what is the use of virtual constructors

[suneel]

hi all
I am new to the C++ please help me what is the use of virtual constructors in c++, and when we can use these virtual constructors.

[Banfa]

I have never seen, nor do I believe that it is correct syntax to have, a virtual constructor.

[vermarajeev]

hi all
I am new to the C++ please help me what is the use of virtual constructors in c++, and when we can use these virtual constructors.

VIrtual constructor is a concept which doesnot exist in C++ but that provides you to implement one.

First thing first
Why do we require a virtual construct?
When you just consider virtual functions, it is concept which is completely based on type of object (not the type of pointer/reference to that object). ie the functions are called at runtime not compile time.

Now conside this example
Base* b = new Derived();

Hope I'm clear and not mistaken. If I'm wrong please comment.

Unlike ordinary member function, a constructor allows you to construct the type of the object at compile time. In above case, the complete object is created at compile time , this does not delay till runtime. So, the concept of virtual constructor is vague in this concept.

But you can provide the concept using virtual functions
Conside this example

class Browser
{
    public: 
     Browser(){}
     Browser(const Browser&){}  
        virtual Browser* construct()
        {
            return new Browser();
        }
 
        virtual Browser* clone()
        {
            return new Browser(*this);
        }
      virtual ~Browser(){}
};
 
class HTML : public Browser
{
  public:
     HTML(){}
     HTML(const HTML&){}  
        HTML* construct()
        {
            return new HTML();
        }
 
        HTML* clone()
        {
            return new Browser(*this);
        }
      virtual ~HTML(){}
};
 
void userCode( Browser& b)
{
    Browser* bw = b.construct();
    //use bw
    delete bw;
} 

This allows you to instatiate a new object of the right type without having to know the type of source object.