Does Emacs Lisp functions return objects by reference or by value?

Question

I tried to determine if Emacs returned object by reference or value, but could not find a clear answer (searching google and Emacs Lisp manual). I guess it must be "by reference", and I just overlooked it somewhere..

For example:

(defun my-get-hash ()
  (let ((h (make-hash-table :test 'equal)))
    (puthash "a" 1 h)
    h))

(let ((h2 (my-get-hash)))
  (message (prin1-to-string h2)))

Here the hash h is created with a let binding, and then return to the caller. Does the caller receive a reference or a copy of h?

Answer 1

It's by reference when it comes to complex types like vectors and lists (or, more correct, cons pairs).

Simple types like symbols and numbers are not mutable, so they can be seen as "by value".