Parameter Passing

Ruby uses the pass by value for parameter passing, but the passing value is the reference to objects [1].

parameter_passing.rb

def change_string(str)
  str = "Modified String"
end

str_a = "String" 
change_string(str_a)
puts str_a  # Print "String"

def change_string_insert(str)
  str.insert(0, "Real Modified ")
end

change_string_insert(str_a)
puts str_a  # Print "Real Modified String"

Source code: https://github.com/yc015/Ruby-Inspect/blob/main/src/parameter_passing.rb

The codes above will print:

String
Real Modified String

In the example above, the argument str passed to the function change_string() is a copy of reference to the str_a. Assigning an address of new string object "Modified String" to the argument str won’t affect the content on the original address, address of str_a, that str previously points to.

In function change_string_insert(), we invoke a method of the string object that str points to, and that method makes an inplace change to the string object. Thus, we can see the content of str_a is changed from "String" to "Real Modified String".

Reference:
[1] https://mixandgo.com/learn/is-ruby-pass-by-reference-or-pass-by-value Is Ruby pass-by-reference or pass-by-value? - Mix & Go