How can I set directory local variable in relative to .dir-locals.el file location?

Question

I often find myself in the situation where I'm told to put absolute paths as directory local variable. For example cmake-ide's cmake-ide-project-dir and cmake-ide-build-dir. Of course this is not very portable.

So instead of

.dir-locals.el:

((nil . ((cmake-ide-project-dir . "/home/user/code/project"))))

I want something like

((nil . ((cmake-ide-project-dir . directory-of-current-dir-locals-file))))

How can I define such a variable directory-of-current-dir-locals-file? And how would I set for example cmake-ide-build-dir, which is typically (concat directory-of-current-dir-locals-file "build")?

Answer 1

My solution so far (based on this Stackoverflow answer):

.dir-locals.el:

;;; Directory Local Variables
;;; For more information see (info "(emacs) Directory Variables")

((nil . ((eval . (set (make-local-variable 'my-project-path)
                      (file-name-directory
                       (let ((d (dir-locals-find-file ".")))
                         (if (stringp d) d (car d))))))
         (cmake-ide-project-dir . my-project-path)
         (eval . (setq cmake-ide-build-dir (concat my-project-path "build")))
         )))

Answer 2

You could define variable with (defvar directory-of-current-dir-locals-file "/home/user/code/project").