Class testing question

**chiefmonkey** · 05-25-2009

Hi,

I am trying to unit test a class I wrote. I wasn't sure how I should go about checking the values of private member variables after calling functions that set them. I was hoping that people with experiences on the subject could help me with it.

Code:

class A
{
public:
    void func()
private:
    int m_var;
};

The class has a public function func() that changes m_var when called. I didn't have m_var made public or expose it through a public getter function because it was necessary not to expose the variable to develoeprs. In this situation, how do I test that func() changed the variable in an expected way?

If m_var were public, I would do

Code:

int main(int argc, char[] argv)
{
    int var = 0; // 0 is the value of m_var supposed to be set by func()
    A a;
    a.func(); // it will call func()
    if(a.m_var != var)
    {
        cout << "func() failed to properly set A::m_var";
    }
}

But I cannot do this because m_var is a private member. Is having a getter for m_var the only option in this situation? or is it recommended to use the "friend" keyword to have a unit test class access the private variable? Thanks!

**dra** · 05-25-2009

It is generally considered good practice to use accessor functions to retrieve members from a class, rather than exposing them directly to the user.

**Programmer_P** · 05-25-2009

Have an accessor function outside int main() that modifies the private variable, like this:

Code:

A::func()  {
m_var = var; //define protected variable inside class "A"
}

or whatever.

**medievalelks** · 05-26-2009

Several options here...

1) Do nothing. Setters and getters are trivial and don't require unit tests.

2) Add a getter function. It matches the setter and is better than a public member variable, but it adds to the public interface for the sole purpose of testing.

3) Make your test class a friend of the class being tested.

Code:

#include <cassert>

class Boo {
	int i;
	friend class BooTest;
public:
	Boo(int j) : i(j) {}
	void set(int p) { i = p; }
};

class BooTest {
public:
	void testSet()
	{
		Boo b(7);
		b.set(0);

		assert(b.i == 0);
	}
};
	
int main()
{
	BooTest bt;
	bt.testSet();

	return 0;
}

4) Write "self-testing" code, as Cline, Lomow, and Girou suggest in the C++ FAQs book:

Code:

#include <cassert>

class Boo {
	int i;
public:
	Boo(int j) : i(j) {}
	void set(int p) 
	{ 
		i = p; 
#ifdef UNITTEST
		assert(i == p);  
#endif
	}
};

int main()
{
	Boo b(4);
	b.set(1);

	return 0;
}

I'd opt for 1) for simple setters (and would try to refactor the setters out of the code).

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