The Book of Ruby

Special Types of eval

There are some variations on the eval theme in the form of the methods named instance_eval, module_eval, and class_eval. The instance_eval method can be called from a specific object, and it provides access to the instance variables of that object. It can be called either with a block or with a string:

instance_eval.rb

class MyClass
 def initialize
   @aVar = "Hello world"
 end
end

ob = MyClass.new
p( ob.instance_eval { @aVar } )         #=> "Hello world"
p( ob.instance_eval( "@aVar" ) )        #=> "Hello world"

The eval method, on the other hand, cannot be called from an object in this way because it is a private method of Object (whereas instance_eval is public):

p( ob.eval( "@aVar" )  )    # This won't work!

In fact, you could explicitly change the ...