Right now it's empty, actually it's not quite empty, it has a pointer back to the class.

当我调用了这个类的，定义声明的时候，它在内存中的特定点。

麻省理工公开课 - 计算机科学及编程导论课程节选

if we move up one of our electrons into an empty p orbital, what were going to see is now we have three unpaired electrons that are ready for bonding.

教授：嗯，如果我们把其中,一个电子填入p轨道。

麻省理工公开课 - 化学原理课程节选