Surprisingly, even I work with the product that is written majorly in C++, I don't have to deal with the stuff that differentiate C++ from C. However, I'm now working on a defect that forces me to simplify C++ objects in order to get the root cause of the problem. That's the place where I have to really need to know how exactly C++ class is structured.

One question I asked myself several years ago: "What is virtual method in C++?" I believed, at that time, I got the answer but I was too lazy to record it somewhere. Now, I have to pay the price by wasting my effort again to dig out the answer. So, I'd better save it at someplace this time and the following is just a simple example to partially show the answer to the question. I know C++ is a monster and I'll definitely need to rewrite the post some day in the future when I know more about the language. However, this answer is good enough for me now.

Virtual methods

Let's consider the following code snippet: we have a base class called Animal and its subclass called Dog

#include <iostream>
using namespace std;

class Animal{
public:
  void getFamily() { cout << "We are animals" << endl; }
  void getClass() { cout << "I'm an Animal" << endl;}
};

class Dog: public Animal{
public:
  void getClass() { cout << "I'm a Dog" << endl;}
};

int main()
{
  Animal *animal = new Animal;
  Dog *dog = new Dog;

  animal->getClass();
  dog->getClass();
}

Now, inside the main, we call animal->getClass() and dog->getClass(). We compile our code using g++ -std=c++11 a.cpp and run the program and get

I'm an Animal
I'm a Dog

As you can see, each object calls their getClass method respectively. Now, let's add a function called whatClassAreYou() to our code above

void whatClassAreYou(Animal *animal)
{
  animal->getClass();
}

and in our main function, we call our newly-added function with

whatClassAreYou(animal);
whatClassAreYou(dog);

and the output looks like below

I'm an Animal
I'm a Dog
I'm an Animal
I'm an Animal

As you can see whatClassAreYou() only calls the getClass() method of our base class Animal even when we pass in Dog class object. Ideally, we want our whatClassAreYou() method call the right getClass() method depending on what class object we pass into. In other words, if our Dog class implements its own getClass() method, we want our whatClassAreYou() method be aware of this fact and call it instead of calling getClass() of our base class Animal. That's why we want to add virtual keyword to the getClass() of our base class. We are essentially telling the compiler that our base class getClass() method might be overridden by its subclass and be aware of this fact when some other method wants to call it.

Now our code looks like below

#include <iostream>
using namespace std;

// Virtual Methods and Polymorphism
// Polymorphism allows you to treat subclasses as their superclass and yet
// call the correct overwritten methods in the subclass automatically

class Animal{
public:
  void getFamily() { cout << "We are animals" << endl; }

  // When we define a method as virtual we know that Animal
  // will be a base class that may have this method overwritten
  virtual void getClass() { cout << "I'm an Animal" << endl;}
};

class Dog: public Animal{
public:
  void getClass() { cout << "I'm a Dog" << endl;}
};

void whatClassAreYou(Animal *animal)
{
  animal->getClass(); // use "virtual", proper getClass() method will be called depending on
                      // the exact type of Animal* animal get passed in (i.e. base class Animal
                      // or subclass Dog)
}

int main()
{
  Animal *animal = new Animal;
  Dog *dog = new Dog;

  // If a method is marked virtual or not doesn't matter if we call the
  // method directly from the object
  animal->getClass();
  dog->getClass();

  whatClassAreYou(animal);
  whatClassAreYou(dog);
}

and the output is

I'm an Animal
I'm a Dog
I'm an Animal
I'm a Dog

The reason behind this scenario is what we called polymorphism, which means "many form" in Greek. Here is how this concept get explained in C++ Primer:

We speak of types related by inheritance as polymorphic types, because we can use the “many forms” of these types while ignoring the differences among them. The fact that the static and dynamic types of references and pointers can differ is the cornerstone of how C++ supports polymorphism.

When we call a function defined in a base class through a reference or pointer to the base class, we do not know the type of the object on which that member is executed. The object can be a base-class object or an object of a derived class. If the function is virtual, then the decision as to which function to run is delayed until run time. The version of the virtual function that is run is the one defined by the type of the object to which the reference is bound or to which the pointer points. On the other hand, calls to nonvirtual functions are bound at compile time. Similarly, calls to any function (virtual or not) on an object are also bound at compile time. The type of an object is fixed and unvarying—there is nothing we can do to make the dynamic type of an object differ from its static type. Therefore, calls made on an object are bound at compile time to the version defined by the type of the object.

