Question

以下代码什么都没打印，我期待welcomeBack。请解释一下。

class Hello

  @instanceHello = "welcomeBack"

  def printHello
    print(@instanceHello)
  end

  Hello.new().printHello();
end

我刚刚开始倾向于红宝石，所以如果问题看起来很愚蠢，请原谅。

Answer 1

如果你只能记住定义方法和设置变量的一件事，那就是：总是问问自己，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

Answer 2

在Ruby中，实例变量是在实例方法中定义和使用的。因此，您需要将作业放在initialize方法中：

class Hello

  def initialize
    @instanceHello = "welcomeBack"
  end

  def printHello
    print(@instanceHello)
  end

end

Hello.new.printHello();

另外，请注意我将printHello调用移到了类定义之外。这是因为该类在第一次关闭之前才实际定义;也就是说，在最后end之后。

Answer 3

class Hello

  @instanceHello = "welcomeBack"

  def printHello
      puts self.class.instance_variable_get(:@instanceHello)
  end

  Hello.new.printHello; #=> welcomeBack
end

这不是一个好的编程，只是为了说明一个类（它是类Class的一个实例）也可以像任何其他实例一样拥有实例变量。它们被称为“类实例变量”，并且优先于类变量。以下示例说明了如何在类级别定义计数器：

class Hello
  @counter = 0

  class << self # access methods for class instance variables must be
                # defined in the singleton class
      attr_accessor :counter
  end

  def printHello
      self.class.counter += 1
      puts "Hello #{self.class.counter}"
  end
end

Hello.new.printHello; #=> Hello 1
Hello.new.printHello; #=> Hello 2
p Hello.singleton_methods #=> ["counter=", "counter"]

Answer 4

将@instanceHello更改为self.class.instance_variable_get（：@ instanceHello）

@instanceHello是一个类实例变体

代码是这样的：

    class Hello
            @instanceHello = "welcomeBack"
            @@classHello = "greeting!"
            def printHello
                    print(self.class.instance_variable_get(:@instanceHello))
                    print(self.class.class_variable_get(:@@classHello))
            end
    end
    Hello.new().printHello();

实例变量声明

4 个答案: