Confusion with instance variables object id allocation in Ruby

Question

I was trying to understand the instance variable initialization and declaration. Doing so I tried the below code. But in the half of my thinking path,I got something interesting as below:

class Person
attr_accessor :name,:age
    def initialize(var)
     @sex = var
    end
    def objctid
        p ":age -> #{@age.object_id}"
        p ":name -> #{@name.object_id}"
        p ":sex -> #{@sex.object_id}"
    end
end
#=> nil

ram = Person.new('M')
#=> #<Person:0x2109f48 @sex="M">

ram.objctid
#":age -> 4"
#":name -> 4"
#":sex -> 17321904"
#=> ":sex -> 17321904"

I didn't use Object#instance_variable_set till now.

How does the symbols :age and :name get the memory references,as I didn't intialize them till above?

Why does the object_id of those instance variables are same ?

ram.instance_variable_set(:@age,'14')
#=> "14"

Now in the below code @age s object_id changed as I used Object#instance_variable_set to set @age.

ram.objctid
#":age -> 17884656"
#":name -> 4"
#":sex -> 17321904"
#=> ":sex -> 17321904"

ram.instance_variable_set(:@name,'Lalu')
#=> "Lalu"

ram.objctid
#":age -> 16884288"
#":name -> 17041356"
#":sex -> 17625228"
#=> ":sex -> 17625228"

* Why such different object_id of @age and @name? *

mu is too short · Accepted Answer · 2013-03-16 19:21:11Z

2

Instance variables are created and initialized to nil when they're first referenced. For example:

class C
    def m
        puts @unset.inspect
    end
end
C.new.m

will say nil.

You don't initialize @age or @name so they're created and initialized to nil when you first reference them inside your objctid method. Since they're both nil, they will have the same object_id since there is only one nil; adding p nil.object_id to your objctid method might be fruitful.

answered Mar 16, 2013 at 19:21

mu is too short

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sunny1304 · Accepted Answer · 2013-03-16 19:31:21Z

2

How does the symbols :age and :name get the memory references ?

as you haven't assigned them any value so by default they get a nil object

Why does the object_id of those instance variables are same ?

because there is only one nil

answered Mar 16, 2013 at 19:31

sunny1304

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