From: Robert Klemme <shortcutter@...>
Date: 2009-08-12T05:10:13+09:00
Subject: Re: Which initialize method? (or, who is self?)

On 11.08.2009 21:23, Daniel Waite wrote:
> Hi. Consider the ruby-hmac gem:
> http://pastie.org/580360
> 
> Line 81 makes the following call:
> hmac = self.new(key)
> 
> But, insofar as I understand Ruby's wonderful world context switching,
> self, in the instance in which it is called, is both unnecessary

On first inspection, yes.  But not if you look closer:

> (because the method in which it is being called is already part of the
> class) and incorrect (because the only initialize method available
> requires more than one argument).

The method invoked is #new and not #initialize!  This means that 
argument lists of new and initialize need not be identical.  You can 
create a class whose Method.new has different arguments than #initialize:

class Foo
   def self.new(x)
     o = allocate
     o.send(:initialize, x, 1, 2)
     o
   end

   def initialize(a, b, c)
     # ...
   end
end

In this case it works differently though:

> The file works (in some instances...), so I'll assume my understanding
> of Ruby is still poor. Which sucks because I thought I knew it fairly
> well.
> 
> I've tried printing out self in each new context, and I get the expected
> result (HMAC::Base). Except, that is, inside a Rails application running
> on Ruby 1.9.1, in which case I get HMAC::SHA1 within the Base.digest
> method. Awesome.
> 
> Any ideas?

The fact that the class is called "Base" indicates that it is intended 
for inheritance.  I don't have a Ruby interpreter at hand right now to 
cook up a tested example, but considering inheritance and the fact that 
class methods are inherited by subclasses the whole thing actually makes 
sense:

class SHA1 < Base
   def initialize(key)
     super(SHA1, 1024, 4096, key)
   end
end

Now line 99 makes sense in combination with 81 / 91: Base.new is private 
which will make self.new() trigger an exception and thus prevent 
invocation of Base.digest() and Base.hexdigest().  But, you can invoke 
SHA1.digest() and it will work.

Arguments algorithm, block_size and output_length to Base.initialize are 
  actually class dependent, i.e. every subclass likely defines their own 
values.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/