Should certain abstract classes be instantiable?

[Jarrett Billingsley]

On Thu, Oct 1, 2009 at 8:49 PM, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org> wrote:
> Jarrett Billingsley wrote:
>>
>> On Thu, Oct 1, 2009 at 4:30 PM, 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?
>>
>> Uh... why?
>
> Because I want to give a good argument one way or another in TDPL. FWIW, "I
> can't imagine why you'd ever..." or "Never needed that" are not strong
> enough arguments.

But.. you mark something abstract when you want it to be .. abstract.
How would you argue that abstract is basically a no-op when used on
methods with bodies? And there's a reasonable use for it, so why
suddenly allow something that.. doesn't really make all that much
sense to begin with?