From: "David A. Black" <dblack@...>
Date: 2007-11-01T05:41:24+09:00
Subject: Re: why, or when to, use a class method?

Hi --

On Thu, 1 Nov 2007, Thufir wrote:

> Because the definition is declared with "def
> SongList.isTooLong(aSong)", this indicates it to be a class variable
> for the SongList class, ok so far.

It declares it to be a class *method*. It may or may not use a class
variable (actually the example you gave does not).

> Why a class variable?  All
> SongList objects will use the same method, but wouldn't they use the
> same method even if it were an instance method?  Methods are declared
> on the class, not the instance.
>
> There is one @plays per instance, but only one @@plays which all Song
> objects share; this I follow.
>
> Why use a class method in the first example, though?

Typically, a class method is used when you have a method that's
directly related to the objects you're using the class to represent,
but that does something at a more general level than a specific
object.

For example, let's say you have a Car class. You want to create
individual cars:

   c = Car.new

and do things with them:

   c.fill_with_fuel
   c.drive(120)

However, you might also want to do things at a more general level that
still have to do with cars. For example:

   class Car
     def self.brands
       ["Ford", "Honda", "Toyota", "Volvo" ...]
     end
   end

This provides a list of all car brands. I've made it a class method
because while every individual car may be of a particular brand, I
don't expect every car to know about every brand. The Car class,
however, is a good place to put general, high-level information about
cars, like a list of all brands.


David

-- 
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