Getting derived classes

[Steven Schveighoffer]

"Ary Borenszweig" wrote
> Steven Schveighoffer wrote:
>> "Ary Borenszweig" wrote
>>> Andrei Alexandrescu escribió:
>>>> dsimcha wrote:
>>>>> I know that, in std.traits, there's a template that spits out all base 
>>>>> classes
>>>>> for a given class.  Based on reading the source, it seems to work 
>>>>> based on a
>>>>> rather interesting use case for is expressions.  Is there any 
>>>>> equivalent way
>>>>> to do the opposite:  For any given position in a class hierarchy, to 
>>>>> get a
>>>>> tuple of all possible descendants?
>>>> That's not possible in general because in D the derived classes form an 
>>>> open set. I guess it could be done at runtime via reflection (not 
>>>> implemented afaik), but not at compile time.
>>> But for a given set of modules, the hierarchy is finite and known.
>>
>> Not at compile time, only at link time.  You can't compile code at link 
>> time.
>
> I still don't understand why you can't do this at compile time, assuming 
> you have the source code for everything that you'll link into your 
> executable.

Let's say this feature theoretically existed.  Here's an example usage:

class X
{
   Tuple!(???) getAllDerivedTypes() { return std.getAllDerivedTypes!(X)();}
}

If you compile this module like:

dmd -c X.d;

At this point, how does the compiler know all the modules that will derive 
from X?

D's compile-time reflection is generally limited to what is at or below a 
certain point in the hierarchy, not what has derived from it.

If you wanted to do something at runtime, that is possible, you might even 
be able to do this today by looking at the global TypeInfo array.

But you certainly cannot get a tuple of the derived classes at compile-time.

>
> How is a class hierarchy different in D than in Java? Eclipse JDT's has a 
> type hierarchy feature, and Bruno was going to implement it in Mrnmhrm, 
> all based on what's defined in the modules (you can say, compilte-time). 
> What's the problem?

Java has runtime reflection.  D does not.  You have to implement runtime 
reflection on top of compile time reflection (or so says Walter).

There should be no problem if you want the information at runtime.  But a 
Tuple is a compile-time entity.  You can probably get the TypeInfos of the 
classes, but not really be able to do much, since D's runtime reflection 
capabilities are limited.

-Steve