Ruby Koans - 词法范围的延续与继承层次结构

Question

我有机会在 StackOverflow 中四处看看，发现了我试图从 Ruby Koans (Ruby Koans: explicit scoping on a class definition part 2) 中更好地理解的同一个问题。

class MyAnimals
  LEGS = 2

  class Bird < Animal
    def legs_in_bird
      LEGS
    end
  end
end

def test_who_wins_with_both_nested_and_inherited_constants
  assert_equal 2, MyAnimals::Bird.new.legs_in_bird
end

# QUESTION: Which has precedence: The constant in the lexical scope,
# or the constant from the inheritance heirarachy?

# ------------------------------------------------------------------

class MyAnimals::Oyster < Animal
  def legs_in_oyster
    LEGS
  end
end

def test_who_wins_with_explicit_scoping_on_class_definition
  assert_equal 4, MyAnimals::Oyster.new.legs_in_oyster
end

# QUESTION: Now Which has precedence: The constant in the lexical
# scope, or the constant from the inheritance heirarachy?  Why is it
# different than the previous answer?

根据链接中的解释，其他人(包括我自己)的主要困惑似乎是因为类定义:

class MyAnimals::Oyster < Animal
  # stuff goes in here
end

我最初的想法是 MyAnimals::Oyster 意味着 Oyster 类是在 MyAnimals 中定义的。换句话说，我认为上面的代码类似于下面的代码:

class MyAnimals
  class Oyster < Animal
    # stuff goes in here
  end
end

为了验证我的想法，我在 IRB 中做了以下操作:

class MyAnimals
  LEGS = 2

  class Bird < Animal
    def legs_in_bird
      LEGS
    end
  end
end

class MyAnimals::Oyster # You should notice that I'm not inheriting from Animal anymore
  def legs_in_oyster
    LEGS
  end
end

如果我的推理是正确的，那么我希望下面的代码返回 2

MyAnimals::Oyster.new.legs_in_oyster # => NameError: uninitialized constant MyAnimals::Oyster::LEGS

因为这不返回 2，有人可以向我解释为什么它不返回 2 吗？

编辑:我忘了添加 Animal 类；这是:

class Animal
  LEGS = 4
  def legs_in_animal
    LEGS
  end

  class NestedAnimal
    def legs_in_nested_animal
      LEGS
    end
  end
end

Answer 1

回到 Ruby 搜索值的顺序。第 1 步是“封闭范围”。当你像这样定义一个嵌套类时

class Animal
    class Bird
    end
end

'Bird' 包含在 'Animal' 范围内。

当你这样做的时候

class Animal
end

class Animal::Bird
end

虽然您已将“Bird”定义为嵌套在“Animal”中，但在您定义“Bird”时，封闭范围是global 范围。返回 IRB 并定义 LEGS = 0(在全局范围内)并再次尝试您的 Oyster。