Classes

Ruby user's guide

Classes

The real world is filled by objects, and we can classify them. For example, a very small child is likely to say "bow-wow" when seeing a dog, regardless of the breed; we naturally see the world in terms of these categories.

In OO programming terminology, a category of objects like "dog" is called a class, and some specific object belonging to a class is called an instance of that class.

Generally, to make an object in ruby or any other OO language, first one defines the characteristics of a class, then creates an instance. To illustrate the process, let's first define a simple Dog class.

ruby> class Dog
    |   def speak
    |     print "Bow Wow\n"
    |   end
    | end
   nil

In ruby, a class definition is a region of code between the keywords class and end. A def inside this region begins the definition of a method of the class, which as we discussed in the previous chapter, corresponds to some specific behavior for objects of that class.

Now that we have defined a Dog class, we can use it to make a dog:

ruby> pochi = Dog.new
   #<Dog:0xbcb90>

We have made a new instance of the class Dog, and have given it the name pochi. The new method of any class makes a new instance. Because pochi is a Dog according to our class definition, it has whatever properties we decided a Dog should have. Since our idea of Dog-ness was very simple, there is just one trick we can ask pochi to do.

ruby> pochi.speak
Bow Wow
   nil

Making a new instance of a class is sometimes called instantiating that class. We need to have a dog before we can experience the pleasure of its conversation; we can't merely ask the Dog class to bark for us.

ruby> Dog.speak
ERR: (eval):1: undefined method `speak' for Dog:class

It makes no more sense than trying to eat the concept of a sandwich.

On the other hand, if we want to hear the sound of a dog without getting emotionally attached, we can create (instantiate) an ephemeral, temporary dog, and coax a little noise out of it before it disappears.

ruby> (Dog.new).speak   # or more commonly, Dog.new.speak
Bow Wow
   nil

"Wait," you say, "what's all this about the poor fellow disappearing afterwards?" It's true: if we don't bother to give it a name (as we did for pochi), ruby's automatic garbage collection decides it is an unwanted stray dog, and mercilessly disposes of it. Really it's okay, you know, because we can make all the dogs we want.

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