C++ Primer, Fifth Edition

14.2.2. Overloading the Input Operator >>

Ordinarily the first parameter of an input operator is a reference to the stream from which it is to read, and the second parameter is a reference to the (nonconst) object into which to read. The operator usually returns a reference to its given stream. The second parameter must be nonconst because the purpose of an input operator is to read data into this object.

The Sales_data Input Operator

As an example, we’ll write the Sales_data input operator:

istream &operator>>(istream &is, Sales_data &item){    double price;  // no need to initialize; we'll read into price before we use it    is >> item.bookNo ...