Interface/abstract constructors

[nrgyzer]

== Auszug aus Steven Schveighoffer (schveiguy at yahoo.com)'s Artikel
> On Mon, 16 May 2011 16:12:05 -0400, nrgyzer <nrgyzer at gmail.com>
wrote:
> > == Auszug aus Steven Schveighoffer (schveiguy at yahoo.com)'s Artikel
> >> On Mon, 16 May 2011 15:32:43 -0400, useo <unknown at unknown.com>
> > wrote:
> >> > Hey guys,
> >> >
> >> > is there any chance to create an abstract constructor like:
> >> >
> >> > abstract class ABC {
> >> >
> >> >    abstract this();
> >> >
> >> > }
> >> >
> >> > DMD always says "...this non-virtual functions cannot be
> > abstract" -
> >> > when I use an interface like:
> >> >
> >> > interface ABC {
> >> >
> >> >    this();
> >> >
> >> > }
> >> >
> >> > I get a similar error: "...constructors, destructors,
postblits,
> >> > invariants, unittests, new and delete functions are not
allowed in
> >> > interface ABC"
> >> >
> >> > Is there any solution or is it possible to create such
> > inheritances
> >> > in DMD?
> >> I think what you are trying to do is say, "if a class implements
> > interface
> >> ABC, it must have a default constructor".  Such a requirement is
> > faulty.
> >> The point of an interface is to able to pass a portion of a
class'
> >> functionality to a function during runtime.  However, the
instance
> > must
> >> *already exist*.  It makes no sense to posit requirements on the
> >> constructor.
> >> What you want is a compile-time requirement using a template
> > constraint.
> >> You may think "damn, but I don't want to make my function a
> > template", I'd
> >> say see previous point ;)
> >> -Steve
> >
> > Okay, thanks... perhaps someone know a better solution: I have one
> > big file which contains some other files (let's say: blocks). Each
> > block has it's own signature... by reading the big file, I read
the
> > signature of each block. Based on the signature, I read block A,
> > block B or another Block. To do that, I want call the block-
specific
> > constructor which reads the next bytes.
> >
> > !Semicode:
> >
> > ...
> > ABC[] blocks;
> > ...
> > while (!eof(bigfile)) {
> >   read(signature);
> >   if (signature==A) blocks ~= new A(bigfile);
> >   else if (signature==B) blocks ~= new B(bigfile);
> >   ...
> > }
> > ...
> No special requirements are necessary.  How would this compile if A
or B
> did not have a bigfile constructor?  The interface specification is
not
> needed.
> If D supported runtime reflection (and it does to a very very small
> degree), then you could use it to ensure the correct constructor is
> available.
> -Steve

It's semicode, so I haven't try to implement it... it should only
show what I'm trying to do.
I'm currently thinking about an empty constructor in an abstract
class like:

abstract class ABC {
   this(Stream) {
      // do nothing
   }
}

class A : ABC {

   this(Stream s) {
      super(s);
      // read my block-specific bytes
   }

}