On Jan 16, 2008 2:29 AM, Johan Nilsson <r.johan.nilsson@gmail.com> 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