To see the final note of the above quote, let's take a look an example

#include <iostream>
using namespace std;

class Animal{
public:
  void getFamily() { cout << "We are animals" << endl; }
  virtual void getClass() { cout << "I'm an Animal" << endl;}
};

class Dog: public Animal{
public:
  void getClass() { cout << "I'm a Dog" << endl;}
};

void whatClassAreYou(Animal *animal)
{
  animal->getClass(); 
}

void whatClassAreYou2(Animal animal)
{
  animal.getClass();
}

int main()
{
  Animal *animal = new Animal;
  Dog *dog = new Dog;

  animal->getClass();
  dog->getClass();

  whatClassAreYou(animal);
  whatClassAreYou(dog);

  Animal animal2;
  Dog dog2;

  whatClassAreYou2(animal2);
  whatClassAreYou2(dog2);
}

In this example, we define another method whatClassAreYou2 calls on object instead of pointers. Now, we apply this method to our newly created objects animal2 and dog2 and we get

I'm an Animal
I'm a Dog
I'm an Animal
I'm a Dog
I'm an Animal
I'm an Animal

As you can see, even we have virtual function in our base class, whatClassAreYou2() invokes only the base class's getClass() method and ignores the subclass overrides.

Aside note, you may notice that we use two ways to initialize our objects. The first way is through Animal *animal = new Animal; and the second way is through Animal animal2;. Initialization is quite complex in C++. These two ways are essentially the same: we use default constructor ¹ to initialize the objects. The only difference is that the first way gives us a pointer to the object and the second way gives object directly. To obtain the pointer to the object, we can do Animal *ptrAnimal = &animal2. Another way to initialize object is through value initilialization ². For example:

string *ps1 = new string;   // default initalized to the empty string
string *ps = new string();  // value initialized to the empty string
int *pi1 = new int;         // default initialized; *pi1 is undefined
int *pi2 = new int();       // value initialized to 0; *pi2 is 0

Pure virtual function & abstract base class

Another important concept related with virtual is called pure virtual. The difference between this concept is that, as stated in wikipedia:

A virtual function or virtual method is a function or method whose behavior can be overriden within an inheriting class by a function with the same signature. A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class that is not abstract.

In short, virtual function can be overriden; pure virtual function must be implemented. Let's take a look a example

#include <iostream>
using namespace std;

// An abstract data type is a class that acts as the base to other classes
// They stand out because its methods are initialized with zero
// A pure virtual method must be overwritten by subclasses

class Car
{
public:
  virtual int getNumWheels() = 0;
  virtual int getNumDoors() = 0;
};

class StationWagon : public Car
{
public:
  int getNumWheels() { cout << "Station wagon has 4 wheels" << endl; }
  int getNumDoors() { cout << "Station wagon has 4 doors" << endl;}
  StationWagon() {}
  ~StationWagon();
};

int main()
{
  // Create a StationWagon using the abstract data type Car
  Car *stationWagon = new StationWagon();

  stationWagon -> getNumWheels();

  return 0;
}

Here, we have a base class called Car and a class that derives from the Car class called StationWagon. Car is a lot similar to our Animal class in the sense that both classes have methods that have keyword virtual. However, Car class's methods have =0. This is exactly how we identify pure virtual: we specify that a virtual function is a pure virtual by writing =0 in place of a function body (i.e., just before the semicolon that ends the declaration).

A class like Car that contains (or inherit without overriding) a pure virtual function is an abstract base class. An abstract base class defines an interface for subsequent classes to override. We cannot (directly) create objects of a type that is an abstract base class.

Key terms summary

Here, I summarize terms appeard in this post as a quick index for future reference:

• virtual: Member function that defines type-specific behavior. Calls to virtual made through a reference or pointer are resolved at run time, based on the type of the object to which the reference or pointer is bound.

• pure virtual: Virtual function declared in the class header using =0 just before the semicolon. A pure virtual function need not be (but maybe) defined. Classes with pure virtuals are abstract classes. If a derived class does not define its own version of an inherited pure virtual, then the derived class is abstract as well.

• polymorphism: As used in object-oriented programming, refers to the ability to obtain type-specific behavior based on the dynamic type of a reference or pointer.

• static type: Type with which a variable is defined or that an expression yields. Static type is known at compile time.

• dynamic type: Type of an object at runtime. The dynamic type of an object to which a reference refers or to which a pointer points may differ from the static type of the reference or pointer. A pointer or reference to a base-class type can refer to an object of derived type. In such cases the static type is reference (or pointer) to base, but the dynamic type is reference (or pointer) to derived.

• abstract base class: Class that has one or more pure virtual functions. We cannot create objects of an abstract base-class type.