使用class_eval和instance_eval访问Ruby类变量 [英] Accessing Ruby Class Variables with class_eval and instance_eval

问题描述

我有以下：

class Test
    @@a = 10

    def show_a()
        puts "a: #{@@a}"
    end

    class << self
      @@b = '40'

      def show_b
        puts "b: #{@@b}"
    end
  end
end

为什么以下工作：

Test.instance_eval{show_b}
b: 40
=> nil

但我无法访问 @@ b 直接？

Test.instance_eval{ @@b }
NameError: uninitialized class variable @@b in Object

同样，以下作品

t = Test.new
t.instance_eval{show_a}
a: 10
=> nil

但以下操作失败

t.instance_eval{ @@a }
NameError: uninitialized class variable @@a in Object

我不明白为什么我不能直接从 instance_eval 块中访问类变量。

I don't understand why I can't access the Class Variables directly from the instance_eval blocks.

推荐答案

我刚刚在RubyKaigi派对时向Matz提出了同样的问题。我是半醉了，但他非常清醒，所以你可以把这作为最终的答案。

I just asked the same question to Matz during the RubyKaigi party. I was half-drunk, but he was perfectly sober, so you can take this as the definitive answer.

Anton是正确的 - 你不能访问类变量的原因instance_eval（）是只是因为。甚至class_eval（）共享同样的问题（Matz自己不完全确定class_eval（），直到我告诉他，我已经尝试了）。更具体地说：范围更广泛，类变量更像是常量而不是实例变量，因此切换self（作为instance_eval（）和class_eval（）do）在访问它们时不会产生任何影响。

Anton is right - the reason why you cannot access class variables through instance_eval() is "just because". Even class_eval() shares the same issue (Matz himself wasn't totally sure about class_eval() until I told him I'd already tried it). More specifically: scope-wise, class variables are more like constants than instance variables, so switching self (as instance_eval() and class_eval() do) is not going to make any difference when it comes to accessing them.

一般来说，避免类变量可能是个好主意。

In general, it might be a good idea to avoid class variables altogether.

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