But to be able to reach any private data within a user defined class you have to declare the global function as a friend within the definition of the class.

为了访问一个用户定义的类里的私有数据，你必须声明全局函数为友元函数，并且在类中定义。

youdao

It is also useful for fine-tuning the exportable interface: when you know that the client, by definition, is unable to access some private data, you need not export the entire class.

这对于微调可导出的接口也很有用：如果根据定义，客户无法访问一些私有数据，你没必要导出整个类。

youdao

Researchers at the Centre for the Economics of Education have used data on earnings, social class and education to distinguish the effects of private schooling from other advantages that students at such schools may enjoy (such as having richer, better-educated parents).

ECONOMIST: Private education