variable project root folder in .dir-locals.el

Question

I'm using company-mode for completion in C++ code. To tell clang backend of company where the include files for the current project are I have to use the following .dir-locals.el file

((c++-mode (eval setq company-clang-arguments (append 
                                               company-clang-arguments
                                               '("-I/full/path/to/project/root/include"))))

I have to specify the full path always. Is there a way to replace full/path/to/project/root with a variable that stores the path the the .dir-locals.el file?

I tried

 (c++-mode (eval setq company-clang-arguments (append
                                               company-clang-arguments
                                               (list concat "-I" default-directory "src")))))

but since default-directory is evaluated in the file that I'm opening it doesn't return to path the the project root but some other path inside the project.

Answer 1

I found the solution with projectile. It has a function projectile-project-root which can be used to get the project path.

I can noe use the following in .dir-locals.el and it will still work when I move the project or use it on another machine.

((nil . ((eval . (progn
                   (require 'projectile)
                   (setq company-clang-arguments (delete-dups (append
                                                  company-clang-arguments
                                                  (list (concat "-I" (projectile-project-root) "src"))))))))))

Only downside is that it you need projectile and that the project has to be a projectile project (not much of a problem since every folder with git/bzr/hg/... is a valid project).

update

As pointed out in the comments it is also possible to use

(locate-dominating-file default-directory ".dir-locals.el")

instead of projectile-project-root.