Ifndef
如果未定义
🗣️ 情景例句
Most people I meet have some English ability, but the ones who work hard at it really stand out from the crowd.
我遇到的多数人都会点英语，但只有那些刻苦学习的人才鹤立鸡群，给我深刻印象。
Shall we take a break?
我们是否应该休息一下？
What is the name of the machine?
机器的名字叫什么呢？