a) Now your singleton is also responsible for providing a mock implementation of itself (which shouldn't really be one of its concerns).
b) You didn't really do anything here: the class under test is still going to use the concrete implementation of C.
You need a way of providing a mock implementation to your class while testing. The obvious answer to how to do it is dependency injection. But if you're going down that route, you can just ditch the whole singleton thing, instantiate a class using "new" somewhere else and pass it to your class. And in your unit test, you simply pass the mock implementation. This is what DI containers like Spring do. We'd be doing the exact same thing here, only manually.
a) True, but it can probably be pushed into the parameterized Singleton class. I didn't try, since that might get more language dependent and since I wanted to keep things explicit-but-short for demonstration purposes anyway.
b) You're touching a tiny part of the implementation of the real C, yes. If you're able to push the Instance and Mock functions into Singleton, you might be touching literally no code of C other than relying on the fact that it in-fact inherits from Singleton.
I nonetheless agree dependency injection is a better solution. The biggest objection I have to this implementation is that there's nothing that tells me I should be mocking C, or catches it when I forget to mock C.
