14.2输入和输出运算符

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输入输出运算符必须是非成员函数

输入时的错误

当流含有错误类型的数据时读取操作可能失败。
当读取操作到达文件尾或者输入流的其他错误时也会失败。

#include <iostream>#include <string>using namespace std;class Sales_item{    friend ostream& operator<<(ostream& os, const Sales_item &item);    friend istream& operator>>(istream& is, Sales_item &item);public:    Sales_item(string &book, unsigned count, double price) :        isbn(book), units_sold(count), revenue(count*price){}    Sales_item() :units_sold(0), revenue(0.0){}private:    string isbn;    unsigned units_sold;    double revenue;};ostream& operator<<(ostream& os, const Sales_item &item){    os << item.isbn << " " << item.units_sold << " "        << item.revenue << " ";    return os;}istream& operator>>(istream& is, Sales_item &item){    double price;//不需要初始化，因为我们将先读取数据到price，之后才使用它    is >> item.isbn >> item.units_sold >> price;    if (is)//检查输入是否成功        item.revenue = item.units_sold*price;    else        item = Sales_item();//输入失败时对象被赋予默认的状态    return is;}int main(){    //Sales_item item(string("99-999-999"), 2, 50.00);    //cout << item << endl;    Sales_item item;    cin >> item;    cout << item;    return 0;}

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