Ifndef
如果未定义
🗣️ 情景例句
You've been very thoughtful.
你一直都很周到。
The chance to enter the college came and he took it.
有一个上大学的机会，他抓住了这个机会。
It will help me understand the point you are trying to make.
这会帮助我了解你们的重点。