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Metaprogramming in Ruby

Welcome back! In our previous article, we delved into Object Oriented Programming (OOP) in Ruby, discussing the core concepts such as classes and objects. Today, we’re moving onto Ruby metaprogramming, a powerful toolset that you can utilize to create dynamic and flexible code.

Metaprogramming is the technique of writing code that writes code by itself dynamically at runtime. This means you can define methods and classes during runtime, modify established classes, and create entirely new methods on the fly as you need them.

Unpacking the Concept of Metaclass

First, it’s important to understand what a metaclass is. In Ruby, every object has its own metaclass, also known as singleton class or eigenclass. The metaclass is simply a class that Ruby creates and inserts into the inheritance hierarchy for holding class methods.

Working with Metaclass

Let’s look at an example:

# Define a string object during runtime
example = "I'm a string object"Code language: PHP (php)

In this simple example, “example” is an instance of the class String. It demonstrates the flexibility of Ruby metaprogramming, where objects can be defined dynamically during runtime.

Class Methods vs. Singleton Methods

Class methods are available to all instances of a class object, while singleton methods are specific to individual instances. Despite different syntax, they function similarly.

However, it’s important to note that class methods are added to the class’s metaclass, not the class itself, which can be confusing.

Discovering the Invisible Metaclass

The concept of an unseen metaclass might sound far-fetched, but there’s a clever trick that vividly showcases its presence:

class << self endCode language: JavaScript (javascript)

This syntax shifts the current context to refer to the metaclass of the current object. Pretty cool, right?

Unlocking the Power of method_missing and define_method

Exploring method_missing

In Ruby, you’re not limited to predefined methods; a significant aspect of Ruby is crafting and employing your own methods. When you invoke a method that Ruby doesn’t recognize, it activates method_missing. This method is part of the Kernel module, inherited by every object in Ruby’s hierarchy.

Getting Creative with define_method

Ever wanted to craft methods on the fly? With define_method, you can do just that:

define_method (:method_name) { |arg| method_band.do_stuff(arg) }

This piece of code lets you call a new method at will, further proving the flexibility and dynamism that makes Ruby so popular.

Practical Usage of Ruby Metaprogramming and Its Limitations

Application in Rails Framework

Ruby metaprogramming plays a significant role in web development frameworks like Rails. For example, gems like RSpec and ActiveRecord employ it to bolster their function and perform the way they do.

You can even create your own Domain-Specific Language (DSL) to simplify complex problems!

Limitations and Cautions

Though powerful, Ruby metaprogramming should be used judiciously. Overuse can complicate your codebase and potentially confuse your teammates.

Like any tool, it’s effective when it’s the right tool for the job—but not every job. Avoid monkey patching unless it’s needed, and as always, leave comments to clarify complex sections of code.


Our foray into Ruby metaprogramming has shown how you can add dynamism and agility to your code. It’s a powerful tool in your arsenal but remember to use it sparingly and responsibly. Stay tuned for more insights into the world of Ruby and other programming languages!

Next: The Only Guide You’ll Need to Decipher Ruby Symbols.


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