A Beginners tutorial in Virtual Destructor

Virtual Destructor

Virtual Destructor is used to release the derived class objects in a proper order....

Consider an Example:

Code:

#include<iostream.h>

class Base
{
public:
	Base()
	{
		cout<<"Base class constructor"<<endl;
	}
	virtual void show()
	{
		cout<<"Base class Inside show function"<<endl;
	}
	void display()
	{
		cout<<"Display method"<<endl;
	}
	[B]~Base ()[/B]
	{
		cout<<"Base class Destructor"<<endl;
	}
};

class Derived : public Base
{
public:
	Derived()
	{
		cout<<"Derived class constructor"<<endl;
	}
	void show()
	{
		cout<<"Inside derived show function"<<endl;
	}
	void display()
	{
		cout<<"Inside derived display function"<<endl;
	}
	~Derived()
	{
		cout<<"Derived class destructor"<<endl;
	}
};

void main()
{
	Base *b=new Derived;
	b->show();
	b->display();
	delete b;
}

Output for the above program is:-

Code:

Base class constructor
Derived class constructor
Inside derived show function
Display function
Base class Destructor

Consider a line

Base *b=new Derived;

Pointer object created for base class (Base class constructor and Derived class constructor automatically called during object creation phase)...

Consider a line

delete b;

In the above line base class object b destructs automatically (without destructing derived class

To avoid it if we have a concept called virtual destructor .

Example:

Code:

#include<iostream.h>
class Base
{
public:
	Base()
	{
		cout<<"Base class constructor"<<endl;
	}
	virtual void show()
	{
		cout<<"Base class Inside show function"<<endl;
	}
	void display()
	{
		cout<<"Display method"<<endl;

	}
	[B]virtual  ~Base ( )//virtual destructor See the virtual keyword[/B]
	{
		cout<<"Base class Destructor"<<endl;
	}
};

class Derived : public Base
{
public:
	Derived()
	{
		cout<<"Derived class constructor"<<endl;
	}
	void show()
	{
		cout<<"Inside derived show function"<<endl;
	}
	void display()
	{
		cout<<"Inside derived display function"<<endl;
	}
	~Derived()
	{
		cout<<"Derived class destructor"<<endl;
	}
};

void main()
{
	Base *b=new Derived;
	b->show();
	b->display();//if a method is not at all virtual only base class function called
	delete b;
}

Output is :-

Code:

Base class constructor
Derived class constructor
Inside derived show function
Display function
Derived class destructor
Base class Destructor

In the above program , Object destructs in a proper order(b'coz base class destructor is made as virtual......)

V-Table:

Normally v_table(virtual table ) is created for each and every class .The v-table normally contains the addresses of the virtual functions and the pointer to point the function(called as v-pointer). Whenever Virtual function is called , v-table decides to the function address..........

 

right said since base class object compatible with derived class object and can store the object of derived class.