Unit test accessing private variables

Question

I have a unit test class Tester; I want it to access private fields of a Working class.

class Working {
    // ...
    private:
    int m_variable;
};

class Tester {
    void testVariable() {
        Working w;
        test( w.m_variable );
    }
}

I have the following options:

• make m_variable public - ugly
• make method test_getVariable() - overcomplicated
• add friend class Tester to Working - then Working "knows" about the Tester explicitly, which is not good

My ideal would be

class Working {
    // ...
    private:
    int m_variable;

    friend class TestBase;
};

class TestBase {};

class Tester : public TestBase {
    void testVariable() {
        Working w;
        test( w.m_variable );
    }
}

where Working knows about TestBase but not each test... but it does not work. Apparently friendship does not work with inheritance.

What would be the most elegant solution here?

Answer 1

I agree with Trott's answer, but sometimes you're adding unit tests to legacy code that wasn't designed for it. In those cases, I reach for #define private public. It's just in unit tests, and it's just for when refactoring is too expensive to bother. It's ugly, technically illegal, and very effective.

Answer 2

Generally, your unit tests should not evaluate private variables. Write your tests to the interface, not the implementation.

If you really need to check that a private variable has a particular characteristic, consider using assert() rather than trying to write a unit test for it.

A longer answer (written for C# rather than C++, but the same principles apply) is at https://stackoverflow.com/a/1093481/436641.