Ruby元编程:初始化singleton_class变量 [英] Ruby metaprogramming: Initialize singleton_class variable

问题描述

• 为什么Foo.val在调用Foo.set之前返回nil而不是"foo"?
• 是否有用于初始化类评估中的@val的机制?

• @val = "foo"存储在哪个范围内?

• Why does Foo.val return nil instead of "foo" before calling Foo.set?
• Is there any mechanism for initializing @val on class evaluation?

• In which scope is @val = "foo" stored into?

class Foo
  class << self
    @val  = "foo"
    attr_reader :val

    def set(val)
      @val = val
    end
  end
end

p Foo.val # nil
Foo.set("bar")
p Foo.val # "bar"

推荐答案

您可以像这样在Foo中初始化@val:

You can initialize @val in Foo like this:

class Foo
  @val  = "foo"
  class << self
    attr_reader :val

    def set(val)
      @val = val
    end
  end
end

p Foo.val         #=> "foo"
Foo.set("bar")
p Foo.val         #=> "bar"

您的代码不是在Foo上而是在Foo的元类上初始化@val

Your code initializes @val not on Foo, but on Foo's metaclass

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