(defun toggle-comment()
  "Toggle the // style comments at the beginning of a line"
  (interactive)
  ; Save cursor position for later.
  (setq cursor (point))
  ; Setup comment string
  (setq comment-str "//")
  (setq comment-size (length comment-str))
  (move-beginning-of-line nil)
  ; Get current line from buffer and check if it is commented or not
  ; and apply or remove comment accordingly
  (setq line-of-code (buffer-substring (line-beginning-position) (line-end-position)))
  (if (not (equal (substring line-of-code 0 comment-size) comment-str))
      (progn 
        (setq line-of-code (concat comment-str line-of-code))
        (setq cursor (+ cursor comment-size))
        )
    (if (string= (substring line-of-code 0 comment-size) comment-str)
        (progn
          (setq line-of-code (substring line-of-code comment-size nil))
          (setq cursor (- cursor comment-size))
          )
      )
    )
  ; Overwrite old line with the newly commented one.
  (kill-line)
  (insert line-of-code)
  (newline)
  (goto-char cursor)
)
 
; Shortcut to call the function
(global-set-key "\C-q" 'toggle-comment)