Is Inheritance really needed?

Question

I must confess I\'m somewhat of an OOP skeptic. Bad pedagogical and laboral experiences with object orientation didn\'t help. So I converted into a fervent believer in Visua

Answer 1

//I found this QA very useful. Many have answered this right. But i wanted to add...

1: Ability to define abstract interface - E.g., for plugin developers. Of course, you can use function pointers, but this is better and simpler.

2: Inheritance helps model types very close to their actual relationships. Sometimes a lot of errors get caught at compile time, because you have the right type hierarchy. For instance, shape <-- triangle (lets say there is a lot of code to be reused). You might want to compose triangle with a shape object, but shape is an incomplete type. Inserting dummy implementations like double getArea() {return -1;} will do, but you are opening up room for error. That return -1 can get executed some day!

3: void func(B* b); ... func(new D()); Implicit type conversion gives a great notational convenience since Derived is Base. I remember having read Straustrup saying that he wanted to make classes first class citizens just like fundamental data types (hence overloading operators etc). Implicit conversion from Derived to Base, behaves just like an implicit conversion from a data type to broader compatible one (short to int).

Answer 2

I think asking for situations where inheritance is really needed is missing the point a bit. You can fake inheritance by using an interface and some composition. This doesnt mean inheritance is useless. You can do anything you did in VB6 in assembly code with some extra typing, that doesn't mean VB6 was useless.

I usually just start using an interface. Sometimes I notice I actually want to inherit behaviour. That usually means I need a base class. It's that simple.

Answer 3

Inheritance is an implementation decision. Interfaces almost always represent a better design, and should usually be used in an external API.

Why write a lot of boilerplate code forwarding method calls to a composed member object when the compiler will do it for you with inheritance?

This answer to another question summarises my thinking pretty well.

Answer 4

Thanks to all for your answers. I maintain my position that, strictly speaking, inheritance isn't needed, though I believe I found a new appreciation for this feature.

Something else: In my job experience, I have found inheritance leads to simpler, clearer designs when it's brought in late in the project, after it's noticed a lot of the classes have much commonality and you create a base class. In projects where a grand-schema was created from the very beginning, with a lot of classes in an inheritance hierarchy, refactoring is usually painful and dificult.

Seeing some answers mentioning something similar makes me wonder if this might not be exactly how inheritance's supposed to be used: ex post facto. Reminds me of Stepanov's quote: "you don't start with axioms, you end up with axioms after you have a bunch of related proofs". He's a mathematician, so he ought to know something.

Answer 5

In the following, inheritance is used to present a particular property for all of several specific incarnations of the same type thing. In this case, the GeneralPresenation has a properties that are relevant to all "presentation" (the data passed to an MVC view). The Master Page is the only thing using it and expects a GeneralPresentation, though the specific views expect more info, tailored to their needs.

   public abstract class GeneralPresentation
    {
        public GeneralPresentation()
        {
            MenuPages = new List<Page>();
        }
        public IEnumerable<Page> MenuPages { get; set; }
        public string Title { get; set; }
    }

    public class IndexPresentation : GeneralPresentation
    {
        public IndexPresentation() { IndexPage = new Page(); }
        public Page IndexPage { get; set; }
    }

    public class InsertPresentation : GeneralPresentation
    {
        public InsertPresentation() { 
          InsertPage = new Page(); 
          ValidationInfo = new PageValidationInfo(); 
        }
        public PageValidationInfo ValidationInfo { get; set; }
        public Page InsertPage { get; set; }
    }

Answer 6

Depends on your definition of "needed". No, there is nothing that is impossible to do without inheritance, although the alternative may require more verbose code, or a major rewrite of your application.

But there are definitely cases where inheritance is useful. As you say, composition plus interfaces together cover almost all cases, but what if I want to supply a default behavior? An interface can't do that. A base class can. Sometimes, what you want to do is really just override individual methods. Not reimplement the class from scratch (as with an interface), but just change one aspect of it. or you may not want all members of the class to be overridable. Perhaps you have only one or two member methods you want the user to override, and the rest, which calls these (and performs validation and other important tasks before and after the user-overridden methods) are specified once and for all in the base class, and can not be overridden.

Inheritance is often used as a crutch by people who are too obsessed with Java's narrow definition of (and obsession with) OOP though, and in most cases I agree, it's the wrong solution, as if the deeper your class hierarchy, the better your software.