c++ - How can I remove/refactor a «friend» dependency declaration properly?

Question

The background of this question is based on a practical sample where I wanted to remove a ?friend? dependency from a pair of classes that are used to manage read/write locked access to a shared resource.

Here's an abstraction of the original structural design for that scenario:

Marked in red, there's this ugly ?friend? dependency I want to remove from the design.

In short, why do I have this thing there:

1. ClassAProvider shares a reference to a ClassA over a number of concurrently accessing Client instances
2. Client instances should access ClassA solely through the ClassAAccessor helper class that manages the internals
3. ClassA hides all methods intended to be used from ClassAAccessor as protected.
4. So ClassA can ensure that Client needs to use a ClassAAccessor instance

This pattern comes primarily useful, when it's about ensuring to leave instances of ClassA in a defined state, if a Client operation bails out (because of e.g. an uncaught exception). Think of ClassA providing (internally visible) paired operations like lock()/unlock() or open()/close().

The (state-)reversing operations should be called in any case, especially when a client crashes due to an exception.
This can be safely handled through the ClassAAcessor's life cycle behavior, the destructor implementation can ensure it. The following sequence diagram illustrates what's the intended behavior:

Additionally Client instances can achieve a fine control of accessing ClassA easily, just using C++ scope blocks:

// ...
{ 
    ClassAAccessor acc(provider.getClassA());
    acc.lock();
    // do something exception prone ...
} // safely unlock() ClassA
// ...

All fine so far, but the ?friend? dependency between ClassA and ClassAAccessor should be removed for a number of good reasons

1. In the UML 2.2 Superstructure, Section C.2 under Changes from previous UML it says: The following table lists predefined standard elements for UML 1.x that are now obsolete. ... ?friend? ...
2. Most coding rules and guidelines I've seen forbid, or strongly discourage using friend, to avoid the tight dependency from the exporting classes to the friends. This thing brings in some serious maintenance issues.

As my question title says

How can I remove/refactor a friend declaration properly (preferably starting out at the UML design for my classes)?

Answer 1

Let's setup some constraints for refactoring first:

1. The ClassAAccessor's publicly visible interface should change in no way
2. The ClassA internal operations should not be visible/accessible from the public
3. The overall performance and footprint of the original design should not be hurt

---

Step 1: Introduce an abstract interface

For a first shot, I factored out the ?friend? stereotype, and replaced it with a class (interface) InternalInterface and the appropriate relations.

What made up the ?friend? dependency, was split up into a simple dependency relation (blue) and a ?call? dependency (green) against the new InternalInterface element.

---

Step 2: Move the operations, that make up the ?call? dependency to the interface

The next step is to mature the ?call? dependency. To do this, I change the diagram as follows:

• The ?call? dependency turned into a directed association from ClassAAccessor to the InternalInterface (I.e. ClassAAccessor contains a private variable internalInterfaceRef).
• The operations in question were moved from ClassA to InternalInterface.
• InternalInterface is extended with a protected constructor, that it's useful in inheritance only.
• ClassA's ?generalization? association to InternalInterface is marked as protected, so it's made publicly invisible.

---

Step 3: Glue everything together in the implementation

In the final step, we need to model a way how ClassAAccessor can get a reference to InternalInterface. Since the generalization isn't visible publicly, ClassAAcessor can't initialize it from the ClassA reference passed in the constructor anymore. But ClassA can access InternalInterface, and pass a reference using an extra method setInternalInterfaceRef() introduced in ClassAAcessor:

---

Here's the C++ implementation:

class ClassAAccessor {
public:
    ClassAAccessor(ClassA& classA);
    void setInternalInterfaceRef(InternalInterface & newValue) {
        internalInterfaceRef = &newValue;
    }
private:  
    InternalInterface* internalInterfaceRef;
};

This one is actually called, when the also newly introduced method ClassA::attachAccessor() method is called:

class ClassA : protected InternalInterface {
public:
    // ...
    attachAccessor(ClassAAccessor & accessor);
    // ...
};

ClassA::attachAccessor(ClassAAccessor & accessor) {
    accessor.setInternalInterfaceRef(*this); // The internal interface can be handed
                                             // out here only, since it's inherited 
                                             // in the protected scope.
}

Thus the constructor of ClassAAccessor can be rewritten in the following way:

ClassAAccessor::ClassAAccessor(ClassA& classA)
: internalInterfaceRef(0) {
    classA.attachAccessor(*this);
}

---

Finally you can decouple the implementations even more, by introducing another InternalClientInterface like this:

---

It's at least necessary to mention that this approach has some disadvantages vs using friend declarations:

1. It's complicating the code more
2. friend doesn't need to introduce abstract interfaces (that may affect the footprint, so constraint 3. isn't fully fulfilled)
3. The protected generalization relationsip isn't well supported by the UML representation (I had to use that constraint)