Re: Class variables

On Tue, Sep 18, 2012 at 12:07 PM, Aleksander Ciesielski <[e-mail]> wrote:
> Is it obligatory to use instance variables in classes? Can't we just write
> something like this:
>
> class Test
>     var = 5
>
>     def show
>         puts var
>     end
> end
>
> test = Test.new
> test.show
> p test.var

This doesn't work. The keyword def starts a new scope, so the "var"
inside "show" is a different local variable than the var in the scope
of the class. Even if this weren't the case, this would resemble more
a class instance variable than an instance variable, since all
instances of the class would have the same value for "var". By the
way, you can achieve this using "define_method" instead of "def",
since define_method is a closure and can see the surrounding scope:

1.9.2p290 :012 > class Test
1.9.2p290 :013?>   var = 5
1.9.2p290 :014?>   define_method :show do
1.9.2p290 :015 >       puts var
1.9.2p290 :016?>     end
1.9.2p290 :017?>   end
 => #<Proc:0x00000000b54938@(irb):14 (lambda)>
1.9.2p290 :018 > Test.new.show
5
 => nil

But as I said, all instances see the same value:


1.9.2p290 :019 > a = Test.new
 => #<Test:0x00000000b42f08>
1.9.2p290 :020 > b = Test.new
 => #<Test:0x00000000b3ac90>
1.9.2p290 :021 > a.show
5
 => nil
1.9.2p290 :022 > b.show
5
 => nil

and you cannot change it on a per instance case. That's what instance
variables are for.
The last part

> p test.var

can never work, since you don't define a method called var.

Jesus.