Question regarding interaction between modules and classes

Ruby
1

Hi,

In the code below, is there a way to learn @foo from $bar_one
using $bar_two?

Many thanks in advance!!

#!/usr/bin/ruby -w

module Foo
def setup
@foo = self.object_id
end
def foo
return @foo
end
end

class Bar
include Foo
end

$bar_one = Bar.new
$bar_one.setup
p $bar_one.foo # => 2148224640

$bar_two = Bar.new
p $bar_two.foo # => warning: instance variable @foo not initialized

2

On Sat, May 7, 2011 at 10:02 PM, Geometric Patterns <
[email protected]> wrote:

#!/usr/bin/ruby -w
class Bar

–
Posted via http://www.ruby-forum.com/.

Hmm. Okay, first a bunch of thoughts, then some potential solutions.

  1. Variables that begin with the $ sigil are global. For a local
    variable,
    no sigil necessary:
    module Foo
    def setup
    @foo = self.object_id
    end
    def foo
    return @foo
    end
    end
    class Bar
    include Foo
    end
    bar_one = Bar.new
    bar_one.setup
    bar_one.foo # => 2151888860

  2. There is an implicit setup method called “initialize”, it is unusual
    to
    me that you have your own, which you invoke explicitly.
    module Foo
    def initialize
    @foo = self.object_id
    end
    def foo
    return @foo
    end
    end
    class Bar
    include Foo
    end
    Bar.new.foo # => 2151889080

  3. When invoking a method on the current object, self is not necessary
    module Foo
    def initialize
    @foo = object_id
    end
    def foo
    return @foo
    end
    end
    class Bar
    include Foo
    end
    Bar.new.foo # => 2151889080

  4. The last line of a method is automatically returned, no explicit
    “return”
    is necessary
    module Foo
    def initialize
    @foo = object_id
    end
    def foo
    @foo
    end
    end
    class Bar
    include Foo
    end
    Bar.new.foo # => 2151889100

  5. When you have a method which returns an instance variable of the same
    name, that is a less efficient version of “attr_reader :ivar_name”
    module Foo
    def initialize
    @foo = object_id
    end
    attr_reader :foo
    end
    class Bar
    include Foo
    end
    Bar.new.foo # => 2151889260

Okay, now that we’re here, I assume you want to know if bar_two can know
bar_one’s foo
bar_one = Bar.new
bar_two = Bar.new
bar_one.foo # => 2151886760
bar_two.foo # => 2151886740

But it is unclear, because you were initially using global variables,
which
breaks scope, allowing a method like
module Foo
def initialize
@foo = object_id
end
attr_reader :foo
def bar_ones_foo
$bar_one.foo
end
end
class Bar
include Foo
end
$bar_one = Bar.new
$bar_two = Bar.new
$bar_one.foo # => 2151884160
$bar_two.foo # => 2151884140
$bar_two.bar_ones_foo # => 2151884160

Which, seems very strange to me, and doesn’t work if you actually meant
to
have local variables (globals are considered a code smell).

Alternatively, you could have been asking whether they could share the
@foo
ivar, in which case the answer is no, that ivar is unique to whichever
object the method is being called on. However, you could use a class
variable

module Foo
def initialize
@@foo = object_id
end
def foo
@@foo
end
end
class Bar
include Foo
end
bar_one = Bar.new
bar_one.foo # => 2151892400
bar_two = Bar.new
bar_two.foo # => 2151891900
bar_one.foo # => 2151891900
Foo.class_variables # => [:@@foo]

Or, more commonly, an instance variable either on Foo or on Bar
module Foo
class << self
attr_accessor :foo
end
def initialize
Foo.foo = object_id
end
def foo
Foo.foo
end
end
class Bar
include Foo
end
bar_one = Bar.new
bar_one.foo # => 2151891940
bar_two = Bar.new
bar_two.foo # => 2151891440
bar_one.foo # => 2151891440

Although, your wording is pretty vague, this would technically satisfy
the
request
module Foo
def initialize
@foo = object_id
end
attr_reader :foo
def foo_of(other)
other.foo
end
end
class Bar
include Foo
end
bar_one = Bar.new
bar_two = Bar.new
bar_one.foo # => 2151894400
bar_two.foo # => 2151894380
bar_two.foo_of bar_one # => 2151894400

So anyway, hopefully something in there addresses what you are asking.
If
not, you’ll have to give more feedback (ie an interface to access this
information).

3

Geometric Patterns wrote in post #997317:

Hi,

Don’t EVER use global variables($).

module Foo
def setup
id = self.object_id
puts id

  self.class.class_eval do
     @foo =  id

     def self.foo
        @foo
     end
  end

end

def foo
return self.class.foo
end
end

class Bar
include Foo
end

bar_one = Bar.new
bar_one.setup
p bar_one.foo # => 2148224640

bar_two = Bar.new
p bar_two.foo # => warning: instance variable @foo not initialized

–output:–
83310930
83310930
83310930