15.1 The Object-Oriented Programming Paradigm
We started out with Procedural Programming, which draws a distinction between data and procedures. Data provides our information, and procedures solve problems by manipulating data into new forms.
Functional Programming puts functions at the center of attention. As much as possible, procedures are wrapped up in function calls, and by their status as first-class citizens (data) functions become king. Everything that happens is a result of function calls on data–where the data can include other functions.
You could say that Object-Oriented Programming reverses the point of view, attempting to put objects–conceived as complex forms of data–at the center of attention.
There are two major types of Object-Oriented (OO) Programming:
- Message-Passing OO
- Generic-Function OO
Message-Passing OO is the type of OO Programming that programmers usually associate with the term “object-oriented”, and since it is the type that you will meet most frequently in other languages that support OO-programming we will discuss it first. Generic-Function OO is actually the older type of OO Programming. Although it is very important for understanding some aspects of how R works, it is met with less frequently in other major programming languages, so we will defer discussion of it until Section 15.7. Accordingly, the following introductory remarks pertain only to message-passing OO.
Objects in the world about us are indeed complex entities: they have particular features. Some of these features they have in virtue of the kind of object that they are, and others are features–attributes–that they have on their own, as individuals. And they can do various things. Again, some of the things they do are things that they do simply because of the kind of thing they are, and others things they do might be unique to them.
Consider a typical object: my dog, for example. He has various attributes: his name is Beau, he has four legs, he weighs about 50 pounds, his hair is black. And he does various things—he has various methods, let’s say, for making his way through life. For example, when we are out of the house he lies in the sofa, when we are home he lies on the floor. He barks. He eats. He annoys cats.
Some of his attributes and methods Beau has simply in virtue of being the kind of thing that he is. He is, after all, a dog—a member of the class Dog, let’s say—and so like all other dogs he will have four legs and he will be able to bark and to annoy cats. Of course all dogs are animals—members of the more general class Animal, let’s say—and all animals eat, so as an animal Beau inherits the method known as eating. Of course all animals are material beings—members of the even-more general class Being, let’s say—and all beings have a weight, and thus Beau has a weight, but the particular value of that weight—50 pounds—is determined by the way he has lived his life thus far. Other dogs, animals and material beings have a weight, but they won’t necessarily have the same weight as Beau.
What emerges from this discussion is an (admittedly oversimplified) view of the world as a collection of complex objects. These objects are related to one another in systematic ways–related closely when they are, for instance, both Dogs, and more distantly, as when one is a Dog and another is a Cat. And these objects can act on themselves and on each other in various ways, through their methods.
The idea of Object-Oriented Programming is that since we are so used to viewing the world as a collection of objects that are constituted of various attributes and methods and that are related to one another through membership in classes of varying levels of generality, then it might help us to be able to view a computer program similarly—as a collection of objects thus constituted and thus related to one another.
Like many other modern programming languages, R provides considerable support for the Object-Oriented paradigm. In fact it provides that support in two different formats:
- Reference Classes
- R6 Classes
Reference Classes are a relatively recent introduction to R. R6 Classes are a simple and lightweight version of Reference Classes. They are not a part of the core of R—they are enabled by a contributed R-package—but they are quite useful as an introduction to object-oriented programming. Accordingly, we will work with them in this Chapter.