Ruby - 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.
