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

Unit tests should 95% of the time only test the publicly exposed surface of a class. If you're testing something under the covers, that's testing implementation details, which is inherently fragile, because you should be able to easily change implementation and still have the tests work. Not only is it fragile, but you could also be tempted into testing things that aren't actually possible in planned usage scenarios, which is a waste of time.

If the point of the accessors you want to add is just to test whether the function had the desired effect, your class design may violate another principle, which is that a state-machine-like class should always make it clear what state its in, if that affects what happens when people interact with the class. In that case, it'd be right to provide those read-only accessors. If it doesn't affect the class's behavior, refer back to my previous bit about implementation details.

And as you rightly said, cluttering up the public surface of a class with unused stuff is also undesirable for its own reasons.

If I had to pick between accessors and friending in your case, I would choose friending, simply because you own your test and can change it in a pinch. You may not own the code by the clown who finds a way to use your extra accessors, and then you'll be stuck.