From: masta Blasta <lists@...>
Date: 2013-02-22T05:47:58+09:00
Subject: Re: How meta class differs from real class ?

Xavier R. wrote in post #1098080:
> In Ruby-
>
> Could anyone help me to understand how meta class differs from real
> class ?
>
> Examples with discussion could be better to understand the core of the
> concept

Not sure what you mean by "real class". Everything in Ruby is an object. 
All objects inherit from BasicObject. Classes in ruby are instances of 
the Class object, meaning:
MyClass = Class.new


Instance methods are on an instance of a class.

Class methods, singleton methods of the eigenclass (metaclass), are
called on the class constant.

A good example is the Math module (Class is a subclass of Module). The
Math module provides a bunch of utility methods, that wouldn't be as
useful if they had to be called on instances of an object.

you can easily do
x = Math.sqrt(144)

otherwise you'd have to open up the Fixnum class and add a sqrt method,
in order to
x = 144.sqrt

but that would only be on Fixnum. What if the number was a Float 144.0, 
or a Bignum ?

Another example,
Let's say we have a class Student.
stu = Student.new #(btw 'new' is a singleton method)
some logical instance methods might be calculate_GPA, show_schedule etc.
stu.calculate_GPA

But what if we wanted to know the total average GPA of all our students?
Calling stu.average_GPA does not make sense, because we're calling it on
a specific student. It makes the code confusing.

Instead we should have that method on the Eigenclass
def Student.average_GPA; ..calculate GPA from all student records; end

so then Student.average_GPA() would give us the average GPA amongst all
our students, and it is namespaced in a way that is logical.

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