I know the question has been discussed before, but it seems always under the assumption that inheritance is at least sometimes preferable to composition. I’d like to challenge that assumption in hopes of gaining some understanding.
My question is this: Since you can accomplish anything with object composition that you can with classical inheritance and since classical inheritance is very often abused[1] and since object composition gives you flexibility to change the delegate object runtime, why the would you ever use classical inheritance?
I can sort of understand why you would recommend inheritance in some languages like Java and C++ that do not offer convenient syntax for delegation. In these languages you can save a lot of typing by using inheritance whenever it is not clearly incorrect to do so. But other languages like Objective C and Ruby offer both classical inheritance and very convienient syntax for delegation. The Go programming language is the only langage that to my knowledge has decided that classical inheritance is more trouble than it’s worth and supports only delegation for code reuse.
Another way to state my question is this: Even if you know that classical inheritance is not incorrect to implement a certain model, is that reason enough to use it instead of composition?
[1] Many people use classical inheritance to achieve polymorphism instead of letting their classes implement an interface. The purpose of inheritance is code reuse, not polymorphism. Furthermore, some people use inheritance to model their intuitive understanding of an “is-a” relationship which can often be problematic.
Update
I just want to clarify what I mean exactly when I talk about inheritance:
I am talking about the kind of inheritance whereby a class inherits from a partially or fully implemented base class. I am not talking about inheriting from a purely abstract base class which amounts to the same thing as implementing an interface, which I for the record am not arguing against.
Update 2
I understand that inheritance is the only way to achieve polymorphism i C++. In that case it’s obvious why you must use it. So my question is limited to languages such as Java or Ruby that offer distinct ways to achieve polymorphism (interfaces and duck typing, respectively).
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Answer
If you delegate everything that you haven’t explicitly overridden to some other object implementing the same interface (the “base” object), then you’ve basically Greenspunned inheritance on top of composition, but (in most languages) with a lot more verbosity and boilerplate. The purpose of using composition instead of inheritance is so that you can only delegate the behaviors you want to delegate.
If you want the object to use all the behavior of the base class unless explicitly overridden, then inheritance is the simplest, least verbose, most straightforward way to express it.