`respond_to?` vs. `respond_to_missing?`

metaprogramming, ruby

Solution

Without `respond_to_missing?` defined, trying to get the method via `method` will fail:

class Foo
  def method_missing name, *args
    p args
  end

  def respond_to? name, include_private = false
    true
  end
end

f = Foo.new
f.bar  #=> []
f.respond_to? :bar  #=> true
f.method :bar  # NameError: undefined method `bar' for class `Foo'

class Foo
  def respond_to? *args; super; end  # “Reverting” previous redefinition

  def respond_to_missing? *args
    true
  end
end

f.method :bar  #=> #<Method: Foo#bar>

Marc-André (a Ruby core committer) has a good blog post on `respond_to_missing?`.

Problem

What is the point of defining `respond_to_missing?` as opposed to defining `respond_to?`? What goes wrong if you redefine `respond_to?` for some class?

Original source