Accessing Class Variables in mixed in define_method

ruby-talk
1

I am using define_method inside a module to dynamically create a class
after the module is mixed in. The module is writing out class variables
for the class it is mixed into (yes I am aware that class variables are
generally frowned upon but they are necessary in this instance) and then
subsequently trying to access them through the method being created with
define_method.

The problem here is that define method is not able to access the class
variables through class_variable_get or through accessing them directly.
In the first case it fails because 'class_variable_get' is not a method
on the class (oMethodError: undefined method ‘class_variable_get’ for
#<Foo:0x100124638>) In the second instance it fails because it thinks
the class variable is uninitialized (NameError: uninitialized class
variable @@c in M)

It seems to be evaluating the '@@c' call in the context of the module
and the class_variable_get call in the context of the class in which
define_method is being called after it is mixed in. Does anyone know how
I can get at the class variable here in define_method? Sample code
below:

Best,
Jamie

module M
  def cv(methodname)
    if class_variable_defined?(:@@c)
      classvar = class_variable_get(:@@c)
      classvar[rand(10000)] = "random"
      class_variable_set(:@cc,classvar)
    else
      class_variable_set(:@@c,{})
    end
    define_method(methodname.to_sym) do
      # need to be able to get class variables for this class
      # @@c does not work because it tries to get the variable for the
module and it exists only in the class
      #class_variable_get(:@@c) #does not work because its an invalid
class method
      #@cc and class_variable_get(:@@c) are getting evaluated in
different contexts which is strange
    end
  end
end

class Foo
  extend M
  cv("mfoo")
end

f = Foo.new
f.mfoo # want this to be able to access and operate on class variables

···

--
Posted via http://www.ruby-forum.com/.

2

There's a typo here: it should be :@@c

This "works" for me:

module M
def cv(methodname)
   if class_variable_defined?(:@@c)
     classvar = class_variable_get(:@@c)
     classvar[rand(10000)] = "random"
     class_variable_set(:@@c,classvar)
   else
     class_variable_set(:@@c,{})
   end
   define_method(methodname.to_sym) do
  self.class.send(:class_variable_get,:@@c)
   end
end
end

class Foo
extend M
cv("mfoo")
end

I say "works" because the first time, @@c is initialized to {} but no
value is set. You need to call cv again inside class Foo in order to
create a value in the hash:

irb(main):021:0* f = Foo.new
=> #<Foo:0xb74aeb30>
irb(main):022:0> f.mfoo
=> {}

if you call it again:

irb(main):024:0> class Foo
irb(main):025:1> cv("mfoo")
irb(main):026:1> end
=> #<Proc:0xb74bb8d0@(irb):10>
irb(main):027:0> f.mfoo
=> {9035=>"random"}

Then it creates an entry in the hash, and then accessing it through
mfoo retrieves it.

Hope this helps,

Jesus.

···

On Fri, Jun 18, 2010 at 10:08 PM, Jamie Quint <jamiequint@gmail.com> wrote:

I am using define_method inside a module to dynamically create a class
after the module is mixed in. The module is writing out class variables
for the class it is mixed into (yes I am aware that class variables are
generally frowned upon but they are necessary in this instance) and then
subsequently trying to access them through the method being created with
define_method.

The problem here is that define method is not able to access the class
variables through class_variable_get or through accessing them directly.
In the first case it fails because 'class_variable_get' is not a method
on the class (oMethodError: undefined method ‘class_variable_get’ for
#<Foo:0x100124638>) In the second instance it fails because it thinks
the class variable is uninitialized (NameError: uninitialized class
variable @@c in M)

It seems to be evaluating the '@@c' call in the context of the module
and the class_variable_get call in the context of the class in which
define_method is being called after it is mixed in. Does anyone know how
I can get at the class variable here in define_method? Sample code
below:

Best,
Jamie

module M
def cv(methodname)
if class_variable_defined?(:@@c)
classvar = class_variable_get(:@@c)
classvar[rand(10000)] = "random"
class_variable_set(:@cc,classvar)