TITLE: An amusing instantiation of an abstract class


PROBLEM:  a260bail@cdf.toronto.edu (Bailin Jeremy)

Just wondering if it's possible to instantiate a class that has a
pure virtual method if you never actually call the method.


RESPONSE: grumpy@cbnewse.cb.att.com (Paul J Lucas)

Just wondering why you didn't read a C++ book which states that
what you ask is impossible.  That's part of the *point* of an
abstract class.


RESPONSE: rmartin@rcmcon.com (Robert Martin), 23 Feb 93

You are of course correct as far as mainstream C++ programming is
concerned.  However perhaps the original poster wanted to know if
there was a "trick" that would allow an instance of an abstract class
to somehow escape into the system.  And, of course, there is.

class Base
{
  public:
    Base();
    virtual void f() = 0;
};

class Derived : public Base
{
  public:
    virtual void f() {}
};

void G(Base& b)
{
   ....  
}

Base::Base() {G(*this);}

main()
{
  Derived d;	
}

When d is created the Base::Base constructor will call G with a
reference to an object which has been constructed as a Base, but not
yet constructed as a Derived.  Thus, it is an instance of an abstract
class.  

Of course, this is very dangerous and I am not recommending it as a
practice.