Marshaling classes

Ruby
1

Guys, how are you?
I’m stuck thinking about how can I save the state of a class. I want to
save the class variables of some classes. Do I have to require to
metaprogramming for this? I mean: save those variables in an object,
then dump it marshaling and then re-construct a class from that object
when is loaded. I’ll try to explain the situation in an example:

class Dog
@@quantity = 5
def self.change_quantity=(n)
@@quantity = n
end
def self.show_quantity
@@quantity
end
end

Dog.change_quantity = 99

File.open(‘anyfile.mrs’, ‘w+’) do |f|
Marshal.dump(Dog, f)
end

Dog.change_quantity = 50

Marshal.load(File.open(‘anyfile.mrs’))

Dog.show_quantity
#=> 50

#I want to show 99

So as you can see I’m very confused. This is probably not the way to do
what I want, do you have any advice for me? The objetive is to save the
“state” of a class, save the values of all his class variables. Thanks.
Damián.

2

On Sep 12, 2012, at 16:13 , Damin M. Gonzlez [email protected]
wrote:

Guys, how are you?
I’m stuck thinking about how can I save the state of a class. I want to
save the class variables of some classes. Do I have to require to
metaprogramming for this? I mean: save those variables in an object,
then dump it marshaling and then re-construct a class from that object
when is loaded. I’ll try to explain the situation in an example:

Two ways I can think of off of the top of my head:

  1. use maglev-ruby -Mpersistent to run your code and you’re done.

class Dog
@@quantity = 5
def self.change_quantity=(n)
@@quantity = n
end
def self.show_quantity
@@quantity
end
end

  1. Rename your methods to reflect good ruby (def self.quantitiy and def
    self.quantity=) and then make them persist your values. Change the
    getter to a lazy accessor that will read in the value from disk (or
    whatever) and change the setter to persist the change back out.

def self.quantity
@@quantity ||= Marshal.load(…) # or whatever
end

3

Unfortunately Marshal.dump doesn’t dump even instance variables of a
class, just the name of the class.

–8<----
class Dog
@quantity = 5
def self.change_quantity=(n)
@quantity = n
end
def self.show_quantity
@quantity
end
end

Dog.change_quantity = 99

File.open(‘anyfile.mrs’, ‘w+’) do |f|
Marshal.dump(Dog, f)
end

Dog.change_quantity = 50

Dog2 = Marshal.load(File.open(‘anyfile.mrs’))

puts Dog.show_quantity # 50
puts Dog2.show_quantity # 50
puts Dog.object_id == Dog2.object_id # true
–8<----

$ hexdump -C anyfile.mrs
00000000 04 08 63 08 44 6f 67 |…c.Dog|
00000007

So I guess it’s up to you to iterate over instance_variables:

–8<----
class IVDumper
def initialize(klass)
@klass = klass
@ivs = {}
klass.instance_variables.each do |k|
@ivs[k] = klass.instance_variable_get(k)
end
end
def restore
@ivs.each do |k,v|
@klass.instance_variable_set(k,v)
end
@klass
end
end
def IVDumper(k)
IVDumper.new(k)
end

Dog.change_quantity = 99

File.open(‘anyfile.mrs’, ‘w+’) do |f|
Marshal.dump(IVDumper(Dog), f)
end

Dog.change_quantity = 50

Marshal.load(File.open(‘anyfile.mrs’)).restore

puts Dog.show_quantity # 99
–8<----

You can probably make this more transparent, e.g using _dump/_load
methods in IVDumper

4

Yes! Both advices help me a lot! Finally I’ve got to require to
metaprogramming for this case. Thank you both guys, for your knowledge.
Damián.