Re: Virtual construtors

On Jan 27, 3:34 am, Mukesh <mukeshs...@gmail.com> wrote:


Been a few comments about this not making sense, but to elaborate with
a framework that might make it easier to explain what you had in mind,
or realise why it might not make sense...

    class Fruit
    {
        Fruit() { }
        virtual ~Fruit() { }
    };

    class Orange : public Fruit
    {
        Orange() { std::cout << "Orange()\n"; }
    }

    class Pear : public Fruit
    {
        Pear() { std::cout << "Pear()\n"; }
    }

....then the Pear constructor can be invoked by...

    new Pear(); // a new object on the heap
    new (address) Pear(); // a new object at an arbitrary memory
location
    Pear(); // a local stack-based temp
    Pear a; // a local stack-based variable

In any of these situations, the Pear's constructor is called without
any need for "virtual", but the actual type Pear class must be
specified. Perhaps you're hoping a virtual constructor would remove
that latter restriction? That's kind of what virtual functions do in
general... let you invoke an Orange or Pear function when you only
know you have a Fruit. But remember that any given Fruit only becomes
an Orange or Pear at the time it's constructed. virtual functions
just harken back to the earlier determination. If you're calling the
constructor saying "make me a Fruit", you can't magically expect the
compiler to intuit that you felt like an Orange rather than a
Pear. :-)

Hence the FAQ's Pear* clone() and static Pear* create() suggestions...
they use the language's "covariant return type" tolerance (so Pear*
successfully overloads Fruit*), and allow an existing object's runtime
type to determine the new object's type, even though you're referring
to the existing object via a general Fruit*. That's about the most
abstract way of specifying the type to be created. If that's not
enough, and you don't have an existing object of the right type, then
you have to create a factory function, returning Fruit*, with a switch
or if/else statement to run the "return new Orange()" or "return new
Pear()" as you feel appropriate....

Cheers,
Tony