Template subclass virtual override

 Dewdman42 <dewdman42@gmail.com>
Mon, 06 Aug 2007 18:46:07 -0700
I have a question for C++ gurus

Assuming I have template class:

   template <class T>
   class Car<T>
       virtual T* factory();

   template <class T>
       return new T;

Here is a class I will use to drive T above:

  class Airplane

To use like this:

  Car<Airplane> myCar;
  Airplane* a = myCar.factory();

Fine so far. Now let's say I want to subclass my template to provide
an ovreride to the factory method because I don't want to allow a
default constructor of Airplane, but rather a one arg constructor:

   class Spaceship
       Spaceship(int arg);
       Spaceship(); // to prevent being able to instantiate without

And finally a subclass of the template that overrides the virtual
factory method:

class Boat : public Car<Spaceship>
       virtual Spaceship* factory();

       return new Spaceship(100);

In theory to use like this:

   Boat<Spaceship> myBoat;
   Spaceship* a = myBoat.factory();

The above does not quite compile because when the compiler uses
Spaceship to generate the Boat<Airplane> class from the template,
which is really a subclass of Car, it expects to find a default
constructor for the Spaceship class in order to satisfy the template
base class virtual factory method, even though there are no clients
that will use Car<Spaceship>, only Boat<Spaceship>

I really need Car() to have this default factory method for cases
where people will not use a subclass or override that virtual factory
method, like my first example of Car<Airplane>. However, I also in
this case don't want to provide a default constructor to my Spaceship
class because it will NEVER be used that way and I want to make sure
it can't be used that way.

Can anyone think of a way to satisfy the template without having to
add the default constructor to Spaceship? Or perhaps a different
class structure that will accomplish what I'm trying to do. ??

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