Bug #11916
closed
Fix delegating to 'args' and 'block'
Description
If you have a class that uses Forwardable to delegate a method to
another object, and the method that returns the delegate object is
called args or block, then Forwardable will fail to work.
Here's a simple example:
class ModelCreator
extend Forwardable
attr_reader :args
def_delegator :args, :model_name
def initialize(args)
@args = args
end
end
ModelCreator.new.model_name
If you run the last line above, then you'll get:
NoMethodError: undefined method `model_name' for []:Array
This error occurs because def_delegator -- as it is written in Ruby --
uses metaprogramming to add methods to the class that will then delegate
to the delegate object. So it's as if we had written:
class ModelCreator
extend Forwardable
attr_reader :args
def model_name(*args, &block)
args.model_name(*args, &block)
end
def initialize(args)
@args = args
end
end
As you can see, def_delegator will not only forward the method call
onto the delegate object, it will also forward any arguments provided as
well. It is here that the bug arises: it splats all of the arguments
into a variable which is called args, and because of how variable
scope works in Ruby, it then attempts to call model_name on this
variable and not our delegate object method.
The fix is to call the delegate object method manually using __send__.
(This assumes, of course, that the given receiver is, in fact, the name
of a method and not the name of an instance variable, which is also a
possibility.) We use __send__ because the delegate object method could
be private.
So, that looks like this:
def model_name(*args, &block)
__send__(:args).model_name(*args, &block)
end
Because def_delegators and delegate use def_delegator internally,
they also get this fix as well.
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