如何根据一个实例变量，使用define_method在ruby中动态创建一个方法？

Question

I just don't get the answer to this question. I want to create a method dinamically with a condition enter in the moment of the instance of the object. I can create the method with other conditions that I tested, but in other circumstances it just doesn't work. Here is my code:

class Animal
  def initialize(live, swim)
    @live = live
    @swim = swim
  end
  def live?
    @live
  end
  def swim?
    @swim
  end
end

class Bird < Animal
  def initialize(live, swim, fly)
    super(live, swim)
    @fly = fly
  end

  define_metod(:flying) {puts "flying high"} if @fly
end

Bird.new(true, true, true).flying

I tried yet to various other ways to do that. The error is that the method "flying" is not created:

Traceback (most recent call last):
main.rb:23:in `<main>': undefined method `flying' for #<Bird:0x000056382015df98 @live=true, @swim=true, @fly=true> (NoMethodError)

Answer 1

@fly in define_method(:flying) {puts "flying high"} if @fly is a class instance variable . It doesn't exist when it's referenced (when the code is parsed), so nil is returned, resulting in the define_method statement not being executed. The class instance variable @fly is unrelated to the instance variables @fly , just as @fly for one instance is unrelated to @fly for another instance.

See @Marcin's answer for how you can achieve your objective.

Answer 2

If you'd still like to do that, try like this (simplified example):

class A
  def initialize(foo)
    define_singleton_method(:flying) { "flying high" } if foo
  end
end

Then:

> A.new(true).flying
 => "flying high"
> A.new(false).flying
NoMethodError: undefined method `flying' for #<A:0x0000000589a148 @foo=false>
  from (irb):24

This way the define_singleton_method is evaluated within the context of specific instance of the class, not the class itself, so you will be able to use any instance's instance variables you wish.