ruby - Difference between @instance_variable and attr_accessor

Question

I Just started learning ruby and I don't see the difference between an @instace_variable and an attribute declared using attr_accessor.

What is the difference between the following two classes:

class MyClass  
  @variable1 
end

and

class MyClass
  attr_accessor :variable1
end

I searched lot of tutorials online and everybody uses different notation, Does it have to do anything with the ruby version? I also searched few old threads in StackOverflow

What is attr_accessor in Ruby?
What's the Difference Between These Two Ruby Class Initialization Definitions?

But still I am not able to figure out what is the best way to use.

Answer 1

An instance variable is not visible outside the object it is in; but when you create an attr_accessor, it creates an instance variable and also makes it visible (and editable) outside the object.

Example with instance variable (not attr_accessor)

class MyClass
  def initialize
    @greeting = "hello"
  end
end

m = MyClass.new
m.greeting #results in the following error:
  #NoMethodError: undefined method `greeting' for #<MyClass:0x007f9e5109c058 @greeting="hello">

Example using attr_accessor:

class MyClass
  attr_accessor :greeting

  def initialize
    @greeting = "hello"
  end
end

m2 = MyClass.new
m2.greeting = "bonjour" # <-- set the @greeting variable from outside the object
m2.greeting #=> "bonjour"   <-- didn't blow up as attr_accessor makes the variable accessible from outside the object

Hope that makes it clear.