From: Robert Klemme <shortcutter@...>
Date: 2006-10-24T05:30:18+09:00
Subject: Re: File::open and File.open

Brad Tilley wrote:
> What exactly is the difference between these? :: is a class method while 
>  is an instance method??? If that is so, could someone clarify this 
> with more detail? I use File.open, but File::open works too.

The same method (a class method) is called.  It is just a syntactic 
difference.  So these are the same

File.open("foo") ...
File::open("foo") ...

 > However
> sometimes, this is not the case which adds to my confusion :) For 
> example, I can only do Digest::MD5.new() and not Digest.MD5.new() why is 
> that?

Because the "." cannot be used for constant lookup:

irb(main):005:0> File.NONBLOCK
NoMethodError: undefined method `NONBLOCK' for File:Class
         from (irb):5
         from :0
irb(main):006:0> File::NONBLOCK
=> 16384

"." is reserved for method invocations.

> I don't fully understand OO programming... perhaps this is my problem.

Nah, I think it is rather a minor ambiguity in the language that causes 
confusion.  Usually people disambiguate it by using "." for methods and 
"::" for constants only although they could use it for class methods as 
well.

Kind regards

	robert