Virtual functions in C++

How do virtual functions work in C++?

My initial thoughts:
Virtual functions are defined in a super class. It does not require implementation for the super class. But all the extended class have to implement the function. For example, a Shape class can have a virtual draw function. Its derived class, e.g., Circle, has to implement its detailed draw function.

A virtual function depends on a “vtable” or “Virtual Table”. If any function of a class is declared as virtual, a v-table is constructed which stores addresses of the virtual functions of this class. The compiler also adds a hidden vptr variable in all such classes which points to the vtable of that class. If a virtual function is not overridden in the derived class, the vtable of the derived class stores the address of the function in his parent class. The v-table is used to resolve the address of the function, for whenever the virtual function is called. Dynamic binding in C++ is therefore performed through the vtable mechanism.
Thus, when we assign the derived class object to the base class pointer, the vptr points to the vtable of the derived class. This assignment ensures that the most derived virtual function gets called.

class Shape {
		int edge_length;
		virtual int circumference () {
			cout << "Circumference of Base Class\n";
			return 0;

class Triangle: public Shape {
		int circumference () {
			cout<< "Circumference of Triangle Class\n";
			return 3 * edge_length;

void main() {
	Shape * x = new Shape();
	x->circumference(); // prints "Circumference of Base Class"
	Shape *y = new Triangle();
	y->circumference(); // prints "Circumference of Triangle Class"

In the above example, circumference is a virtual function in shape class, so it becomes virtual in each of the derived classes (triangle, rectangle). C++ non-virtual function calls are resolved at compile time with static binding, while virtual function calls are resolved at run time with dynamic binding.

Apparently I mixed the concept of virtual function and abstract function in C++. A abstract function in C++ is defined as:

virtual void foo() = 0;

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: