Re: [boost] [review] Fast track review of Boost.Utility/Singleton begins today

From: Pierre-Jules Tremblay (boost_at_[hidden])
Date: 2008-01-17 12:19:36

On Jan 17, 2008 2:15 AM, Johan Nilsson <r.johan.nilsson_at_[hidden]> wrote:
> >> Did you notice that it is possible to use multiple inheritance? This
> >> way you can implement and test your domain-specific logic as a
> >> "normal" class, and then just expose a singleton of that specific
> >> type using e.g. boost::singleton<>.
> >
> > Yes, it makes it possible to test the class to be wrapped as a
> > singleton.  That's the easy part.  The hard part is to make the
> > singleton's client code testable.
> >
> > If I have a singleton class A that wraps a "regular" class B, I can
> > test B independently of A.  However, I have no way to test any code
> > that calls A::instance to get to B.
>
> That's why you should use dependency injection instead of designing the
> client code to directly call A::instance(). Why does the client code need to
> know it is accessing a singleton, when all it wants is the domain-specific
> logic?

I agree and advocate for dependency injection myself.  But your
original comment was that somehow Tobias' singleton's
multiple-inheritance support makes it possible to test the code.  I'm
saying it only enables you to test the
class-to-be-used-as-a-singleton's code, not the client code.  Now
you're saying the client code shouldn't depend on the class being used
as a singleton.  I agree, but Tobias' design doesn't make that use
case possible, so what are you suggesting (in the context of this
particular review)?

pj