I am just looking up abstract class information and i notice that there is no information on dlang.org/spec

Yet using this as a resource:

https://www.tutorialspoint.com/d_programming/d_programming_abstract_classes.htm

It can be found, with a proper example. Am i blind and not seeing it in the specs?

Even Ali's book barely mentions it ( 55 ).

And yet abstract classes exit and function in D. So strange.

On Tuesday, May 30, 2017 22:13:54 Wulfklaue via Digitalmars-d wrote:
> I am just looking up abstract class information and i notice that there is no information on dlang.org/spec
>
> Yet using this as a resource:
>
> https://www.tutorialspoint.com/d_programming/d_programming_abstract_classe s.htm
>
> It can be found, with a proper example. Am i blind and not seeing it in the specs?
>
> Even Ali's book barely mentions it ( 55 ).
>
> And yet abstract classes exit and function in D. So strange.

http://dlang.org/spec/attribute.html#abstract

That part of the spec talks about the class becoming abstract if any of its members are. Note that marking a class as abstract is equivalent to marking all of its member functions with abstract, just like marking class with @safe would make all of its member functions @safe. So, there isn't really any special handling of marking the class itself as abstract.

- Jonathan M Davis

On Tuesday, 30 May 2017 at 22:13:54 UTC, Wulfklaue wrote:
> I am just looking up abstract class information and i notice that there is no information on dlang.org/spec

They are defined where the abstract attribute is defined[1], but there is not much to them.

[1] http://dlang.org/spec/attribute.html#abstract

On 05/31/2017 12:19 AM, Jonathan M Davis via Digitalmars-d wrote:
> Note that marking a class as abstract is equivalent to marking
> all of its member functions with abstract, just like marking class with
> @safe would make all of its member functions @safe. So, there isn't really
> any special handling of marking the class itself as abstract.

That's not true. A class's `abstract` attribute does not transfer to its methods. It's the other way around: Having an abstract method makes the class abstract.

On Wednesday, May 31, 2017 07:30:20 ag0aep6g via Digitalmars-d wrote:
> On 05/31/2017 12:19 AM, Jonathan M Davis via Digitalmars-d wrote:
> > Note that marking a class as abstract is equivalent to marking
> > all of its member functions with abstract, just like marking class with
> > @safe would make all of its member functions @safe. So, there isn't
> > really any special handling of marking the class itself as abstract.
>
> That's not true. A class's `abstract` attribute does not transfer to its methods. It's the other way around: Having an abstract method makes the class abstract.

Hmmm. That would make abstract inconsistent with other attributes on a class, but after messing around with it a bit, it looks like the only effect that marking a class with abstract has is how many times you get a linker error about the member function not being defined. So, it looks like a I did indeed remember incorrectly (or just misunderstood), but the behavior that we do get with regards to putting abstract on the class is rather weird as well as inconsistent with what happens with other attributes on a class. But on reflection, that's probably because you wouldn't want to mark a function with a body as abstract, meaning that marking abstract on a class would either have to be ignored as it seems to be, or it would have to be ignored on any function that has a body, which would probably cause problems with .di files. So, I stand corrected.

- Jonathan M Davis

On 2017-05-31 09:09, Jonathan M Davis via Digitalmars-d wrote:

> Hmmm. That would make abstract inconsistent with other attributes on a
> class, but after messing around with it a bit, it looks like the only effect
> that marking a class with abstract has is how many times you get a linker
> error about the member function not being defined. So, it looks like a I did
> indeed remember incorrectly (or just misunderstood), but the behavior that
> we do get with regards to putting abstract on the class is rather weird as
> well as inconsistent with what happens with other attributes on a class. But
> on reflection, that's probably because you wouldn't want to mark a function
> with a body as abstract, meaning that marking abstract on a class would
> either have to be ignored as it seems to be, or it would have to be ignored
> on any function that has a body, which would probably cause problems with
> .di files. So, I stand corrected.

The behavior actually makes sense.

If you mark the whole class as "abstract", you cannot create instances of the class.

If you mark a method "abstract" it will force any sub classes to override that method. Since a sub class has to override the method it doesn't make sense to allow to create instances of the class, implicitly making the whole class abstract.

-- 
/Jacob Carlborg