That way, if the file we are compiling happens to include this header a second time, the #ifndef directive will discover that SALESITEM_H is defined and skip the rest of the header file.

• 那样的话，如果我们编译的文件恰好又一次包含了该头文件。%23ifndef指示会发现SALESITEM_H已经定义，并且忽略该头文件的剩余部分。