From: Alex Fenton <alex@...>
Date: 2006-07-24T08:40:10+09:00
Subject: Re: Metaprogramming problems

Hi

Leslie Viljoen wrote:
> I am pretty new to metaprogramming, this is my first shot. I want to
> be able to take some binary data, split it into parts based on a
> template and make an object with those parts.
> 
...

You can do the kind of thing you want using eval, but there's easier ways in Ruby.

> #Apply the template to a (binary data) string, producing an object
>  def apply(className, string)
>    evalString = "class #{className}\n"
>    @map.each do |attribute|
>      evalString << "attr_accessor :#{attribute[0]}\n"
>    end
>    evalString << "end\n" << "o = #{className}.new"
>    eval(evalString)
<snip>
>      if attribute[1] == "a*"
>        eval("o.#{attribute[0]} =
> string[#{currentPos}..-1].unpack(\"#{attribute[1]}\")")
>      else
>        eval("o.#{attribute[0]} = string[#{currentPos},
> #{size}].unpack(\"#{attribute[1]}\")")
>        currentPos += size
>      end
>    end
> 
>    return eval("o")
>  end
> 

These are some ways of doing the same thing using Ruby methods:

# Create a new class, assign it to a variable.
new_class = Class.new()

# Add an attribute accessor to the class a based on the template
new_class.class_eval { attr_accessor :unit_id }

# Create an instance of the new class
item = new_class.new

# Assign a value based on the data in your binary format
item.unit_id = 666

> 1. I use the final eval("o") to return my object from eval-world. How
> do I get my class from there? I want the new class I make to be
> persistent at the top level so that I can make several objects of that
> class later.

If you want to use the class again later, store a reference to it in a 
constant or an instance variable.

@my_class = Class.new()
MyClass = Class.new()

> 2. Once the class is created, I want to be able to go back and parse
> another binary string, producing further fields and add those to the
> existing class. My object would need to have the initial accessors
> plus the new ones, and the new variables must be set.

You can add methods dynamically to your new class at any time, using
class_eval as above, or other techniques. Another alternative might
be to create subclasses.

cheers
alex