I used a function to test private class members which was just called TestInvariant().
It was a private member of the class and, in debug mode, was called at the beginning and end of every function (except the beginning of the ctor and end of the dctor).
It was virtual and any base class called the parent version before it's own.
That allowed me to verify the internal state of the class all of the time without exposing the intenals of the class to anyone. I had very simple tests but there is no reason why you could not have complicated ones, or even set it on or off with a flag etc.
Also you can have public Test functions which can be called by other classes which call your TestInvariant() function. Therefore when you need to change inner class workings you do not need to change any user code.
Would this help?
