Interface/abstract constructors

[Steven Schveighoffer]

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