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 ...
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