Metaprogramming problems

[Leslie Viljoen]

Hello!

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.

Basically, I have this template array which defines a binary format:

@map =
[["length", "S"],["unit_id", "L"],["date", "L"],["transaction_id", "L"],
["type", "c"], ["subtype", "c"], ["version", "c"],
["body", "a*"]]


Then I have this in my template class:


#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)

currentPos = 0
@map.each do |attribute|
case attribute[1]
when "L": size = 4
when "S": size = 2
when "c": size = 1
end

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


...which produces a class with accessors for each "field" in the
binary data. An object is created and returned - exactly what I want.
Two problems though:

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.

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.

I was thinking of a method similar to the one above:

def applyMore(object, className, string)

But I don't know how to add accessors to an already created object,
and since the class above is lost in eval-world (where is that??) - I
can't add to it.

I need this because all my binary files have the same header, but
different bodies. I want to apply the header template and then a body
template to produce the finished object.


So, can anyone help with these problems? If I am going completely off
the track here, can anyone suggest a better way to do all this? I am
reading a lot on meta-programming but I'm slow to catch on.

Les

 

[Alex Fenton]

Hi

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)

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

 

[Leslie Viljoen]

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.

Holy macaroni that's cool.
Thanks Alex!