《C++ Primer (5th edition)》 P230 ，CHAPTER 7 中提到类似于<<,>>的read，print函数是作为Sales_data的接口组成部分的非成员函数，请问该如何理解这句话？
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In human words, read and write are non-member functions for sales_data
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In human words, read and write are non-member functions for sales_data