From: Anurag Priyam <anurag08priyam@...>
Date: 2011-02-23T23:09:48+09:00
Subject: Re: Calling class method from instance method

> You see, there's stuff that need to be donne in both cases, in instances
> and in classes. For example:
>
> class SomeClass
>  def self.method
>    if file_exists?
>      #do stuff
>    end
>    #something need to be donne here, and I need to check if the file
> exists
>    #I dont want to define a nother class method exactly the same as the
> already declared instance method.
>  end

Since file_exists? is an instance method, it is very likely to return
true/false based on the state of the instance (see below). Calling
file_exists in self.method does not make sense unless its
responsibility is to act on an instance of your class - like a
factory, or processing an instance passed to it as a parameter.

def self.method(object)
  if object.file_exists?
    # do something on that object
    object.statistics
  end
end

or,

def self.create(*args)
  object = SomeClass.new(*args)
  return object if object.file_exists?
end

>  def file_exists?
>    #there are instance methods depending on this one
>    File.exists?("some_file_name.txt")
>  end

Umm, is the file name hard coded, or does it depend on the state of
the instance, like:

def file_exists?
  File.exists?(@file)
end

If it is hard coded, or depends on a constant, you might as well
define a class method:

class SomeClass
  CONFIG = "~/.foo.rc"

  def self.config_exists?
    File.exists?(CONFIG)
  end

  def init
    raise "Config file not found" unless SomeClass.config_exists?
    ...
  end
end

> end

> How do you deal with this kind of thing?

Depends on the context.

-- 
Anurag Priyam
http://about.me/yeban/