Ruby’s Pass by Reference of the Value

As seen in Pass by Reference vs Pass by Value, there are mainly two ways of dealing with objects passed into methods: Pass by Reference and Pass by Value.

However, Ruby does things slightly differently:

When passing a object to a method, the object is scoped to this method definition level. Reassigment of the object won’t affect the original object. However, when performing an action that mutates the caller, the original object is altered as well.

This is called Pass by Reference Value, Pass by Reference of the Value, Pass by Value of the Reference or Call by Sharing.

The different naming are utterly confusing but they mean that Ruby passes copies of the references.

Caveat

Immutable object, like literals or boolean, can’t be mutated and are therefore not affected (see Mutable and Immutable Objects).
Only operations which mutate the caller (destructive) alter the original object. Unfortunately there is no rule as to which operation are destructive or not. Some methods end with a ! but this is just a naming convention, not a hard rule.
Ruby’s variable don’t contains objects but references to objects. A literal will be converted first into an object, then, internally, create a reference to the object. Those literal references are called anonyous references.

Examples

Equivalent of a Pass by Value in Ruby:

def assign(list, animal)
  list + [animal]
end

list = %w(flamingo capybara)
assign(list, 'red panda')
p list # ["flamingo", "capybara"]

Equivalent of a Pass by Reference in Ruby:

def mutate(list, animal)
  list << animal
end

list = %w(flamingo capybara)
mutate(list, 'red panda')
p list # ["flamingo", "capybara", "red panda"]

Tips

Every object in Ruby has a unique object ID: strings, literals, booleans, nil, range etc. Using Ruby’s $object_id is useful to inspect an unexpected value in a program.

>> 7.object_id
=> 15

>> true.object_id
=> 20

>> nil.object_id
=> 8

>> 'flamingo'.object_id
=> 180

>> (1..100).object_id
=> 13320

Mental Model

Ruby is neither Pass by Value or Pass by Reference, Ruby is:

Pass by Value for immutable objects:

def increment(a)
  a = a + 1
end

b = 3
puts increment(b)  # 4
puts b             # 3

Pass by Reference for mutable objects:

def append(s)
  s << '*'
end

t = 'abc'
puts append(t)  # abc*
puts t          # abc*

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