Faking private and protected methods

Sometimes, you need to test code that internally calls private methods, and you want to control or fake those private methods during testing. Isolator++ provides the CallToPrivate API to help you fake private methods, even if they are not directly accessible.

The following APIs are used to define the private method to be faked: (ICallContext)
 

The API can be used like this:

a.CallToPrivate(private_method, args...).<behaviour>; 

Change the behavior of a private method

class MyClass

{

private:

   int PrivateMethod() { return -1; }

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

// Arrange

auto a = Isolator();

MyClass* myClass =  new MyClass();

PCSTR szMessage = "PrivateMethod was called";

a.CallToPrivate(A::Member(myClass, PrivateMethod)).WillThrow(szMessage);

In this example:

1. We use a.CallToPrivate(A::Member(myClass, PrivateMethod)) to fake the private method.

2. We specify the behavior WillThrow(szMessage) to throw an exception when the private method is called.

3. This approach is refactor-safe; if PrivateMethod is later made public, the test will still work.

Here is another example where we fake a local instance while using & in A::Memeber:

// Arrange

auto a = Isolator();

MyClass myClass = MyClass();

a.CallToPrivate(A::Member(&myClass, PrivateMethod)).WillReturn(10);

Change the behavior of a private static method

To fake a private static method, you use A::Global.

For example, if MyClass has a private static method StaticPrivateMethod, you can fake it as follows: (see Conditional Behavior)

a.CallToPrivate(A::Global(MyClass::StaticPrivateMethod) ,A::Any()).WillThrow(szMessage);

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

Change the behavior of a static C function (Free Function)
 

Static c functions are visible only within the specific module they are defined in. To fake these use A::Global:

a.CallToPrivate(A::Global(staticFunctionInModule)).WillReturn(20);

Out Parameters

To change behavior of private method with an out parameters use A::SetOut(), see here.

Faking a specific overloaded method

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