Counterproposal for extending static members and constructors

[Timon Gehr]

On 07/12/2012 09:54 PM, H. S. Teoh wrote:
> 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
>

Note that this is 'supported' today because of a compiler bug.

class C{
     this(){
         this = new D;
     }
     protected this(int){}
}
class D : C{
     this(){super(0);}
}


void main(){
     C c = new C;
     assert(typeid(c) is typeid(D));
}