Is there a way to forward-declare interfaces to avoid module interdependencies?

Per Ångström d-news at
Thu Nov 11 08:07:42 PST 2010

On 2010-11-11 15:51, Jacob Carlborg wrote:
> First you have to import ITeacher in Student.d (if I'm reading this
> right).

No, the code works fine as is, but if I move the import of ITeacher from 
IStudent.d to Student.d, IStudent.d won't compile:

IStudent.d(4): Error: identifier 'ITeacher' is not defined
IStudent.d(4): Error: ITeacher is used as a type

> Second, D has (generally) no problems with circular references.

No, but I have! ;-) To be honest, I would have wanted D to outright 
outlaw circular references.

> You only get problems when two modules is a part of a circular reference
> and both have module constructors. If you porting C++ code you will not
> have this problem since C++ doesn't have module constructors.

Actually, I'm more thinking whether D would be a good language for 
building really big systems. If the language more or less requires many 
modules to import each other, that's a big drawback in my opinion, since 
that makes a mess of the dependency graph. It also makes independent 
testing of individual modules a lot harder.

> BTW, you generally don't separate your interface and implementation in D
> (you can to that if you want to hide your implementation).

I think information hiding is essential for a large-scale design to be 
comprehensible as a whole. Agreed, the module system in D hides a lot of 
implementation detail from being included in each compilation unit, but 
we must also consider not overloading the programmer with needless 

But then again, maybe I just need to stop thinking in C++.

> The import/module system in D is more like the one in Java than the one
> in C/C++.

OK, I'm more familiar with C++. But I really like the concept of 
importing modules instead of file inclusion.

Per Å.

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