Should certain abstract classes be instantiable?

[Steven Schveighoffer]

On Thu, 01 Oct 2009 16:30:43 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Consider:
>
> class A {
>      abstract void fun() {}
> }
>
> The class defines a function that is at the same time abstract (so it  
> requires overriding in derivees) and has implementation.
>
> Currently the compiler disallows creation of objects of type A, although  
> technically that is feasible given that A defines the abstract method.
>
> Should A be instantiable? What designs would that help or hinder?

If you want to force a class to be abstract, even though it technically  
could be concrete given it has implementations for all functions, you  
could allow that by marking the class abstract.  i.e.:

abstract class A {
    void fun() {}
}

The other side, allowing A to be instantiated, makes no sense whatsoever.   
If you (as a class designer) want something to be instantiated, and it has  
all methods implemented, why mark it abstract?  IMO, it should be a  
compiler error to give implementation to an abstract method.

-Steve