Private function can access private variables.
这说明私有函数能访问私有变量。
Why can't this friend function access the private variables?
为什么这不可以友元函数访问私有变量？
The Book class is then defined, with private member variables for each field.
然后定义book类，用私有成员变量代表每个字段。
The Book class is then defined, with private member variables for each field.
然后定义book类，用私有成员变量代表每个字段。
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