C++ Primer, Fifth Edition

14.2.1. Overloading the Output Operator <<

Ordinarily, the first parameter of an output operator is a reference to a nonconst ostream object. The ostream is nonconst because writing to the stream changes its state. The parameter is a reference because we cannot copy an ostream object.

The second parameter ordinarily should be a reference to const of the class type we want to print. The parameter is a reference to avoid copying the argument. It can be const because (ordinarily) printing an object does not change that object.

To be consistent with other output operators, operator<< normally returns its ostream parameter.

The Sales_data Output Operator ...