instance_eval and class_eval allow you to execute a chunk of code. So what you might say? The old-fashioned eval can do this. But instance_eval and class_eval accept a block argument for the chunk of code. So the chunk of code does not need to be a string. Also instance_eval and class_eval allow a receiver (unlike the old eval). Consequently, you can invoke these two modern methods on a class object or even on an instance object.
class A
end
A.instance_eval do
# self refers to the A class object
self
end
a = A.new
a.instance_eval do
# self refers to the a instance object
self
end
Also remember in ruby if we call a method without a receiver, then the method would be invoked on self, which in the instance_eval block is the object we invoked instance_eval on. instance variables are private in ruby. You cannot access them outside the class they are defined in. But since instance variables are stored in self, we can access them in instance_eval (the same applies for private methods which cannot be invoked with a receiver):
class A
def initialize
@a = “a”
end
private
def private_a
return “private a”
end
end
a = A.new
puts a.instance_eval { @a }
# => “a”
puts a.instance_eval { private_a }
# => “private a”
We can also add methods to the receiver in instance_eval and class_eval. Here we add it to instance_eval:
class A
end
A.instance_eval do
def a_method
puts “a method”
end
end
A.a_method
# => a method
Now think what we just did for a moment. We used instance_eval, defined a method in its block, and then invoked the method on the class object itself. Isn’t that a class method? Think of it as a “class” method if you want. But all we did was define a method on the receiver in the instance_eval block, and the receiver happened to be A. We can easily do the same on a instance object:
a.instance_eval do
def a_method
puts "a method"
end
end
a.a_method
# => a method
And it works just the same. Don’t think of class methods as class methods in other languages. They are just methods defined on self, when self happens to be a class object (extending from Class.new as in class A end).
But I want to take this answer a little bit more deeper than the accepted answer. Where does instance_eval actually stick the methods you place into them? They go into the singleton class of the receiver! As soon as you invoke instance_eval on the receiver, the ruby interpreter opens the singleton_class and places the methods defined in the block inside this singleton_class. It is just like using extend in a class (because extend opens up the singleton class and places the methods in the module passed to extend into the singleton class)! It opens up the singleton_class, which is part of the inheritance hierarchy (right before the parent class): A -> singleton_class -> Parent
Now what makes class_eval different? class_eval can only be called on classes and modules. self still refers to the receiver:
class A
end
A.class_eval do
# self is A
self
end
But unlike instance_eval, when you define methods in the class_eval block, they will be available on instances of the class not the class object itself. With class_eval, the methods are not added to the singleton class in the inheritance hierarchy. Instead the methods are added to the current class of the receiver! So when you define a method in class_eval, that method goes directly into the current class and, thus, it becomes an instance method. So you cannot call it on the class object; you can only call it on instances of the class object.
