How to use an istream in a class member function?

Hi,

I learn C++ from a C++ Primer book. I find its class header file having a member
function:

    Sales_item(std::istream &is) { is >> *this; }

but I cannot find its application code to call it. I have tried myself, but it
fails because I uses file input stream does not match the std::istream type.
I do not know an appropriate input stream even after I see the inheritance map
of istream.

From this definition, it should be called.

Can you show me how to call this member function?

Thanks,

#include <iostream>
#include <string>

class Sales_item {
friend bool operator==(const Sales_item&, const Sales_item&);
// other members as before
public:
    // added constructors to initialize from a string or an istream
    Sales_item(const std::string &book):
              isbn(book), units_sold(0), revenue(0.0) { }
    Sales_item(std::istream &is) { is >> *this; }
    friend std::istream& operator>>(std::istream&, Sales_item&);
    friend std::ostream& operator<<(std::ostream&, const Sales_item&);
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);
    // other members as before
    
public:
    // operations on Sales_item objects
    double avg_price() const;
    bool same_isbn(const Sales_item &rhs) const
        { return isbn == rhs.isbn; }
    // default constructor needed to initialize members of built-in type
    Sales_item(): units_sold(0), revenue(0.0) { }
// private members as before
private:
    std::string isbn;
    unsigned units_sold;
    double revenue;

};