ruby - `respond_to?` 与 `respond_to_missing?`

Question

与定义 respond_to? 相比，定义 respond_to_missing? 有什么意义？如果为某些类重新定义 respond_to? 会出什么问题？

Answer 1

没有respond_to_missing?已定义，试图通过 method 获取方法会失败:

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 (Ruby 核心提交者)有一个很好的 blog post on respond_to_missing? .