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Advanced C++ - virtual functions

Today I am going to explain some basic but very important C++ feature - polymorphism and inheritance which is based on virtual functions in C++.

Virtual functions can be declared by preceding the function with virtual keyword. The importance of Virtual functions can be understood, when we design classes using inheritance. Virtual functions are special functions which are called using late binding concept.

Late binding or dynamic binding means that the binding happens during run time using vtable (virtual table) to select the correct virtual method to be called as the code runs.


Let us look at an example for getting more clarity on how Virtual functions work in reality: Output of that example is: Point 1 shows how virtual function is declared and defined.

Point 2 shows the inheritance concept i.e class Derived is inherited from class Base using public access specifier.

In point 3, virtual function of class Base is redefined in class Derived which leads to function overriding. Note that the use of keyword virtual is not required if other classes does not derive from Derived class.

From point 4, you can check which function is called for Base and Derived class(look at output). We see that based on the address what base pointer holds, such respective function gets called.

You can also run the same sample program after deleting the virtual keyword. Check the output then: This is because without using the concept of virtual functions, the compiler does early binding.

Early binding or Static binding is nothing but binding the function call to function body during compile time. Irrespective of what base pointer holds (value), the method matching the type of pointer gets called, in this case it is Base class method.

Now, let us understand how the concept of virtual functions work. Using virtual functions, we can achieve run time selection of methods which needs to be called based on object type. Late binding happens using vtable. Once the compiler sees that a class has a virtual function, it constructs a vtable which consist of addresses of virtual functions of that class.

Also, compiler places a special pointer called as vpointer i.e VPTR which points to the vtable which happens when an object of that class is constructed. Hence, when we call a virtual function using base class pointer, the compiler fetches the VPTR from the object’s memory, gets the function address from the vtable, thus resolving the virtual function call during run time as shown in the first output.

Code of this example can be found on our GitHub account: https://github.com/xmementoit/CppAdventureExamples/tree/master/advancedCpp/virtualFunction


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