In C++ there is a concept of Virtual Distributor but there is no virtual constructor or constructor are not virtual why??????

Hi,

In C++, Virtual is Used foe polymorphism, which is calling a Derived class function using the Base class pointer.

Since all the functions in the Public, protected are inherited we may have virtual functions.

BUT Constructor of a class is never and ever extended by a derived, hence we will never heed to have a Virtual constructor.....

ie . U will never need to call a derived class constructor using the Base class Pointer...


Thanks & Regards,
Riju.

While it is true that using the keyword 'virtual' when declaring a constructor results in a syntax error at compile time, the concept of a virtual constructor exits in C++ as a design pattern, sometimes called the Factory Method. See Duffy "Financial Instrument Pricing Using C++."

what is virtual distructor?

Hi,

A destructor can be defined as virtual or even pure virtual. You would use a virtual destructor if you ever expect a derived class to be destroyed through a pointer to the base class. This will ensure that the destructor of the most derived classes will get called:

Eg:

A* b1 = new B;
delete b1;

If A does not have a virtual destructor, deleting b1 through a pointer of type A will merely invoke A's destructor. To enforce the calling of B's destructor in this case we must have specified A's destructor as virtual:

virtual ~A();

Thanks,
Riju.

The concept of polymorphism is by itself done in the constructor of each class.
The constructor owns the responsibility of initialising the Vtable and vptr which actually implements the Polymorphism technology. So constructor cannot be virtual. And it is also not required.
What is the concept of Polymorphism???
to call the respective functions of the object at run time rather than the static type of the object.
Imagine if I need to create an inheritted class object and the call does not go the base class constructor?? will my object be complete... then y is the necessity for such....

Virtual destructor is a must eg

Base *b;
b = new Derived();
delete b; // if my destructor is not virtual then it will invoke Base destructor
this is a big mess. Suppose I make them virtual then my call goes
to derived.

Neither constructors nor destructors are inherited.
Though virtual destructors are used for polymorphic approach in case of inheritance, but the main reason of using virtual destrustor is used for effective memory management.
It makes sure that all the relevant memory released associated with the class & its subclasses at the time of stack unwinding when destructor is called.