From: Joel VanderWerf <vjoel@...>
Date: 2006-12-12T05:29:32+09:00
Subject: Re: Question on IO objects, initialize, and yaml

Joel VanderWerf wrote:
> Mark Noworolski wrote:
>> Say I have a class with pseudo code as follows:
>> class Test
>> def initialize
>>  @a=1
>>  @b=File.open("file+Time.now or similar")
>> end
>> end
>>
>> Now, imagine that I use this class for a while and then I want to dump 
>> it to
>> a YAML file.
>>
>> When I restart my code, I want it to read in stuff from the YAML file 
>> (if it
>> exists) and use that as a starting point to work from (basically, I am
>> saving state in the yaml file). You'll note that I cannot just do this as
>> is, because reading in from YAML does not initialize the class, so @b 
>> would
>> not be initialized correctly on restart.
>>
>> I'd like to just initialize the thing, then read in the YAML file to
>> overwrite the state with the saved values. Is there some extremely 
>> obvious
>> ruby way to do this or something equivalent?
> 
> A possible alternative is to be lazy about constructing the non-dumpable 
> stuff:
> 
> [~] cat test.rb
> require 'yaml'
> 
> class Test
>   def initialize
>     @a = 1
>   end
>   def b
>     @b ||= File.open(__FILE__) # just for testing
>   end
> end
> 
> t = Test.new
> t2 = YAML.load(YAML.dump(t))
> 
> line = t2.b.gets
> p line
> 
> [~] ruby test.rb
> "require 'yaml'\n
> 
> 
> If you do this, other parts of your code shouldn't access @b directly. 
> Instead, call the #b method.
> 

That's not quite right. If you use #b and then dump/load and try to use 
#b on the new object, you get an error:

   t = Test.new

   t2 = YAML.load(YAML.dump(t))
   line = t2.b.gets
   p line

   t3 = YAML.load(YAML.dump(t2))
   line = t3.b.gets
   p line

Output:

   "require 'yaml'\n"
   test.rb:23:in `gets': uninitialized stream (IOError)
           from test.rb:23

You can "fix" this by adding a #to_yaml method to your class that clears @b:

   class Test
     def to_yaml(*)
       @b = nil
       super
     end
   end

Then t3.b.gets doesn't fail. But whether this "fix" is right for your 
class is another question...

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407