Demystifying class_eval and instance_eval in Ruby

Ruby gives you the ability to modify a class or an object on-the-fly. You may have seen examples of code written like the examples below.

Example 1: instance_eval

Example 2: class_eval

At first glance, the two blocks of code look very similar. But if you look closely, you’ll see that the first example is calling instance_eval on person, an instance of the class Person. The second example is calling class_eval on the actual Person class.

So what is the difference, you might ask?


Let me illustrate.

Suppose we have the Person class defined and have created two instances of Personperson1 and person2.

Now let’s apply this code:

What happens when we call How about

If you guessed returns “Bob”, then you are correct. But does return “Jane”?

Let’s see:

That’s right – instance_eval only evaluates in the context of one object. Only the particular instance that instance_eval is called on is injected with the name method. In this case, person1 has access to the name method, but not person2.

Now you might be tempted to call instance_eval on person2, which will make the name method accessible to person2. But what happens if you have person3 and person4? It doesn’t make sense to duplicate code. We want to make sure that the method is shared by all objects of the Person class.

If you guessed to use class_eval, you’re right. class_eval defines an instance method for the class, so that all instances of that class have access to the method. Let’s see:

All instances of the Person class, new and old, can now call name. Defining a method with class_eval is syntactically the same as defining it in the actual class.

is exactly equivalent to:

I hope this has helped your understanding of Ruby’s instance_eval and class_eval methods. Just remember that you can only call instance_eval on an object of a class and class_eval on a class. The instance_eval method defines a method for one object only, whereas the class_eval method defines it for ALL objects or instances of a class.

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