web-dev-qa-db-ja.com

/home
/日本語
/ifndef

ifndef

マクロがまだ定義されていない場合にのみマクロを定義するのはなぜですか？

Content dated before 2011-04-08 (UTC) is licensed under CC BY-SA 2.5. Content dated from 2011-04-08 up to but not including 2018-05-02 (UTC) is licensed under CC BY-SA 3.0. Content dated on or after 2018-05-02 (UTC) is licensed under CC BY-SA 4.0. | Privacy