Why use inheritance at all?

Asked 2010-07-28T09:48:44.690

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 ab

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Every one knows polymorphism is a great advantage of inheritance. Another Benefit which i find in inheritance is that helps to create replica of real world. For example in pay roll system we deal managers developers ,office boys etc. if we inherit all these class with super class Employee. it makes our program more understandable in context of real world that all these classes are basically employees. And one more thing classes not only contain methods they also contain attributes. So if we contain attributes generic to employee in the Employee class like social security number age etc. it would provide greater code reuse and conceptual clarity and of course polymorphism. However while using inheritance things we should keep in mind is the basic design principle "Identify the aspects of your application that vary and separate them from that aspects which change". You should never implement those aspect of application that change by inheritance instead use composition. And for those aspects which are not changeable u should use inheritance ofcourse if an obvious "is a" relation lies.

answered 2012-11-25T15:44:07.180

Inheritance is to be preferred if:

You need to expose the whole API of the class you extend (with delegation, you will need to write lots of delegating methods) and your language doesn't offer a simple way to say "delegate all unknown methods to".
You need to access protected fields/methods for languages that have no concept of "friends"
The advantages of delegation are somewhat reduced if your language allows multi-inheritance
You usually have no need delegation at all if your language allows to dynamically inherit from a class or even an instance at runtime. You don't need it at all if you can control which methods are exposed (and how they are exposed) at the same time.

My conclusion: Delegation is a workaround for a bug in a programming language.

answered 2010-07-28T09:56:16.233

Why use inheritance at all?

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