From: Ralph Shnelvar <ralphs@...32.com>
Date: 2009-11-30T06:02:09+09:00
Subject: Re: semantics of @x v. $x

SC> On Sunday 29 November 2009, Ralph Shnelvar wrote:
>> |Newbie here:
>> |
>> |Consider
>> |irb(main):001:0> @xyzzy = 5
>> |=> 5
>> |irb(main):002:0> defined? @xyzzy
>> |=> "instance-variable"
>> |irb(main):003:0> $xyzzy = 5
>> |=> 5
>> |irb(main):004:0> defined? $xyzzy
>> |
>> |
>> |What, semantically, is the difference between $x and @x when at top
>> |level?  (Am I at top level?)
>> |
>> |Do $xyzzy and @xyzzy have global scope at statement 5 and beyond?
>> |

SC> @xyzzy = 5 defines an instance variable for the global object. That instance
SC> variable is accessible only from the top level. $xyzzy = 5, instead, defines a
SC> global variable, which can be accessed from everywhere.

SC> Here's an example:

SC> @x = 1
SC> $y = 2

SC> class C
SC>   
SC>   def test
SC>     p defined?(@x)
SC>     p defined?($y)
SC>     p @x
SC>     p $y
SC>   end
SC>   
SC> end

SC> C.new.test

SC> The output is:

SC> nil
SC> "global-variable"
SC> nil
SC> 2

SC> This shows that, while $y can be accessed even from instances of the C class,
SC> @x is accessible only from top level, that is only from the main object.

SC> By the way, I suspect you forgot to paste a last line of output in your
SC> example. defined?($xyzzy) should have given you "global-variable".

SC> I hope this helps

SC> Stefano

I thank you greatly for you explanation and your time.

Is there a document that you know of that describes these scoping
rules for beginners?