C++ VIRTUAL

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VIRTUAL FUNCTION

Q: What is virtual function?
A: When derived class overrides the base class method by redefining the same function, then if
client wants to access redefined the method from derived class through a pointer from base class
object, then you must define this function in base class as virtual function.
class parent
{
void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls parent->show()
now we goto virtual world...
class parent
{
virtual void Show()
{
cout << "i'm parent" << endl;
}
};
class child: public parent
{
void Show()
{
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls child->show()

Q: What is a "pure virtual" member function?
A:  The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.

Q: How virtual functions are implemented C++?
A: Virtual functions are implemented using a table of function pointers, called the vtable. There
is one entry in the table per virtual function in the class. This table is created by the constructor
of the class. When a derived class is constructed, its base class is constructed _rst which creates
the vtable. If the derived class overrides any of the base classes virtual functions, those entries in
the vtable are overwritten by the derived class constructor. This is why you should never call
virtual functions from a constructor: because the vtable entries for the object may not have
been set up by the derived class constructor yet, so you might end up calling base class
implementations of those virtual functions

Q: What is pure virtual function? or what is abstract class?
A: When you de_ne only function prototype in a base class without implementation and do the
complete implementation in derived class. This base class is called abstract class and client won't
able to instantiate an object using this base class. You can make a pure virtual function or
abstract class this way..
class Boo
{
void foo() = 0;
}
Boo MyBoo; // compilation error

Q: What is Pure Virtual Function? Why and when it is used?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined  as pure
virtual in base class.

Q: How Virtual functions call up is maintained?
A: Through Look up tables added by the compile to every class image. This also leads to
performance penalty.

Q: What is a virtual destructor?
A: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
The behavior of a virtual destructor is what is important. If you destroy an object through a caller
or reference to a base class, and the base-class destructor is not virtual, the derived-class
destructors are not executed, and the destruction might not be complete.

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