ruby - 实例变量声明

Question

以下代码不打印任何内容，我期待着 welcomeBack。请解释。

class Hello

  @instanceHello = "welcomeBack"

  def printHello
    print(@instanceHello)
  end

  Hello.new().printHello();
end

我刚开始学习 ruby​​，所以如果问题看起来很愚蠢，请原谅。

Answer 1

如果关于定义方法和设置变量你只能记住一件事，那就是:总是问自己，此时 self 是什么？

class Hello
  # This ivar belongs to Hello class object, not instance of Hello.
  # `self` is Hello class object.
  @instanceHello = "welcomeBack" 

  def printHello
    # Hello#@instanceHello hasn't been assigned a value. It's nil at this point.
    # `self` is an instance of Hello class
    print @instanceHello
  end

  def self.printSelfHello
    # Now this is Hello.@instanceHello. This is the var that you assigned value to.
    # `self` is Hello class object
    print @instanceHello
  end

end

Hello.new.printHello # >> 
Hello.printSelfHello # >> welcomeBack

如果要为 ivar 设置默认值，请在构造函数中执行:

class Hello

  def initialize
    @instanceHello = "welcomeBack"
  end

  def printHello
    print @instanceHello
  end
end

Hello.new.printHello # >> welcomeBack