rg_home_test.rb

--- ~ » irb
1.9.3p286 :001 > class A
1.9.3p286 :002?>   def to_modify
1.9.3p286 :003?>     "Not modified"
1.9.3p286 :004?>     end
1.9.3p286 :005?>   end
 => nil 
1.9.3p286 :006 > def redefine_method(klass, &block)
1.9.3p286 :007?>   if klass.new.respond_to?(:to_modify)
1.9.3p286 :008?>     klass.send(:define_method, :to_modify, &block)
1.9.3p286 :009?>     end
1.9.3p286 :010?>   end
 => nil 
1.9.3p286 :011 > A.new.to_modify
 => "Not modified" 
1.9.3p286 :012 > redefine_method(A) do
1.9.3p286 :013 >     puts "modified"
1.9.3p286 :014?>   end
 => #<Proc:0x007f945409bfb0@(irb):12 (lambda)> 
1.9.3p286 :015 > A.new.to_modify
modified
 => nil