From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2011-02-07T17:57:09+09:00
Subject: Re: Setter method for Hash value

On Sun, Feb 6, 2011 at 10:23 PM, Rolf Pedersen <rolfhsp@gmail.com> wrote:
> 2011/2/4 Jesús Gabriel y Galán <jgabrielygalan@gmail.com>
>
>> For the other two, you need a proxy object that can delegate [] and
>> []= to DataClass too. For this to work, MyClass#props needs to wrap
>> the actual hash returned by DataClass with a wrapper object (Props).
>> As you need this proxy object to have some hash like behaviour, you
>> can implement whatever methods you need to behave like a hash. As an
>> example, apart from [] and []= I have implemented inspect and to_s.
>>
>>
> Instead of implementing other methods to make the wrapper object behave more
> like a Hash object, why not let Props inherit Hash, and just redefine those
> methods that I need to behave differently? Perhaps like this:
>
> class Props < Hash
>  def initialize h
>    replace h
>  end
>  def [](key)
>    ...
>  end
>  def []=(key, value)
>    ...
>  end
> end

Hi,

In fact I was going to follow up the discussion with a further idea.
What I sent has a flaw, and it's the fact that the Props object gets
initialized with the hash, while ideally it should delegate also all
reads to the DataClass. It doesn't make sense to have a reference to
the resulting hash. This way, if the hash in DataClass is modified by
other means, the new values will be available to all instances of
Props (in the current implementation this might be true, because we
are returning references to the same hash, but this can be a weak
point). It's cleaner this way, in my opinion:

class MyClass
 def props
   Props.new
 end

 def props=(h)
   DataClass::set_props h
 end
end

class Props
 def [](key)
   DataClass::get_prop key
 end

 def []=(key,value)
   DataClass::set_prop key,value
 end

 def inspect; DataClass::get_props.inspect; end
 def to_s; DataClass::get_props.to_s; end
end


class DataClass
 @data = {:a=>1, :b=>2, :c=>3}
 def self.get_props()
   @data
 end
 def self.get_prop(key)
   @data[key]
 end
 def self.set_props(data)
   @data = data
 end
 def self.set_prop(key, value)
   @data[key] = value
 end
end

As for your question about inheriting from Hash: read a bit about
Inheritance vs Composition. It's generally considered that Composition
is more flexible.

Jesus.