Set encoding new file for some extensions.

set-encoding.el

(defun my-set-encoding ()
  "新規ファイル生成時のみ、エンコードを強制する Thanks for Syohei YOSHIDA"
  (let ((bufname (buffer-file-name)))
		(when (not (file-exists-p buffer-file-name))
			(when (and bufname (string-match-p ".+\\.\\(cpp\\|c\\|hpp\\|h\\|mm\\)$" bufname))
				(set-buffer-file-coding-system 'utf-8-with-signature)))))

(add-hook 'find-file-hook 'my-set-encoding)