Access to module instance variable

Question

I'm trying to access an instance variable in the methods of a module.

module Filter
  def self.included(target)
    puts "INCLUDED: #{@var}"
    target.instance_variable_set(:@var, @var) # doesn't seem to work
  end

  def self.var=(var)
    @var = var
  end

  def self.var
    @var
  end

  def hello
    @var
  end

end

f = Filter
f.var = "hello"

puts "SET: #{f.var}"

class Main
  include Filter
end

m = Main.new

puts "HELLO: #{m.hello}"

It produces this output:

ruby test2.rb
SET: hello
INCLUDED: hello
HELLO:

The last line "HELLO:" needs to output "HELLO: hello". How do I initialize the @var instance variable to make this happen?

Sergio Tulentsev · Accepted Answer · 2015-07-31 13:06:47Z

2

As many-many others, you too are confused by the term "instance variable" here. It's an instance variable, all right, but it's not an instance you think about.

Remember, everything in ruby is an object. Even classes themselves.

  def self.included(target)
    puts "INCLUDED: #{@var}"
    target.instance_variable_set(:@var, @var) # doesn't seem to work
  end

So here you're setting instance variable on target (which would be a Class, most likely). Yet you're trying to read from instance of target.

m = Main.new

puts "HELLO: #{m.hello}"

This is where your variable is:

puts "The right HELLO: #{Main.instance_variable_get(:@var)}"

Naturally, it is not possible to set an instance variable (in this manner) on an instance that does not yet exist! Depending on what exactly you're after, different strategies can be implemented. I suggest you ask another, more refined question.

edited Jul 31, 2015 at 13:06

answered Jul 13, 2015 at 11:38

Sergio Tulentsev

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5 Comments

Vidar Over a year ago

Thanks, yeah, it's a little confusing. I just need to be able to access @var when I call m.hello, which is the method included from the module. Any ideas?

Sergio Tulentsev Over a year ago

@Vidar: this one option - def hello; self.class.instance_variable_get(:@var); end

Sergio Tulentsev Over a year ago

Beware, though. If you change that @var, all instances of target will see the updated version. Not sure if it's desirable behaviour for you or not.

Vidar Over a year ago

So how do I avoid that? That's exactly what I'm trying not to do. I used a class variable before (@@var) and got a race condition.

Sergio Tulentsev Over a year ago

That's another question. Make it focused.

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