Counterproposal for extending static members and constructors

[H. S. Teoh]

On Thu, Jul 12, 2012 at 03:27:06PM -0400, Jonathan M Davis wrote:
> On Thursday, July 12, 2012 18:25:03 David Piepgrass wrote:
> > I'm putting this in a separate thread from
> > http://forum.dlang.org/thread/uufohvapbyceuaylostl@forum.dlang.org
> > because my counterproposal brings up a new issue, which could be
> > summarized as "Constructors Considered Harmful":
> > 
> > http://d.puremagic.com/issues/show_bug.cgi?id=8381
> 
> I think that adding constructors to a type from an external source is
> downright evil. It breaks encapsulation. I should be able to constrain
> exactly how you construct my type. If you want to create a free
> function (e.g. a factory function) which uses my constructors, fine.
> But I'm completely against adding constructors externally.
[...]

Yeah, I think free-function ctors are not a good idea.

But unifying ctor syntax with object factories is a good idea IMO.  It
helps encapsulation: the users of class C don't have to know what the
_actual_ object instance is, they just get a C reference, which could be
an instance of D (which inherits from C). For example, I can write:

	string url = ...;
	auto connection = new DBConnection(url);

If url points to a SQLite database, the DBConnection ctor can return an
instance of SQLiteDBConnection; if url points to an Oracle database, the
ctor can return an instance of OracleDBConnection. But the ctor could
just as easily return an instance of DBConnection itself, if the class
is designed to work across different database backends in a generic way.
The user doesn't have to know this implementation detail.

The only concern is how this would interact with derived class ctors,
since calling superclass ctor may not return what the derived class ctor
is expecting. Other than this concern, though, I really like this idea.


T

-- 
"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." -- Bertrand Russell. "How come he didn't put 'I think' at the end of it?" -- Anonymous