Pass By Reference

Objectives

1. Get a refresher on method scope in Ruby.
2. Learn how Ruby variables can differ from the objects to which they point.
3. Understand the concept of mutability in Ruby.

str = "Hello"

...variables in Ruby (with a few exceptions, most notably variables bound to integers) don’t hold object values. str doesn’t contain “Hello”. Rather, str contains a reference to a string object. It’s the string object that has the characteristic of containing the letters that make up “Hello”.
—David Black, The Well Grounded Rubyist

Method Scope in Ruby

Let's take a moment to refresh our memory about method scope. In Ruby, methods create their own scope. By default, any variable from outside a method will be unavailable inside that method. In addition, local variables created inside a method "fall out of scope" (become unavailable) once we're outside that method.

Unavailable Variables

Let's make Douglas Adams proud and save the coveted answer to everything (42) in a variable at the top of a file. Now, let's try to print it in a method:

the_answer_to_the_ultimate_question_of_life_the_universe_and_everything = 42

def print_the_answer
  puts the_answer_to_the_ultimate_question_of_life_the_universe_and_everything
end

print_the_answer
NameError: undefined local variable or method 'the_answer_to_the_ultimate_question_of_life_the_universe_and_everything' for main:Object

Why can't the method see the answer? It's right there! It's because methods drop an imaginary "gate" around their environment, closing out all other local variables.

Hmm... well what about a variable declared inside a method body?

def what_are_you_thinking_about?
  want = "Chocolate"
  need = "A drink"
  hear = "Someone in the kitchen"
  "nothing"
end
what_are_you_thinking_about? #=> "nothing"
want
NameError: undefined local variable or method 'want' for main:Object

We encounter a similar problem when going in the opposite direction.

Opening The Gates

Methods can take arguments, and here lies something very curious. What happens if we pass a variable in as an argument to a method? What happens when that reference is mutated (changed)?

names = []

def add_to_roster(list_of_names, name)
  list_of_names << name
end

add_to_roster(names, "Steven")
add_to_roster(names, "Avi")
add_to_roster(names, "Joe")
names #=> ["Steven", "Avi", "Joe"]

Arguments act as a gateway into a method body, allowing a variable to be passed into its local scope from the external scope that calls the method.

When you pass in a variable, you're not passing in a value. You're passing in a reference to an object. If we send a message to that variable, such as << ("shovel"), it's able to respond with its programmed instructions!

The "exit gate" is the method's return, which can pass a reference out from its internal local scope back to the scope that called the method. Refer back to the what_are_you_thinking_about? method definition above. We did actually get something back: the return object, in this case a string of "nothing".

Pass by Reference vs. Pass by Value

In Ruby, objects are either passed by value or passed by reference. Primitive data like integers, floats, fixnums, and symbols require a fixed, small amount of memory. Consequently, when you pass around an integer, you are passing around the actual value. Objects that can grow and change, like arrays and strings, are never a fixed size. They are instead always accessed by a reference pointer in order to save memory use in a program. Passing around objects means passing around this reference pointer.

Mutability

Because passing by references allows the object to be of any size, objects like strings and arrays can change; they are said to be "mutable". This means that if you pass such a variable into a method, the method can operate on the value of that variable and change its value while still keeping the reference intact.

Let's take a look:

def change_the_array(array, string)
  array << string
end

some_words = ["hi", "there"]
change_the_array(some_words, "quizmaster")

puts.array.inspect
  #=> ["hi", "there", "quizmaster"]

Since all arrays in Ruby are mutable, when we pass the variable some_words (which is a reference to an array) into the method change_the_array, the method actually changes the array!

On the other hand, variables bound to objects that are passed by value, like integers, are not mutable. This means that if you pass such a variable into a method, the method cannot operate on the original variable's value to change it.

Let's take a look:

def age_me_by(start_age, age_amount)
  start_age = start_age + age_amount
end

age = 31

age_me_by(age, 60) #=> 91
age #=> 31

Here, when we passed the variable, age, into the method, age_me_by, we passed in the actual value of 31, not a reference to age. Since variables that point to integers are not mutable, our method's attempt to change the value of age did not work, so when we return to the original scope, age still has a value of 31.

Here's a handy reference regarding different datatypes and their mutability:

Data Type	Pass by...
string	reference
array	reference
hash	reference
integer	value
fixnum	value
float	value
boolean	value

Resources

• Pass By Value vs. Pass By Reference