dynamically defining instance methods but not class methods

Question

Ruby 1.9.3: I am trying to implement a method which takes as argument a class, a symbol and a proc and defines (eventually overwriting) a new instance method for that class, but not a class method so that this test should pass:

require 'test/unit'

include Test::Unit::Assertions

class String
  def my_method
    'my_method'
  end
end

def define_instance_method(klass, method, &block)
  # klass.send :define_method, method, &block
  # ...
end

define_instance_method(Object, :my_method) { 'define_instance_method my_method' }
define_instance_method(String, :my_method) { 'define_instance_method my_method' }

assert Object.new.my_method == 'define_instance_method my_method'
assert_raise(NoMethodError) { Object.my_method }

assert String.new.my_method == 'define_instance_method my_method'
assert_raise(NoMethodError) { String.my_method }

I did many tryings (define_method, define_singleton_method, class_eval, instance_eval, instance_exec, module_eval...) but with no success; do you have any ideas?

score 3 · Accepted Answer · 2012-12-02 00:36:52Z

@ProGNOMmers, recall that objects in Ruby are classes and classes are objects.

So everything in Ruby are objects, even classes :)

And when you define a method inside Object it will also be available to Class as well as to any class that inherits from Object.

That's mean to all classes.

Some proof:

define_instance_method(String, :my_method) { 'define_instance_method my_method' }

p String.new.my_method
p String.my_method

# =>  "define_instance_method my_method"
# =>  stackoverflow.rb:11:in `<main>': undefined method `my_method' for String:Class (NoMethodError)

See, your method not yet defined inside Object, thus not available inside String.

Now let's do it:

define_instance_method(Object, :my_method) { 'define_instance_method my_method' }

p String.my_method
# => "define_instance_method my_method"

p Array.my_method
# => "define_instance_method my_method"

p NilClass.my_method
# => "define_instance_method my_method"

So just comment define_instance_method(Object... in your tests
and String specs will pass just well.

Here are some crash tests

sohaibbbhatti · Accepted Answer · 2012-12-01 23:51:52Z

1

class_eval should work for you. Let me know why it isn't working for you

class A
end

A.class_eval do
  define_method(:foo) {puts 'hello'}
end

A.new.foo #hello
A.foo     #NoMethodError: undefined method `foo' for A:Class

answered Dec 1, 2012 at 23:51

sohaibbbhatti

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1 Comment

mdesantis Over a year ago

It doesn't work: Object.my_method does not raise a NoMethodError

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Here are some crash tests

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