Re: 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;

};