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Question

I have two classes A and B. Currently, B is constructed inside of A. Since I want to verify in my unit tests that A calls B's methods correctly, I want to inject B into A instead to enable mocking.

The problem is: Upon construction, B needs one of A's fields as a constructor parameter. Which means that if I inject B and therefore move its construction out of A, I would also have to move that field's construction out of A.

What's the most elegant solution here? I hope I explained the problem properly. If not, please feel free to ask any questions.

An attempt at explaining the problem with code snippets:

What it's currently like:

// constructor of class A. b_ is being constructed. field_of_a_ is initialized.
A::A() : b_{field_of_a_}, field_of_a_{5} {

}

What I want to avoid:

// In the class constructing A...
int field_of_a;
B b(field_of_a);
A a(b, field_of_a);

// and inside A...
A::A(B& b, int& field_of_a) : b_{b}, field_of_a_{field_of_a} {

}

EDIT 1: Clarification from inside one of my comments:

Yeah so currently B holds all kinds of timers and timer related methods. Its methods are called by A during message handling. B needs access to a queue and a few other resources located inside of A. When testing A, I want to verify that during message handling the timers are started/stopped correctly.

Answer 1

Instead of passing B, you should pass a factory that generates B. It still maintains the dependency injection. The factory could be a virtual class, a function pointer, a std::function, ...

It is then up to A to use the factory. Your test would simply verify that the factory is used during construction of A.

Update : your factory should be abstract, and return an instance to the public interface of B that A requires. During testing the factory can be programmed to spawn mocked instances of B.

Also, make sure that in the class definition of A the member passed to B is declared first, then the member holding a B instance. This in order to make the first member's lifetime outlive that of B.