;; The help for 'c++-mode' says
;;
;;    The hook `c-mode-common-hook' is run with no args at mode
;;    initialization, then `c++-mode-hook'.
;;
;; So, this could be dlr-c-mode-hook, or dlr-c-mode-common-hook, or
;; whatever, depending on how many hooks you plan to add it to.  Its
;; name doesn't matter, of course, it's just a convention.
(defun dlr-c++-mode-hook ()
  (setq indent-tabs-mode nil))

(add-hook 'c++-mode-hook 'dlr-c++-mode-hook)