How to access instance_variables of a class and why does this method call return nil?

Question

class Human
  @core = "heart"

  def cardiovascular
    arr = ['heart','blood','lungs']
    core = @core
  end
end

Is the only way I am able to access @core directly by using this:

Human.instance_variable_get(:@core)  #=> "heart"

My understanding is that an instance variable is accessible from anywhere within the scope of the Class.

I am able to access the method by: Human.new.cardiovascular and I expect the return to be "heart" but the return I get is nil

How come I can't access the instance variable as Human.core or Human.new.core?
Why does the Human.new.cardiovascular return nil? (Shouldn't core == @core?)

Update

After putting @core within an initialize block I see the following output in IRB:

Human.new
=> #<Human:0x2f1f030 @core="heart">

This makes sense as it's now available to the entire Class, but how do I access the specific instance variables within the initialize block? Meaning, how do I get just: @core without calling the cardiovascular method in this case?

using this ruby-doc.org/core-2.1.0/Module.html#method-i-attr_accessor — Arup Rakshit
– Arup Rakshit, Commented Jan 24, 2014 at 20:09
Thanks @Arup, I see adding: attr_accessor :core Allows me to access @core as Human.new.core — MrPizzaFace
– MrPizzaFace, Commented Jan 24, 2014 at 20:12

Sergio Tulentsev · Accepted Answer · 2014-01-24 20:00:01Z

5

Do this:

class Human
  def initialize
    @core = "heart"
  end

  def cardiovascular
    arr = ['heart','blood','lungs']
    core = @core
  end
end

In short, you're setting instance variable on a class itself (which is an object too), but you want it on an instance.

answered Jan 24, 2014 at 20:00

Sergio Tulentsev

231k43 gold badges381 silver badges373 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

user2864740 · Accepted Answer · 2014-01-24 20:10:36Z

Also..

How come I can't access the instance variable as Human.core or Human.new.core?

Because, in Ruby, only methods can be accessed as such - not variables/fields. In this aspect, Ruby acts like SmallTalk (and unlike Python, Java, and many other OOP languages). The "accessor" (e.g. attr_accessor) methods create applicable proxies.

Why does the Human.new.cardiovascular return nil? (Shouldn't core == @core?)

Because @core in context is nil as explained in the other answers (and observations), however, the assignment core = @core is useless in the context (and this ties in with the previous question) because the local variable assignments are only accessible within that scope.

Note the correctness of the observation(s) - e.g. Human.instance_variable_get(:@core) works because it is an instance variable for the receiver (the Human class which is also an object), even if not for any instance of said class. Continue with this logic analysis, it's right :)

Collectives™ on Stack Overflow

How to access instance_variables of a class and why does this method call return nil?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related