type, the base class reference, will be changed many ways depending on the derived type. Continuing with the Hollywood example, we can tell an actor to act out a role and, based on what they've been cast in, they will do something different.

By using the virtual keyword on a base class function and overriding that function in a derived class, we can gain the ability to call that derived class function from a base class reference. While it may seem a bit complex at first, this will seem clearer with an example. For instance, if we have the following class:

class Animal 
{ 
  public: 
    virtual void Speak(void) const //virtual in the base class 
    { 
      //Using the Mach 5 console print 
      M5DEBUG_PRINT("...\n"); 
    } 
};

I could create a derived class with its own method, without modifying the base class in any way. In addition, we have the ability to replace or override a method within a derived class without affecting the base class. Let's say I wanted to change this function:

class Cat: public Animal 
  { 
  public: 
    void Speak(void) const //overridden in the derived class 
  { 
  M5DEBUG_PRINT("Meow\n"); 
  } 

void Purr(void) const //unrelated function 
  { 
    M5DEBUG_PRINT("*purr*\n"); 
  } 
}; 
class Dog: public Animal 
  { 
    public: 
    void Speak(void) const //overridden in the derived class 
    { 
      M5DEBUG_PRINT("Woof\n"); 
    } 
};

Since a derived class can be used anywhere a base class is needed, we can refer to derived classes using a base class pointer or an array of pointers and call the correct function at runtime. Let's have a look at the following code:

void SomeFunction(void) 
{ 
  const int SIZE = 2; 
  Cat cat; 
  Dog dog; 
  Animal* animals[SIZE] = {&cat, &dog}; 

  for(int i = 0; i < SIZE; ++i) 
  { 
    animals[i]->Speak(); 
  } 
}

The following is the output of the preceding code:

Meow 
Woof

As you can see, even though we have an array of base class pointers, the correct derived class function is called. If the functions weren't marked as virtual, or if the derived classes didn't override the correct functions, polymorphism wouldn't work.

An interface implements no functions, but simply declares the methods that the class will support. Then, all of the derived classes will do the implementation. In this way, the developer will have more freedom to implement the functions to fit each instance, while having things work correctly due to the nature of using an object-oriented language.

Interfaces may contain only static final variables, and they may contain only abstract methods, which means that they cannot be implemented within the class. However, we can have interfaces that inherit from other interfaces. When creating theses classes, we can implement whatever number of interfaces we want to. This allows us to make classes become even more polymorphic but, by doing so, we are agreeing that we will implement each of the functions defined in the interface. Because a class that implements an interface extends from that base class, we would say that it has an IS-A relationship with that type.

Now, interfaces have one disadvantage, and that's the fact that they tend to require a lot of coding to implement each of the different versions as needed, but we will talk about ways to adjust and/or fix this issue over the course of this book.

In C++, there isn't an official concept of interfaces, but you can simulate the behavior of interfaces by creating an abstract class.

Here's a simple example of an interface, and an implementation of it:

class Enemy 
{ 
  public: 
    virtual ~Enemy(void) {/*Empty virtual destructor*/} 
    virtual void DisplayInfo(void) = 0; 
    virtual void Attack(void)      = 0; 
    virtual void Move(void)        = 0; 
}; 

class FakeEnemy: public Enemy 
{ 
  public: 
  virtual void DisplayInfo(void) 
  { 
    M5DEBUG_PRINT("I am a FAKE enemy"); 
  } 

  virtual void Attack(void) 
    { 
      M5DEBUG_PRINT("I cannot attack"); 
    } 

  virtual void Move(void) 
  { 
    M5DEBUG_PRINT("I cannot move"); 
  } 
};

And here's how it looks in UML: