How to understand the difference between class_eval() and instance_eval()?

Foo = Class.new
Foo.class_eval do
  def class_bar
    "class_bar"
  end
end
Foo.instance_eval do
  def instance_bar
    "instance_bar"
  end
end
Foo.class_bar       #=> undefined method ‘class_bar’ for Foo:Class
Foo.new.class_bar   #=> "class_bar"
Foo.instance_bar       #=> "instance_bar"
Foo.new.instance_bar   #=> undefined method ‘instance_bar’ for #<Foo:0x7dce8>

Just based on the name of the methods, I would expect class_eval to allow you to add a class method to Foo and instance_eval to allow you to add an instance method to Foo. But they seem to do the opposite.

In the example above if you call class_bar on the Foo class you get an undefined method error and if you call instance_bar on the instance returned by Foo.new you also get an undefined method error. Both errors seem to contradict an intuitive understanding of what class_eval and instance_eval should do.

What is really the difference between these methods?

Documentation for class_eval:

> mod.class_eval(string [, filename [, > lineno]]) => obj > > Evaluates the string or block in the > context of mod. This can be used to > add methods to a class.

Documentation for instance_eval:

> obj.instance_eval {| | block } => obj > > Evaluates a string containing Ruby > source code, or the given block, > within the context of the receiver > (obj). In order to set the context, > the variable self is set to obj while > the code is executing, giving the code > access to obj’s instance variables.

As the documentation says, class_eval evaluates the string or block in the context of the Module or Class. So the following pieces of code are equivalent:

class String
  def lowercase
    self.downcase
  end
end

String.class_eval do
  def lowercase
    self.downcase
  end
end

In each case, the String class has been reopened and a new method defined. That method is available across all instances of the class, so:

"This Is Confusing".lowercase 
=> "this is confusing"
"The Smiths on Charlie's Bus".lowercase
=> "the smiths on charlie's bus"

class_eval has a number of advantages over simply reopening the class. Firstly, you can easily call it on a variable, and it's clear what your intent is. Another advantage is that it will fail if the class doesn't exist. So the example below will fail as Array is spelt incorrectly. If the class was simply reopened, it would succeed (and a new incorrect Aray class would be defined):

Aray.class_eval do
  include MyAmazingArrayExtensions
end

Finally class_eval can take a string, which can be useful if you're doing something a little more nefarious...

instance_eval on the other hand evaluates code against a single object instance:

confusing = "This Is Confusing"
confusing.instance_eval do
  def lowercase
    self.downcase
  end
end   

confusing.lowercase
=> "this is confusing"
"The Smiths on Charlie's Bus".lowercase
NoMethodError: undefined method ‘lowercase’ for "The Smiths on Charlie's Bus":String

So with instance_eval, the method is only defined for that single instance of a string.

So why does instance_eval on a Class define class methods?

Just as "This Is Confusing" and "The Smiths on Charlie's Bus" are both String instances, Array, String, Hash and all other classes are themselves instances of Class. You can check this by calling #class on them:

"This Is Confusing".class
=> String

String.class
=> Class

So when we call instance_eval it does the same on a class as it would on any other object. If we use instance_eval to define a method on a class, it will define a method for just that instance of class, not all classes. We might call that method a class method, but it is just an instance method for that particular class.

The other answer is correct, but allow me to go in depth a little.

Ruby has a number of different kinds of scope; six according to wikipedia, though detailed formal documentation seems to be lacking. The kinds of scope involved in this question are, not surprisingly, instance and class.

The current instance scope is defined by the value of self. All unqualified method calls are dispatched to the current instance, as are any references to instance variables (which look like @this).

However, def is not a method call. The target for methods created by def is the current class (or module), which can be found with Module.nesting[0].

Let's see how the two different eval flavors affect these scopes:

String.class_eval { [self, Module.nesting[0]] } => [String, String] String.instance_eval { [self, Module.nesting[0]] } => [String, #<Class:String>]

In both cases, the instance scope is the object on which *_eval is called.

For class_eval, the class scope also becomes the target object, so def creates instance methods for that class/module.

For instance_eval, the class scope becomes the singleton class (aka metaclass, eigenclass) of the target object. Instance methods created on the singleton class for an object become singleton methods for that object. Singleton methods for a class or module are what are commonly (and somewhat inaccurately) called class methods.

The class scope is also used to resolve constants. Class variables (@@these @@things) are resolved with class scope, but they skip over singleton classes when searching the module nesting chain. The only way I have found to access class variables in singleton classes is with class_variable_get/set.

I think you got it wrong. class_eval adds the method in the class, so all instances will have the method. instance_eval will add the method just to one specific object.

foo = Foo.new
foo.instance_eval do
  def instance_bar
    "instance_bar"
  end
end

foo.instance_bar      #=> "instance_bar"
baz = Foo.new
baz.instance_bar      #=> undefined method

instance_eval effectively creates a singleton method for the object instance in question. class_eval will create a normal method in the given class's context, available to all objects of that class.

Here's a link regarding singleton methods and the singleton pattern(non-ruby specific)

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 initialzie
    @a = “a”
  end

  private

  def private_a
    puts “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.