respond_to_missing.rb

class Wrong
  def method_missing(m, *)
    if m =~ /\Ahello_(.+)\z/
      puts "Hello, #{$1.capitalize}"
    else
      super
    end
  end
end

w = Wrong.new
w.hello_world
p w.respond_to? :hello_world
p defined? w.hello_world
begin
  p w.method(:hello_world)
rescue NameError
  puts "not defined!"
end

puts

class Right
  def method_missing(m, *)
    if m =~ /\Ahello_(.+)\z/
      puts "Hello, #{$1.capitalize}"
    else
      super
    end
  end

  def respond_to_missing?(m, include_all)
    m =~ /\Ahello_(.+)\z/ || super
  end
end

r = Right.new
r.hello_world
p r.respond_to? :hello_world
p defined? r.hello_world
begin
  p r.method(:hello_world)
rescue NameError
  puts "not defined!"
end