Virtual function in c++ with example program pdf

The keyword virtual has to be preceded the normal declaration. As you see, because of the virtual nature of the function in class a, similar function of class c was executed. Defining in a base class a virtual function, with another version in a derived class, signals to the compiler that we dont want static linkage for this function. Pdf virtual functions make code easier for programmers to reuse but. By including one or more virtual functions will increase the size of the class by 4 bytes. There is a necessity to use the single pointer to refer to all the objects of the different classes. The function definition of sqrt body of that function is present in the cmath header file. Virtual function is the member function of a class that can be overriden in its derived class. In the following sample code, we have got two virtual. Cpp, you will notice that there is one small change in line 8.

While calling the derived class, the overwritten function will be called. A virtual method has an implementation in a base class as well as derived the class. Because, the function address would have been statically bound during compile time. Virtual functions can only be members of a class, they cannot be nonmember functions. It is used to tell the compiler to perform dynamic linkage or late binding on the function. Virtual function call is resolved at runtime dynamic binding whereas the nonvirtual member functions are resolved at compile time static binding. Before diving into the how virtual function works internally. Describe the virtual function and virtual function table. But now, as the function is declared virtual it is a candidate for runtime linking and the derived class function is being invoked.

Defining in a base class a virtual function, with another version in a derived class. After you compile and execute the current program, we will go on to study our first virtual function. We finally come to an example program with a virtual function that operates as a virtual function and exhibits dynamic binding or polymorphism as it is called. I have explained how the concept of virtual function works in derived classes with example. A function call is resolved at runtime in late binding and so compiler determines the type of object at runtime. Now lets understand why making the function fetchclassname impacted the output. Typically, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance.

In short, a virtual function defines a target function to be executed, but. A virtual function is a function in a base class that is declared using the keyword virtual. Try to remove info member function from derivedclass1 and compile this code again then output will be. You can see that, there is no body of function in prototype. A virtual function is a function whic is declared in base class using the keyword virtual.

If you fail the override it, the derived class will become an abstract class as well. A virtual method is a method that can be redefined in derived classes. You need to declare the function virtual only in the base class. Consider the following simple program showing runtime behavior of virtual functions. To invoke the derived class overriding member function b base class pointer when i t is assigned with derived class address the base class member function has to made virtual. An abstract class is a class that is designed to be specifically used as a base class. The keyword virtual has been added to the declaration of the method named message in the parent class. Virtual function in cpp with simple example codeverb. Virtual function is a function in base class, which is overrided in the derived class, and which tells the compiler to perform late binding on this function. A virtual function is a member function in base class that you expect to redefine in derived classes. Even though we have the parent class pointer pointing to the instance object of child class. Lets say that a person knows how to calculate areas of various polygons. If you want to define the function, youll need to use. This 4 bytes are used to store the address of the array of function pointers.

An abstract class contains at least one pure virtual function. Virtual function is the member function defined in the base class and can further be defined in the child class as well. See in this case the output is woof, which is what we expect. Since we marked the function animalsound as virtual, the call to the function is resolved at runtime, compiler determines the type of the object at runtime and calls the appropriate function. As you can see, compiler first looks for info member function in the appropriate derived class. Let us assume, we are working on a game weapons specifically. However, i wanted to take some time to write about dealing with virtual functions in large, enterprisy codebases. This concept is an important part of the runtime polymorphism portion of objectoriented programming oop. Its purpose is to tell the compiler that what function we would like to call on the basis of the object of derived class. To allow to change the behavior, you must declare the function as virtual in the base class. In the example above, sqrt library function is invoked to calculate the square root of a number.

One important thing to note is that, you should override the pure virtual function of the base class in the derived class. Also, there are only return type of arguments but no arguments. One exception to the identical declaration rule is that if the member. When a virtual function is called, it is first looked up in the child and up the inheritance chain.

Use of virtual function allows the program to decide at runtime which function is to be called based on the type of the object pointed by the pointer. However, basegetname is virtual, which tells the program to go look and see if there are any morederived versions of the function available between base and derived. Virtual keyword is used to make a member function of the base class virtual. It is used when a methods basic functionality is the same but. Please note that in order to work properly, the virtual function and all its counterparts in derived classes should have same prototype. The compiler binds virtual function at runtime, hence called runtime polymorphism.

Before going into detail, lets build an intuition on why virtual functions are needed in the first place. As you examine the next example program named virtual2. You have already declared the function inside the body of the class definition. In this program, pure virtual function virtual float area 0. Virtual function are function pointers, just implemented in the viable of that class. The virtual function must be members of some class. When a class declares a virtual member function, most of the compilers add a hidden member variable that represents a pointer to virtual method table vmt or vtable. When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived classs version of the function. To create a virtual function, just place the reserved word virtual before the return type in the function prototype. Pure virtual function and abstract class in above example, base class shape member function area do not need any implementation because it is overriding in derived class. After making the function virtual, the program generates the correct expected output. A virtual function is a member function which is declared within a base class and is redefinedoverriden by a derived class. If it cannot find member function in the derived class, it will call member function of the base class. We write the body of virtual function in the derived classes.

Virtual functions are without a doubt the most difficult. The goal of objectoriented programming is to divide a complex problem into small sets. You say the name of the polygon, and tells you the area accordingly. If not declared with the same arguments in the subclasses, the member functions are not overridden polymorphically, whether or not they are declared virtual. Nonvirtual members can also be redefined in derived classes, but nonvirtual members of derived classes cannot be accessed through a reference of the base class.