Why do we need virtual functions in C++?

Question

I\'m learning C++ and I\'m just getting into virtual functions.

From what I\'ve read (in the book and online), virtual functions are functions in the base class that

Answer 1

Why do we need Virtual Methods in C++?

Quick Answer:

1. It provides us with one of the needed "ingredients"¹ for object oriented programming.

In Bjarne Stroustrup C++ Programming: Principles and Practice, (14.3):

The virtual function provides the ability to define a function in a base class and have a function of the same name and type in a derived class called when a user calls the base class function. That is often called run-time polymorphism, dynamic dispatch, or run-time dispatch because the function called is determined at run time based on the type of the object used.

2. It is the fastest more efficient implementation if you need a virtual function call ².

To handle a virtual call, one needs one or more pieces of data related to the derived object ³. The way that is usually done is to add the address of table of functions. This table is usually referred to as virtual table or virtual function table and its address is often called the virtual pointer. Each virtual function gets a slot in the virtual table. Depending of the caller's object (derived) type, the virtual function, in its turn, invokes the respective override.

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^{1.The use of inheritance, run-time polymorphism, and encapsulation is the most common definition of object-oriented programming.}

^{2. You can't code functionality to be any faster or to use less memory using other language features to select among alternatives at run time. Bjarne Stroustrup C++ Programming: Principles and Practice.(14.3.1)}.

^{3. Something to tell which function is really invoked when we call the base class containing the virtual function.}