Why cool guys use obj.instance_eval instead of class << obj to define singletons?

Or why one should use class << obj if you have instance_eval? After all instance_eval is more convenient and is executed in the ‘obj’ scope (e.g. you can use instance methods), while class << obj is something odd as it is executed in the scope of the ghost class. What can be done in this scope except of defining singletons? Well, just about my favorite @@var – @@var defined inside class << obj becomes common for all classes!

class << "dog" 
  @@var = 1
end

class << Fixnum
  puts @@var     # 1
end

class String
  puts @@var     # 1
end

This means there is not ghost class between singleton ghost class and Class while there is between String and Class for example. Confusing…

And even more confusing here

def String.scream
  puts 'wow'
end

String.instance_eval { scream } # wow
String.class_eval { scream } # wow

So when we execute instance_eval and class_eval both blocks are executed in the context of String class (self = String), but ‘def method’ does different things! Well, concept ‘current class’ is reasonable, but how far this concept goes? It means there are two contexts – ‘self’ for calling methods and accessing variables and ‘current class’ for defining methods? Where else these contexts are different?

As to your first point: that’s one reason I think @@var is unusable in real code. I also suspect that example will no longer work in 1.9. In general, @@var should be avoided.

For the second point: the biggest place the two are different is at the top level of your program…

Actually @@var can be found in many gems, including ActiveSupport, ActiveRecord, haml etc. Of course usage I showed in this post is quite bizzare, but how are you going to share variable between several instances of the same class if you need to access it from instance methods? Well, through class method and class level @var, but it requires additional lines of code. And even if you do this for objects, you can’t do it for classes like it is done in ActiveRecord when all subclasses share the same variable. Maybe it is just wrong use of inheritance, don’t know.

Wow, I just realized that top level works as Object.class_eval block!

Victor:

I know people use it. I just think they shouldn’t… :)