Virtual Function in C++

virtual function

A virtual function is a member function of a class that is declared using the keyword virtual and is overridden in a derived class.
The function call is resolved at runtime based on the type of object, not the pointer.

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Need for Virtual Function

• Function calls are decided at runtime
• This is called Late Binding (Dynamic Binding)

 

Syntax of Virtual Function

class Base {

public:

    virtual void show() {

        cout << "This is Base class show function";

    }

};

• virtual keyword is written in the base class
• Derived class overrides the function

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Rules of Virtual Function

1. Declared using the virtual keyword
2. Must be a member function
3. Cannot be static
4. Call is resolved at runtime
5. A function call is made using a base class pointer
6. Virtual function remains virtual in derived classes

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Example  – With Virtual Function

Program

#include <iostream>

using namespace std;

 

class Base {

public:

    virtual void show() {

        cout << "Base class show function" << endl;

    }

};

 

class Derived : public Base {

public:

    void show() {

        cout << "Derived class show function" << endl;

    }

};

 

int main() {

    Base* b;

    Derived d;

    b = &d;

    b->show();

    return 0;

}

Output

Derived class show function

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Process of Virtual Function Works

Virtual Table (V-Table) Concept

• The compiler creates a V-Table for each class
• V-Table stores addresses of virtual functions
• Each object has a V-Pointer pointing to its V-Table
• At runtime, the function address is fetched from the V-Table

  

Pure Virtual Function

A function with no definition in the base class.

virtual void display() = 0;

• Makes class Abstract Class
• The derived class must override the function

Example

class Animal {

public:

    virtual void sound() = 0;

};

 

class Dog : public Animal {

public:

    void sound() {

        cout << "Dog barks" << endl;

    }

};

Difference: Virtual Function vs Non-Virtual Function

Feature	Virtual Function	Non-Virtual Function
Binding	Runtime	Compile Time
Polymorphism	Yes	No
Keyword	virtual	No keyword
Flexibility	High	Low

 

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