From: Robert Klemme <bob.news@...>
Date: 2004-02-26T20:04:53+09:00
Subject: Re: Why don't $global and @instance variables need declaration?


"David Garamond" <lists@zara.6.isreserved.com> schrieb im Newsbeitrag
news:403DC272.10009@zara.6.isreserved.com...
> Sorry if the answer is obvious, but I can't find a satisfactory or more
> verbose explanation about this. The only thing I found is from the
> Ruby's User Guide:
>
> "As with globals, instance variables have the nil value until they are
> initialized. Instance variables of ruby do not need declaration. This
> implies a flexible structure of objects. In fact, each instance variable
> is dynamically appended to an object when it is first referenced."
>
> Could someone explain the above? How is it more flexible?

You don't need the declaration.  That saves typing and also you can add
them at runtime as you like:

irb(main):003:0> class Foo
irb(main):004:1>   def set(name, val)
irb(main):005:2>     instance_eval "#{name} = val"
irb(main):006:2>   end
irb(main):007:1>
irb(main):008:1*   def get(name)
irb(main):009:2>     instance_eval name
irb(main):010:2>   end
irb(main):011:1> end
=> nil
irb(main):012:0>
irb(main):013:0* f=Foo.new
=> #<Foo:0x101724a0>
irb(main):014:0> f.set "@foo", "bar"
=> "bar"
irb(main):015:0> f.get "@foo"
=> "bar"
irb(main):016:0> f.set "@hello", "world"
=> "world"
irb(main):017:0> f.instance_variables
=> ["@hello", "@foo"]
irb(main):018:0>

If you would need to declare globals then you either need to include a
declaration for each global you need or you had to add code in each place
that would do the declaration on the fly when needed.  That's tedious and
error prone.

> Also, how do
> we differentiate between an unitialized global/instance variable and a
> variable that has explicitly been set to 'nil'?

Typically you don't.  There are certain situations where you can use
"defined?" but these are rare.  AFAIK it's seldom used.  The more usable
option is to use "global_variables":

irb(main):002:0> puts global_variables
$-w
$stderr
$LOAD_PATH
$<
$-0
$+
$-p
$$
$-i
$deferr
$~
$!
$-v
$stdout
$_
$-K
$-F
$SAFE
$*
$-I
$/
$'
$-d
$FILENAME
$defout
$@
$VERBOSE
$stdin
$.
$KCODE
$;
$binding
$0
$:
$,
$`
$DEBUG
$>
$-a
$"
$\
$=
$-l
$?
$&
=> nil
irb(main):003:0>

Regards

    robert