Try assigning one string to two variables, then changing it through one of them. Both variables appear to change. Ruby has not copied the string: each variable holds a reference to the same object. Assigning one variable to another copies the reference, not the object. section 2.5.1
def say_goodbye
str = "Hello"
abc = str
str.replace("Goodbye")
puts str
puts abc
end
say_goodbye
Here, `abc = str` gives both variables a reference to one string. `replace` changes that string in place, so both `puts` calls print `Goodbye`. section 2.5.1
Reassignment is different
With `str = "Hello"; abc = str; str = "Goodbye"`, `str` prints `Goodbye` but `abc` still prints `Hello`. The second assignment binds `str` to a different string; it neither changes the old string nor redirects `abc`. The same assignment rule applies beyond local variables. section 2.5.2
Method arguments also pass object references. If a method calls `replace` on its string argument, the caller can see the changed string afterward. Pass `s.dup` when the method should receive a separate object, or use `freeze` to prevent changes to an object. `clone` differs from `dup` when the original is frozen: the clone stays frozen. section 2.5.3
>> numbers = ["one", "two", "three"]
=> ["one", "two", "three"]
>> numbers.freeze
=> ["one", "two", "three"]
>> numbers[2] = "four" 1
RuntimeError: can't modify frozen array 1
>> numbers[2].replace("four") 2
=> "four"
>> numbers
=> ["one", "two", "four"] 3
Freezing an array prevents replacing one of its elements, but does not freeze the strings it references. Changing an element’s string with `replace` remains possible. section 2.5.3
To check whether two variables identify one object, compare their objects’ `object_id` values. Two strings can display identical text yet be different objects. There are also exceptions to the usual reference explanation: Ruby handles certain values, including some integers, symbols, `true`, `false`, and `nil`, as immediate values. section 2.3.1 section 2.5.1
