Should certain abstract classes be instantiable?

[Andrei Alexandrescu]

Jeremie Pelletier wrote:
> Andrei Alexandrescu 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?
>>
>>
>> Andrei
> 
> What's the point of marking fun() abstract if it has an implementation, 
> I thought the compiler disallowed that.

It may offer incomplete functionality that is to be reused and enhanced 
by descendants.

Andrei