将实例变量添加到Ruby中的类 [英] Adding an instance variable to a class in Ruby

问题描述

如何在运行时上将实例变量添加到已定义的类中，然后从该类外部获取并设置其值?

How can I add an instance variable to a defined class at runtime, and later get and set its value from outside of the class?

我正在寻找一种元编程解决方案，该解决方案允许我在运行时修改类实例，而不是修改最初定义该类的源代码.一些解决方案说明了如何在类定义中声明实例变量，但这不是我要问的.

I'm looking for a metaprogramming solution that allows me to modify the class instance at runtime instead of modifying the source code that originally defined the class. A few of the solutions explain how to declare instance variables in the class definitions, but that is not what I am asking about.

推荐答案

您可以使用属性访问器:

You can use attribute accessors:

class Array
  attr_accessor :var
end

现在您可以通过以下方式访问它:

Now you can access it via:

array = []
array.var = 123
puts array.var

---

请注意，您也可以使用attr_reader或attr_writer来仅定义吸气剂或设置器，也可以这样手动定义它们:

---

Note that you can also use attr_reader or attr_writer to define just getters or setters or you can define them manually as such:

class Array
  attr_reader :getter_only_method
  attr_writer :setter_only_method

  # Manual definitions equivalent to using attr_reader/writer/accessor
  def var
    @var
  end

  def var=(value)
    @var = value
  end
end

---

如果只想在单个实例上定义它，也可以使用单例方法:

---

You can also use singleton methods if you just want it defined on a single instance:

array = []

def array.var
  @var
end

def array.var=(value)
  @var = value
end

array.var = 123
puts array.var

---

仅供参考，针对此答案的评论，单例方法很好用，以下是证明:

---

FYI, in response to the comment on this answer, the singleton method works fine, and the following is proof:

irb(main):001:0> class A
irb(main):002:1>   attr_accessor :b
irb(main):003:1> end
=> nil
irb(main):004:0> a = A.new
=> #<A:0x7fbb4b0efe58>
irb(main):005:0> a.b = 1
=> 1
irb(main):006:0> a.b
=> 1
irb(main):007:0> def a.setit=(value)
irb(main):008:1>   @b = value
irb(main):009:1> end
=> nil
irb(main):010:0> a.setit = 2
=> 2
irb(main):011:0> a.b
=> 2
irb(main):012:0> 

如您所见，单例方法setit会设置与使用attr_accessor定义的字段相同的字段@b.因此，单例方法是解决此问题的完美方法.

As you can see, the singleton method setit will set the same field, @b, as the one defined using the attr_accessor... so a singleton method is a perfectly valid approach to this question.

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