Ifndef
如果未定义
🗣️ 情景例句
How long does your vacation last?
你的假期有多长？
I get to my office at 10:30.
十点半到办公室。
A day may begin well enough, but suddenly everything seems to get out of control.
一天开始时，可能还不错，但突然间似乎一切都失去了控制。