User Guide #12 - Ruby - classes

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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

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