I’ve recently discovered a new trend in object-oriented programming. These days, some people are saying that when writing a class, we should always prefer to use composition over inheritance. This means that whenever we feel tempted to inherit from another class, we should instead make that class an instance variable of our new class and write wrapper methods for the methods that we need to import into our new class. If you need polymorphic behavior, use interfaces to fit your static types into polymorphic variables.
When we design an inheritance hierarchy, we have to make a lot of decisions about the relationships between types that will be hard to change later. Composition requires a bit more code, but it is much more flexible because each class can independently decide how it uses other classes.
What if my language doesn’t have interfaces?
C++ and Python do not have a built-in concept of interfaces. This is because they both allow multiple inheritance, and abstract classes can be written to bahave as interfaces. When using these languages under a composition paradigm, you may feel free to inherit from abstract classes that do not have any instance variables.
A few years ago, I dicovered the language Go. Go does not have inheritance, and it takes a unique approach to composition.
In the above code, type
B embeds type
A. This means we don’t have to
write wrapper methods to access fields or methods of
B. The name of
an embedded field is simply the type name. For example,
myB.A.x == myB.x.
Furthermore, if we call
myB.getX(), the reciever of
getX is the
myB, so the method cannot affect or be affected by other fields in the
I enjoyed using Go for a few years before I found out about the trend toward composition in other OO languages. Before I found Go, I thought that polymorphism was the same thing as inheritance. Go taught me that polymorphism (embodied by interfaces) is a completely independent feature that works perfectly well without inheritance, at least in Go. I didn’t realize until recently that this principal applies just as well to other languages.