; warn about long lines
(defun font-lock-set-up-width-warning (width)
  (require 'font-lock)
  (font-lock-mode 1)
  (make-local-variable 'font-lock-keywords)
  (font-lock-add-keywords
   nil
   `((,(format "^.\\{%d\\}\\(.+\\)" width)
      1 font-lock-warning-face t))))
(add-hook 'c++-mode-hook
         '(lambda () (font-lock-set-up-width-warning 80)))
(add-hook 'java-mode-hook
         '(lambda () (font-lock-set-up-width-warning 100)))
(add-hook 'python-mode-hook
         '(lambda () (font-lock-set-up-width-warning 80)))