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Asked: 2026-06-16T04:17:41+00:00

I have a simple question, and I’m not even sure it has an answer

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I have a simple question, and I’m not even sure it has an answer but let’s try.
I’m coding in C++, and using dependency injection to avoid global state. This works quite well, and I don’t run in unexpected/undefined behaviours very often.

However I realise that, as my project grows I’m writing a lot of code which I consider boilerplate. Worse : the fact there is more boilerplate code, than actual code makes it sometimes hard to understand.

Nothing beats a good example so let’s go :

I have a class called TimeFactory which creates Time objects.

For more details (not sure it’s relevant) : Time objects are quite complex because the Time can have different formats, and conversion between them is neither linear, nor straightforward. Each “Time” contains a Synchronizer to handle conversions, and to make sure they have the same, properly initialized, synchronizer, I use a TimeFactory. The TimeFactory has only one instance and is application wide, so it would qualify for singleton but, because it’s mutable, I don’t want to make it a singleton

In my app, a lot of classes need to create Time objects. Sometimes those classes are deeply nested.

Let’s say I have a class A which contains instances of class B, and so on up to class D. Class D need to create Time objects.

In my naive implementation, I pass the TimeFactory to the constructor of class A, which passes it to the constructor of class B and so on until class D.

Now, imagine I have a couple of classes like TimeFactory and a couple of class hierarchies like the one above : I loose all the flexibility and readability I’m suppose to get using dependency injection.

I’m starting to wonder if there isn’t a major design flaw in my app …
Or is this a necessary evil of using dependency injection ?

What do you think ?

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1 Answer

  1. Editorial Team

    In my naive implementation, I pass the TimeFactory to the constructor
    of class A, which passes it to the constructor of class B and so on
    until class D.

    This is a common misapplication of dependency injection. Unless class A directly uses the TimeFactory, it should not ever see, know about, or have access to the TimeFactory. The D instance should be constructed with the TimeFactory. Then the C instance should be constructed with the D instance you just constructed. Then the B with the C, and finally the A with the B. Now you have an A instance which, indirectly, owns a D instance with access to a TimeFactory, and the A instance never saw the TimeFactory directly passed to it.

    Miško Hevery talks about this in this video.

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