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