Here are couple more simple functions, used for commenting out a block of C++ code.

c++-comment-line below simple inserts "//" at the start of a line

c++-comment region will take the current region and run the c++comment-line on each line.


(defun c++-comment-region()
 "Comment a region out"
 (interactive)
 (save-excursion
   (save-restriction
     (narrow-to-region (point) (mark))
     (goto-char (point-min))
     (while (> (point-max) (point))
 (c++-comment-line)
 (forward-line)))))

(defun c++-comment-line()
 "Comment a line of C++ out"
 (beginning-of-line)
 (insert "//"))