Inheritance: is-a has-a

Lots of things we learn in school turn out to be naive simplifications of how the real world works, and sometimes we later learn, to our chagrin, that the way we thought about the world really isn’t true at all. Take that familiar organization of life into a giant tree: kingdom, phylum, genus, species. It seems neat enough, but in the grown-up world, people can spend lifetimes arguing about where things fit in this classification.

A related simplification that I learned in school was the rule of when to use inheritance versus composition. It went like so: in this assignment, you simulate a world full of monsters. Zombie is a type of monster, so zombie should inherit from monster. On the other hand, vampires have a coffin, so vampire should have a field that refers to coffin. Now make a UML diagram.

This makes sense as far as it goes, but there’s a major problem: it’s not usually a useful way to think about inheritance when building real programs.

Is-a v has-a perspective makes most sense when thinking about type systems. If a function takes an argument of type monster, it can also take any type of monster, either vampire or zombie. The trouble starts when you use the same reasoning to design a program and it comes back to our taxonomy problem.

You start designing a system by figuring out what your different things are: zombies, vampires, ghosts, coffins and so on. It’s easy enough: three types of monsters, each a class that inherits from monster, and coffin, its own thing. Naturally, you also need people; people need places to live and ghosts need places to haunt, so you have houses. But wait, people aren’t monsters, but they have a lot in common, so they need a base class, say living things. But that’s not quite right; the monsters aren’t technically alive, so maybe they are dead things. Also, houses and coffins seem to be of a non-living type, so that’s another base class. Should it be dead things? If the coffin is made of wood, it used be alive, so maybe that makes sense.

Most real-world characteristics of things are completely irrelevant to most programs. In our simulation, perhaps the only thing ghosts do is haunt, whereas vampires and zombies bite people but don’t haunt. It’s confusing and wasteful to worry about how they are all types of monsters, who are types of dead things and so forth.

Now, occasionally, it does make sense to think of inheritance as an is-a relationship. The cf0x10 parse tree, for example, is a pile of subclasses. When this type of design makes sense, however, it will be obvious; no need to shoehorn everything into it.

What about other metaphors? It’s common, for example, to say that instances of classes are receivers while method calls are messages to that receiver. That’s a useful perspective for language design and it’s useful to have a name for that bit before the dot – receiver.message() – but, again it’s not so helpful a metaphor when designing a program.

In real programs, metaphors like these just tend to cause trouble. Software isn’t made of physical things. A class, in reality, is just a way to group related bits of a program. I prefer not to start by creating any design for a class hierarchy; instead I write code that does the things I need it to do. A class hierarchy, if any, usually emerges from unifying the bits that make sense to put together.

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