From: Jeremy Bopp <jeremy@...>
Date: 2010-12-10T08:26:12+09:00
Subject: Re: is my instance variable being masked?

On 12/09/2010 04:20 PM, Sylvester T Cat wrote:
> This is probably trivial for the pros here, but
> 
> I have a class
> 
> class MyClass
>     attr_accessor :raw_vehicle_data
> 
>   def parse_raw_vehicle_data
>     @raw_vehicle_data = [0,1,2,3,4]
>     puts raw_vehicle_data.size # => gives proper size of the array
> (i.e. size > 0)
>     # do other stuff with raw_vehicle_data
>   end
> end
> 
> irb(main):019:0> my_class = MyClass.new
> => #<MyClass:0x00000102005f98>
> irb(main):020:0> my_class.parse_raw_vehicle_data
> 5  #<= this value I expect, but it's not assigned to my "instance
> variable"
> => nil
> irb(main):021:0> my_class.raw_vehicle_data.size
> => nil
> 
> If I change the line from raw_vehicle_data = [0,1,2,3,4] to
> @raw_vehicle_data = [0,1,2,3,4], then everything works properly, but I
> thought attr_accessor gave me access to the function
> "raw_vehicle_data=" that would assign to an instance variable.

Your code listing above has the @raw_vehicle_data reference, but I think
you meant to leave off the @ there for your problem demonstration.
Leaving off the @ introduces an ambiguity for Ruby in assignment
statements like yours.  The left hand side is seen as a local variable
reference rather than a call to the attr_writer method of the same name.

You can either use the @ as you discovered, or you can explicitly call
the attr_writer:

self.raw_vehicle_data = [0,1,2,3,4]

-Jeremy