ruby - 实例变量声明

Question

下面的代码什么也没打印，我期待welcomeBack. 请解释。

class Hello

  @instanceHello = "welcomeBack"

  def printHello
    print(@instanceHello)
  end

  Hello.new().printHello();
end

我刚开始学习红宝石，所以如果问题看起来很愚蠢，请原谅。

Answer 1

如果你只能记住关于定义方法和设置变量的一件事，那就是：总是问自己，此时此刻是什么self？

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，请在构造函数中进行：

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。所以你需要把你的任务放在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。这是因为该类直到第一次关闭后才真正定义；也就是在最后一个之后end。

Answer 3

class Hello

  @instanceHello = "welcomeBack"

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

  Hello.new.printHello; #=> welcomeBack
end

这不是好的编程，只是为了说明一个类（它是类 Class 的一个实例）也可以有实例变量，就像任何其他实例一样。它们被称为“类实例变量”，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();