How are respond_to and respond_to_missing different?

Question

I\'m confused when to use each of this methods. From respond_to? documentation:

Returns true if obj responds to the given meth

Answer 1

respond_to_missing? is supposed to be updated when you make available additional methods using the method missing technique. This will cause the Ruby interpreter to better understand the existence of the new method.

In fact, without using respond_to_missing?, you can't get the method using method.

Marc-André posted a great article about the respond_to_missing?.

In order for respond_to? to return true, one can specialize it, as follows:

class StereoPlayer
  # def method_missing ...
  #   ...
  # end

  def respond_to?(method, *)
    method.to_s =~ /play_(\w+)/ || super
  end
end
p.respond_to? :play_some_Beethoven # => true

This is better, but it still doesn’t make play_some_Beethoven behave exactly like a method. Indeed:

p.method :play_some_Beethoven
# => NameError: undefined method `play_some_Beethoven'
#               for class `StereoPlayer'

Ruby 1.9.2 introduces respond_to_missing? that provides for a clean solution to the problem. Instead of specializing respond_to? one specializes respond_to_missing?. Here’s a full example:

class StereoPlayer
  # def method_missing ...
  #   ...
  # end

  def respond_to_missing?(method, *)
    method =~ /play_(\w+)/ || super
  end
end

p = StereoPlayer.new
p.play_some_Beethoven # => "Here's some_Beethoven"
p.respond_to? :play_some_Beethoven # => true
m = p.method(:play_some_Beethoven) # => #<Method: StereoPlayer#play_some_Beethoven>
# m acts like any other method:
m.call # => "Here's some_Beethoven"
m == p.method(:play_some_Beethoven) # => true
m.name # => :play_some_Beethoven
StereoPlayer.send :define_method, :ludwig, m
p.ludwig # => "Here's some_Beethoven"

See also Always Define respond_to_missing? When Overriding method_missing.