How to unit test if a private variable has changed

Question

Let's say that I have this class in C++:

class ExampleClass{
private:
    int example_var;
public:
    void exampleMethod(){
         example_var = other_value; // other value will be always different
    }
}

How can I unit test exampleMethod()? I would like to do something like this:

void testExampleMethod(){
    ExampleClass obj;
    int before_call_value = obj.example_var;
    obj.exampleMethod();
    int after_call_value = obj.example_var;
    ASSERT_NOT_EQUALS(before_call_value, after_call_value);
}

But example_var is private.

So, what is the right way to do this unit test? How can I test if a private example_var has changed?

Answer 1

It is bad approach to test private variable/methods. But if you need there are a lot of options:

1. You can make Your test class as friend of ExampleClass

2. You can grab information using moc object

Answer 2

If you want to access example_val, there are one of two things you can do. The first is by making testExampleMethod() a friend method, as follows:

class ExampleClass{
private:
    int example_var;
public:
    void exampleMethod(){
         example_var = other_value; // other value will be always different
    }
    friend void testExampleMethod(); //Now you can use the function as is.
}

On the other hand, you could just add a getter to your ExampleClass to access the variable, such as the following:

class ExampleClass{
private:
    int example_var;
public:
    void exampleMethod(){
         example_var = other_value; // other value will be always different
    }
    inline void getExampleVar() const { return example_var; }
}

And then change testExampleMethod() to:

void testExampleMethod(){
    ExampleClass obj;
    int before_call_value = obj.getExampleVar();
    obj.exampleMethod();
    int after_call_value = obj.getExampleVar();
    ASSERT_NOT_EQUALS(before_call_value, after_call_value);
}

I would honestly use the second method, since accessing a class's private variables is generally not recommended.

Answer 3

You just simply implement get function for that private variable you want to get.

class ExampleClass{
private:
    int example_var;
public:
    void exampleMethod(){
         example_var = other_value; // other value will be always different
    }

    int GetExampleVar(){
         return example_var;
    }
}

And call it like

void testExampleMethod(){
    ExampleClass obj;
    int before_call_value = obj.GetExampleVar();
    obj.exampleMethod();
    int after_call_value = obj.GetExampleVar();
    ASSERT_NOT_EQUALS(before_call_value, after_call_value);
}

---

Or make testExampleMethod friend function (friend function can access private variables of friend class even if its not its method).

class ExampleClass{
private:
    int example_var;
public:
    void exampleMethod(){
         example_var = other_value; // other value will be always different
    }

    friend void testExampleMethod();
}

---

In my opinion first example would be more suitable, but if you cannot modify ExampleClass, you can turn off access control for gcc -- -fno-access-control.