Instance Variables

Now that we’ve learned how the string that we pass to the new method passes to the new object’s initialize method, we can start improving initialize. We want it to do something with the string and actually initialize our new object:

class Person
  def initialize(name)
    @name = name
  end
end

This introduces a new type of variable called an instance variable. The @name is what we call an instance variable. The @ symbol at the beginning of the variable name is what tells us that @name is an instance variable.

The body of the initialize method now does nothing else but assign the value of the local name variable to the @name instance variable.

Local scope versus an object’s scope

Remember that each method has its own local scope, which is created when it’s called and populated with local variables from the arguments list. We’ve also learned that this scope is erased when Ruby exits the method body and returns from the method.

Note: Local variables that are visible in one method aren’t visible in other methods. That’s why they’re called local.

Every object also has its own scope.

An object’s scope is populated with instance variables at the moment we assign something to them. They’re also visible everywhere in the object, that is, in every method that the object has.

Remember: Instance variables live in, and are visible everywhere in, the object’s scope.

Think of an object’s scope as its own knowledge, or memories.

Analogy

For example, you know your name, your email address, and your email password. You keep this knowledge around, and you can use it when you do things (such as responding to another person). Likewise, an object keeps its instance variables around as long as the object exists.

Let’s see how that works in practice.

If we create and output an instance of our class Person, we’ll see that Ruby now prints out the instance variable too:

The first line creates a new instance of the Person class, passing the "Ada" string and assigning this new object to the person variable. The second line then prints it out:

#<Person:0x007fd8947aa868 @name="Ada">

Notice that this includes the @name instance variable with the "Ada" value. This specific, concrete instance of the Person class knows their name.

Think of this process as a programmer creating and immediately naming a new person.