DelegateClass and "superclass mismatch for class" errors

problem.rb

require 'delegate'

class Klass < DelegateClass(String)
  # ...
end

class Klass < DelegateClass(String)
  # Re-open the class
end
# => TypeError: superclass mismatch for class Klass

solution.rb

require 'delegate'

DELEGATE_STRING = DelegateClass(String) unless defined?(DELEGATE_STRING)

class Klass < DELEGATE_STRING
  # ...
end

class Klass < DELEGATE_STRING
  # Re-open the class
end

why.rb

> require 'delegate'
 => true
> DelegateClass(String).object_id == DelegateClass(String).object_id
 => false

Thanks for posting this. Ran into this issue today.

Just to note why this happens, DelegateClass(SomeClass) is really doing Class.new(SomeClass) and assigning it as the superclass. That's why it's a different object and not the same superclass each time.

Thanks for the explanation of the issue -- had me scratching my head. I was running into this when reloading source files from a pry session to pick up changes. Chose to do something similar in a module, to eliminate the ...unless defined? boilerplate per class. It may well be bad ruby - I'm a dabbler.

# This module wraps DelegateClass to eliminate the error on reload, which is an aggravation during development
#
# To use it, just replace any incidence of
#
#   class MyClass < DelegateClass(SomeOtherClass)
#
# with
#
#   class MyClass < NMD::ExplicitDelegator.to(SomeOtherClass)

module NMD
  module ExplicitDelegator
    class << self
      attr_accessor :class_cache
      def to superclass
        superclass_id = superclass.to_s
        @class_cache = {} if @class_cache.nil?
        @class_cache[superclass_id] = DelegateClass(superclass) if @class_cache[superclass_id].nil?
        @class_cache[superclass_id]
      end
    end
  end
end