instance_eval and class_eval in ruby

In an attempt to better understand instance_eval and class_eval, I just read Khaled’s post on Ruby reflection. It helped, and I came up with a memory crutch I can use to remember when to use each of them:

Use ClassName.instance_eval to define class methods.

Use ClassName.class_eval to define instance methods.

That’s right. Not a typo. Here are some examples, shamelessly stolen from his post:

# Defining a class method with instance_eval

Fixnum.instance_eval { def ten; 10; end }

Fixnum.ten #=> 10



# Defining an instance method with class_eval

Fixnum.class_eval { def number; self; end }



7.number #=> 7

I Like Stuff that’s Backwards

Why is it the reverse of what you might expect? Because Fixnum.instance_eval treats Fixnum as an instance (an instance of the Class class), thus any new functions you define can be called on that instance. So it’s equivalent to this:

class Fixnum

def self.ten

10

end

end



Fixnum.ten #=> 10

Fixnum.class_eval treats Fixnum as a class and executes the code in the context of that class, thus any “def” statements are treated exactly as if they were in normal code without any reflection. It’s equivalent to this:

class Fixnum

def number

self

end

end



7.number #=> 7

There are still some things about Ruby reflection that mystify me but at least I think I’ve got this one nailed.