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

From: Pierre-Jules Tremblay (boost_at_[hidden])
Date: 2008-01-16 09:23:49

On Jan 16, 2008 2:29 AM, Johan Nilsson <r.johan.nilsson_at_[hidden]> wrote:
> Pierre-Jules Tremblay wrote:
> > My biggest problem with this design is that it is unsuitable for
> > library writers: types derived from typical singleton class designs,
> > including boost::singleton and friends under review, invariably give
> > rise to
> > untestable code.  To make a singleton's client code testable involves
> > exposing some kind of overridable factory so that the singletonized
> > type can be replaced with a test mockup.  So far I have found this to
> > be a hard problem to solve in general.
>
> 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.

pj