Ruby instance reference in class method?

Question

I'm trying to fill in class variables for a class instance from a file, and the only way I've managed to figure how to do this is something like

a=Thing.new
File.read("filename.ext").each_line do |arguments| #this will => things like @variable=str\n
eval( "a.instance_eval {" + arguments.chop + "}")   #hence the awkward eval eval chop
end

The only problem I've found is that in trying to impliment this in a class method (to do this for several instances in a go), I don't know how to make this happen:

class Thing
attr_accessor :variable

 def self.method
  File.read("filename.ext").each_line do |arguments|
   eval("instance.instance_eval{" + arguments.chop + "}")   #this line
  end
 end
end

namely, the reference to the instance calling the method. self will just just be Thing in this case, so is there any way to do this? More pertinent might be a better way to go about this overall. I only just learned ruby last night, so I haven't had an opportunity to see some of the neater tricks that are to be had, and my language maturity is a little fresh yet as a result.

For context, Thing is a character in a game, loading its base values from a savefile.

A better way to go about this would be, IMO, to use YAML and marshall it to and from a file. Here is a good blog post about it skorks.com/2010/04/… — ian
– ian, Commented Oct 23, 2012 at 5:33

Chris Heald · Accepted Answer · 2012-10-23 05:43:25Z

1

Well, first off, take a look at Marshal. It's specifically used for dumping data structures to serialized formats and loading them back.

The said, if you want to persist in your direction, then try something like this:

class Thing
  attr_accessor :variable

  def self.method
    File.read("filename.ext").each_line do |arguments|
      ivar, val = arguments.strip.split("=", 2)
      instance.instance_variable_set(ivar, val)
    end
  end
end

#instance_variable_set allows you to...well, set instance variables on an object by name. No ugly eval necessary!

By way of demonstration:

class Foo
  attr_accessor :bar
end

foo = Foo.new
foo.instance_variable_set("@bar", "whatzits")
puts foo.bar # => whatzits

answered Oct 23, 2012 at 5:43

Chris Heald

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3 Comments

Ryan Wood Over a year ago

I like it. Honestly I'm probably going to end up using Marshal and YAML to do this at this point, but for the sake of curiosity though: instance.instance_variable_set(ivar, val) will this substitute the class instance into the method? As in, will, a=Thing.new\n a.method process a.instance_variable_set(ivar, val) or instance.instance_variable_...

Chris Heald Over a year ago

It'd run a.instance_variable_set.

Ryan Wood Over a year ago

Ok, something about it bugged be, so I went to test it out in irb. These came up everywhere NameError: undefined local variable or method `instance' for #<Thing:0x7f7ca3df7c68> But it gave me some insight, instead of using a class method a la self.method, I was mismatching my scope, so by making an instance method of it instead, self is instance instead of class like I was thinking, and I can even use self to pull the correctly associated file if I chose to go this way ^^. Maybe I was using a different version, moot though, the insight was valuable

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