Mocking data members

[Thomas W.]

In "A Case for Mock Turtles" is a pretty simple example of defining an interface for your dependency.  But what if my class has data members of my dependency? You can't declare data members of an abstract class. 

I can make my class a template and resolve the data member on instantiation of the class. So during tests I can instantiate with the mock class, and during production I can instantiate with the real class. I can use the "real" class as a default template parameter so it doesn't muck up my API.

But this seems to mean I have to use a lot of dynamic casting on my data members if I want code completion.  And all of this feels funny to me.  Am I going down the right path here?

[Thomas W.]

Instead of forcing my dependencies to be virtual classes - I build the project twice. For test I like with mocks and for production I link to the actual dependencies.  This seems way easier and is less invasive to forcing changes on production code.