|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
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
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
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
should have. Since our idea of
Dog-ness was very
simple, there is just one trick we can ask
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.