Bulldozer00's Blog

Check out the simple unit test code fragment below. What the hell is the #define private public preprocessor directive doing in there?

Now look at the simple definition of MyClass:

The purpose of the preprocessor #define statement is to provide a simple, elegant way of being able to unit test the privately defined MyClass::privateImpl() function. It sure beats the kludgy “friend class technique” that blasphemously couples the product code base to the unit test code base, no? It also beats the commonly abused technique of specifying all member functions as public for the sole purpose of being able to invoke them in unit test code, no?

Since the (much vilified) C++ preprocessor is a simple text substitution tool, the compiler sees this definition of MyClass just prior to generating the test executable:

Embarassingly, I learned this nifty trick only recently from a colleague. Even more embarassingly, I didn’t think of it myself.

In an ideal world, there is never a need to directly call private functions or inspect private state in a unit test, but I don’t live in an ideal world. I’m not advocating the religious use of the #define technique to do white box unit testing, it’s just a simple tool to use if you happen to need it. Of course, it would be insane to use it in any production code.

Arne Mertz has a terrific post that covers just about every angle on unit testing C++ code: Unit Tests Are Not Friends.