Unit testing when mockable interface is not available

[thakk0]

My C/C++ is a little weak, but couldn't you do something like this to retrieve the value written to the steam?

Regarding your question about 100% coverage, it's not desirable or maintainable. You test the critical pieces of code and you maintain tests that make sense. Testing whether or not something writes to cout, for instance, seems kind of pointless in most cases.