Let me clarify what I think you're saying. It's not about testing class X, it's about testing code that uses class X. As long as X is a class, then we can cre

Let me clarify what I think you're saying. It's not about testing class X, it's about testing code that uses class X. As long as X is a class, then we can create a mock version and pass around instances of it. If our "X functionality" is stored somewhere else (ie a bare function), then that gets harder.