Add ruby class methods or instance methods dynamically

Question

I am quite new to Ruby, so still learning. I was researching quite a bit about how to add methods dynamically, and I was successful to create instance methods, but not successful when creating class methods.

This is how I generated instance methods:

  class B
    def before_method
      puts "before method"
    end

    def self.run(method)
        send :define_method, method do
          before_method
          puts "method #{method}"
        end
    end
  end

  class A < B
    run :m
    run :n
  end

Any idea about the best ways to create static methods?

My final task is to look for the best way to create "before" and "after" tasks for class methods.

possible duplicate stackoverflow.com/questions/752717/…

rwilliams
– rwilliams

2011-09-12 19:02:20 +00:00
Commented Sep 12, 2011 at 19:02 — rwilliams
– rwilliams, Commented Sep 12, 2011 at 19:02

Jon Snow · Accepted Answer · 2013-02-22 13:10:06Z

27

To create instance methods dynamically, try

class Foo
  LIST = %w(a b c)

  LIST.each do |x|
    define_method(x) do |arg|
      return arg+5
    end
  end
end

Now any instance of Foo would have the method "a", "b", "c". Try

Foo.new.a(10)

To define class methods dynamically, try

class Foo
  LIST = %w(a b c)

  class << self
    LIST.each do |x|
      define_method(x) do |arg|
        return arg+5
      end
    end
  end
end

Then try

Foo.a(10)

answered Feb 22, 2013 at 13:10

Jon Snow

12k9 gold badges74 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Confusion · Accepted Answer · 2011-09-13 07:35:14Z

7

Instance methods of an objects singleton class are singleton methods of the object itself. So if you do

class B
  def self.run(method)
    singleton_class = class << self; self; end
    singleton_class.send(:define_method, method) do
        puts "Method #{method}"
    end
  end
end

you can now call

B.run :foo
B.foo 
=> Method foo

(Edit: added B.run :foo as per Lars Haugseth's comment)

edited Sep 13, 2011 at 7:35

answered Sep 12, 2011 at 19:45

Confusion

17k8 gold badges51 silver badges73 bronze badges

4 Comments

Lars Haugseth Over a year ago

You'll want to do B.run :foo before you can call B.foo.

Lars Haugseth Over a year ago

You can also avoid send by using eigenclass.class_eval { define_method(method) { ... }}

Marc-André Lafortune Over a year ago

Edited to use the official terminology of singleton_class, introduced in Ruby 1.9.2

Confusion Over a year ago

@Marc-André: ah, I wasn't aware there finally was a standard; we're still at 1.8.7. Now I'll have to unlearn our company standard 'eigenclass' :).

tadman · Accepted Answer · 2014-01-21 20:03:22Z

2

Here's something re-worked to use class methods:

class B
   def self.before_method
     puts "before method"
   end

  def self.run(method)
    define_singleton_method(method) do
      before_method
      puts "method #{method}"
    end
  end
end

Update: Using define_singleton_method from Ruby 1.9 which properly assigns to the eigenclass.

edited Jan 21, 2014 at 20:03

answered Sep 12, 2011 at 19:07

tadman

212k23 gold badges237 silver badges266 bronze badges

9 Comments

purbon Over a year ago

Could you please tell me why its an edge case? I am quite new and would love to learn, xD!

tadman Over a year ago

People rarely create dynamic instance methods, and dynamic class methods are even rarer still. Sadly, the semantics are kind of ugly because of how infrequently it comes up. There's nothing wrong with doing it, but most people never will, that's all.

Confusion Over a year ago

This probably isn't what you want: you've now added the method to Class, which means it is available in every class (B.class == Class). This is a problem especially because not every class has a 'before_method'. Try calling B.run 'foo', defining a class A and calling A.foo. It will fail with NameError: undefined local variable or method before_method' for A:Class`

Dave Newton Over a year ago

I thought people created dynamic methods fairly frequently in Ruby; that's where a lot of the "magic" comes from. Am I confusing terminology?

tadman Over a year ago

The edit here with define_singleton_method puts the method in the correct context.

|

Collectives™ on Stack Overflow

Add ruby class methods or instance methods dynamically

3 Answers 3

Comments

4 Comments

9 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

4 Comments

9 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related