What I have learned so far about OO Ruby

From what I’ve understood Object-Oriented programming is very popular and it has lots of advantages. The programmer may have a closer feeling of working with real-life entities due to the multitask nature of the objects in OOP. From my point of view OOP is a good way to keep all your code organise. Each object contains its data and functionality within its scope and if its methods are written in an abstract way they can be reused for other objects or for a different project.

I am still working in the OO Ruby section and it is taking me some time to understand all the concepts and why or when to use them. Here it is what I have learned so far:

Creating a class

Class names begin with capital letters because they are stored in Ruby constants. If your class name contains two words, the name should be CamelCased.


Every time you want to represent a new individual in your program you should instantiate an object of that type (Dog.new). We call these individuals instances.

Instances variables

An instance variable represents a singular property of an individual, of an instance. This variable is accessible in any instance method in a particular instance of a class. The name of the instance variabe is written with a @ at the beginning of the word: @instance_variable_name.

Instance methods

We call the methods defined within the object’s class instance methods because they are methods that belong to any instance of the class.

They provide a mechanism for an object to expose to the outside world its data. They can manipulate the internal state of an object and provide some functionality for an individual object too.

Attribute readers, writers and accesors

Our object’s instances can have attributes, like a name or hair colour in the case of our Dog class. In order to set these attributes we have to use a writer and in order to have these data available in the outside world we have to have a reader. It is very important to use an instance variable(@name), otherwise the getter method couldn’t read the data that we set in the writer method because it is out of scope.

This is the explicit method for the attribute @name. There is another faster way to do it using a macro: the attr_writer, the attr_reader and the attr_accessor that does the work of the two previous macros. In Ruby a macro is like a method that instead of returning a Ruby datatype returns more Ruby code. The implementation of macros is considered metaprogramming.

When to use attr_accessor vs a custom reader/writer.

The attr_accessor macro will provide a naïve interface for reading and writing to an instance variable. Anytime we need something conceptually different or more complex, we should build our own reader or writer.


Whenever an object needs to refer to itself, the keyword “self” is used.

In an instance method, self will always refer to the instance itself and in a class method, self will always refer to the class itself.

Class variable

Class variables are for properties that belong to the entire class. A class level variable can be used for example for instance memoization, for keeping track of all instances of a class or for collaborating data. The name of the class variable is written with a double @ at the beginning of the word: @@class_variable_name.

#self.all is a class method for reading data stored in the class variable @@all. This is a class reader, very similar to an instance reader method that reads an instance property.

Class methods

With these methods we can access any class variable. A class method operate on the entire class and expose the class’ scope to the rest of our program. They don’t have access to the instance scope.

The most common class methods are:

Class finders

A class finder method returns existing instances.

Custom class constructors

A class constructor is any class method that instantiates an instance of the class.

Class operators

A class operator is perfect to manipulate any class level data.