From: Dan Doel <djd15@...>
Date: 2003-08-22T13:34:22+09:00
Subject: Re: Class variables - a surprising result

Patrick Bennett wrote:

> Ryan Pavlik wrote:
>
>> Basically, in Ruby, you _do_ have what C++/Java would call a "static
>> class member", and you also have something else---@@class variables.
>> It's mostly the term here that's confusing, I think.
>>
> No, @variables are like normal instance variables in C++.  They 
> *definitely* aren't like C++ 'static's. @@variables aren't like C++ 
> statics either, but that's not necessarily a *bad* thing - it's 
> just... different ;) 


@variables are only like C++ instance variables when used within 
instance methods.  If you do:

class Foo
    @var = 6
   
    def Foo.print
       p @var
    end

    def print
       p @var
    end
end

Then:

Foo.print # => 6
Foo.new.print # => nil

If you want to access these in instance methods, you need to do:

class Foo
    def instance_meth
       p self.class.class_eval { @var }
    end
end

You cannot do:

class Foo
    def instance_meth
       p class << self; @var; end
    end
end

Because @var is an instance variable in the Foo class object, not in the 
singleton class object
of an instance of Foo (or maybe there's some other explanation I can't 
come up with).

So it is effectively a class instance variable, although that's still 
not the same as a Java static
variable, since it's not inherited.

- Dan