Faking private and protected methods

Sometimes you'd like to test code that calls internally a private method, and you'd like to fake that private method.

Change the behavior of a private method of our class

class __declspec(dllexport) MyClass

{

private:

   int PrivateMethod() { return -1; }

To change the behavior of PrivateMethod, for example to throw an exception, we would write the following:

#include "MyClass.h"

MyClass* myClass =  new MyClass();

PCSTR szMessage = "PrivateMethod was called";

PRIVATE_WHEN_CALLED(myClass, PrivateMethod).Throw(szMessage);

We use the PRIVATE_WHEN_CALLED macro on the live instance to declare the behavior we want. In this case, we're throwing an exception with a string.

Using this macro has a future-benefit: If in the future you decide to expose the PrivateMethod as public, the tests still work!

Change the behavior of a private static method

What if we had a private static method, like StaticPrivateMethod was ? The same applies. Simply supply the static method signature too:

PRIVATE_WHEN_CALLED(_, MyClass::StaticPrivateMethod).Throw(szMessage);

As with the instance behavior setting, if the method goes public, it will continue to work.

Change the behavior of a static C function
 

Static c functions are visible only within the specific module they are defined in. To fake these functions simply pass the static function:

PRIVATE_WHEN_CALLED(_, staticFunctionInModule).Throw(szMessage);

How to change behavior of private method with an out parameters see here.

To fake a specific overload of a private method, see Faking Overloaded methods.