Dependency injection in C++

Question

This is also a question that I asked in a comment in one of Miško Hevery's google talks that was dealing with dependency injection but it got buried in the comments.

I wonder how can the factory / builder step of wiring the dependencies together can work in C++.

I.e. we have a class A that depends on B. The builder will allocate B in the heap, pass a pointer to B in A's constructor while also allocating in the heap and return a pointer to A.

Who cleans up afterwards? Is it good to let the builder clean up after it's done? It seems to be the correct method since in the talk it says that the builder should setup objects that are expected to have the same lifetime or at least the dependencies have longer lifetime (I also have a question on that). What I mean in code:

class builder {
public:
    builder() :
        m_ClassA(NULL),m_ClassB(NULL) {
    }
    ~builder() {
        if (m_ClassB) {
            delete m_ClassB;
        }
        if (m_ClassA) {
            delete m_ClassA;
        }
    }
    ClassA *build() {
        m_ClassB = new class B;
        m_ClassA = new class A(m_ClassB);
        return m_ClassA;
    }
};

Now if there is a dependency that is expected to last longer than the lifetime of the object we are injecting it into (say ClassC is that dependency) I understand that we should change the build method to something like:

ClassA *builder::build(ClassC *classC) {
    m_ClassB = new class B;
    m_ClassA = new class A(m_ClassB, classC);
    return m_ClassA;
}

What is your preferred approach?

Answer 1

Based on my own experience, it is best to have clear ownership rules. For small concrete objects, it is best to use direct copy to avoid cross dependency.

Sometimes cross dependency is unavoidable, and there is no clear ownership. For example, (m) A instances own (n) B instances, and certain B instances can be owned by multiple As. In this case, the best approach is to apply reference counting to B, in the way similar to COM reference counting. Any functions that take possession of B* must increase reference count first, and decrease it when releasing the possession.

I also avoid using boost::shared_ptr as it creates a new type (shared_ptr and B* become two distinct types). I found that it brings more headaches when I add methods.

Answer 2

You can also check the FFEAD Dependency Injection. It provides DI on the lines of Spring for JAVA and has a non-obtrusive way of dealing with things. It also has a lot of other important features like in-built AJAX Support,Reflection,Serialization,C++ Interpreter,Business Components For C++,ORM,Messaging,Web-Services,Thread-Pools and an Application Server that supports all these features.

Answer 3

I've recently been bitten by the DI bug. I think it solves a lot of complexity problems, especially the automated part. I've written a prototype which lets you use DI in a pretty C++ way, or at least I think so. You can take a look at the code example here: http://codepad.org/GpOujZ79

The things that are obviously missing: no scoping, no binding of interface to implementation. The latter is pretty easy to solve, the former, I've no idea.

I'd be grateful if anyone here has an opinion on the code.

Answer 4

In C++, normally, when you done things right, you don't need to write destructors at all in most cases. You should use smart pointers to delete things automatically. I think, builder don't looks like the owner of the ClassA and ClassB instances. If you don't like to use smart pointers, you should think about objects life time and their owners.

Answer 5

Things get complicated if you don't settle on the question of ownership once and for all. You will simply have to decide in your implementation if it's possible that dependencies live longer than the objects they are injected into.

Personally I'd say no: the object into which the dependency is injected will clean up afterwards. Trying to do it through the builder means that the builder will have to live longer than both the dependency and the object into which it is injected. This causes more problems than it solves, in my opinion, because the builder does not serve any more useful purpose after the construction with the dependency injection has been completed.

Answer 6

Use RAII.

Handing a raw pointer to someone is the same as handing them ownership. If that's not what you want to do, you should give them some kind of facade that also knows how to clean up the object in question.

shared_ptr<> can do this; the second argument of its constructor can be a function object that knows how to delete the object.