Using a friend class vs. adding accessors for unit testing in C++?

Question

Is it better to add functions that return the internal state of an object for unit testing, as opposed to making the testing class a friend? - especially, when there is no use f

Answer 1

I'll disagree with the accepted answer and instead recommend the use of a friend class.

Part of the state you are testing is probably specific to the implementation of your class; you're testing details that other code normally doesn't know about or care about and shouldn't rely on. Public accessor functions make these implementation details part of the class's interface. If the internal state you are testing is not part of the intended interface, it shouldn't be visible through public functions. From a purist point of view you're stuck between two wrong answers, as friend classes are also technically part of the public interface. In my mind the question becomes, which option is less likely to lead to poor coding choices down the road? Going with a set of implementation-dependent public accessor functions will inadvertantly encourage an implementation-dependent conceptual model of the the class, leading to implementation-dependent use of the class. A single friend class, appropriately named and documented, is less likely to be abused.

While in general I strongly agree with the recommendation to prefer accessor functions over direct access to member variables, I don't agree that this best practice applies to unit testing of implementation-dependent internal state. A reasonable middle ground is to use private accessor functions to those pieces of state your unit test will care about, and be disciplined enough to use the accessor functions in your unit tests. Just my opinion.