Composition Over Inheritance

Clean Code
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Ben and Joe compare a number of code samples and compare solutions written using both inheritance and composition. Learn how to use composition to place an emphasis on objects and flexible runtime state.
This is a companion discussion topic for the original entry at https://thoughtbot.com/upcase/videos/composition-over-inheritance
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Great episode. 'Prefer Composition over Inheritance` is something that I hear often but usually neglect to apply it, I guess because my brain is wired with a more ā€˜inheritanceā€™-like style of thinking. Iā€™ll need to watch this a few more times to help it sink in.

A couple of questions:

  • Are there any situations where traditional inheritance still makes sense? Or is it better to always go with composition to provide for more flexibility as code grows over time?
  • And what about Ruby modules, should they generally treated as a code smell? How do you evaluate whether they are a good choice or not?
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@andyw8 Iā€™ve seen Sandi Metz (I recommend watching that whole video if you havenā€™t already) say that inheritance is good when a subclass basically overrides some methods and very little else, kind of Interface Segregation Principle related - as in donā€™t inherit from a super class just to get a tiny subset of behaviour thatā€™s not core to the purpose of that class, prefer composition in that case.

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By coincidence just I watched that just a few days ago. It was a very useful companion to the ideas presented in the episode.

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Hi,

Great discussion. I feel like there was a lot of agreement that we should almost always use composition over inheritance. Having said that I wanted to challenge that there are cases where one would use inheritance/include.

For example when creating an API wrapper it seems to be common practice to include all functionality required in a client via includes. I thought maybe there is a way to leverage composition, but I havenā€™t been able to come up or find any examples. I also checked what APIs thoughtbot has written and found that they have worked on the Yam Gem which has been contributed to by thoughtbot and I wanted to find out why is it ok to use includes in the client library? This seems to contradict what is said in the discussion.

Any help would be appreciated.

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I guess you mean yam/client.rb at 11eb501363b4dc1e3de4117066e8c1e2f2842503 Ā· yammer/yam Ā· GitHub ?

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Yes, sorry I thought I included that link. That is what I am referring to.

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I really enjoyed this video. Thank you @r00k and @jferris for putting together a great explanation of composition and why it supports more flexible and reliable systems than inheritance.

A lot Rails consists of inheritance. I was wondering if either of you, @r00k or @jferris, could comment on software like, Rails. Here are some questions I have for you both:

  1. Do you two think that Railsā€™s use of inheritance is a poor design choice?
  2. Or is there something about the type of software that Rails is ā€“ a web framework ā€“ which makes it a strong and practical candidate for inheritance?
  3. If Railsā€™s use of inheritance is a good design choice, then do we have a use case where we choose Inheritance Over Composition?

Also, the above questions are not just for Ben and Joe. Feel free to chime in anyone and everyone.

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Do you think Composition over inheritance contradicts Convention over configuration?