//在C++ Primer中文版230页Sales_data类中，有个问题不太明白。当接口函数只有 combine的时候，编译成功，一旦加入 read 和 print两个非成员接口函数，则编译报错，请指教

ifndef SALES_DATA_H_INCLUDED

define SALES_DATA_H_INCLUDED

include <iostream>

include<string>

struct Sales_data{

std::string isbn()const {return bookNo;}
Sales_data& combine(const Sales_data &rhs);
std::string bookNo;
unsigned uints_sold =0;
double revenue = 0.0;

};
Sales_data& Sales_data::combine(const Sales_data &rhs)

{
uints_sold += rhs.uints_sold;
revenue += rhs.revenue;
return *this;

}
std::istream &read (istream &is , Sales_data &item)
{

double price = 0;
is >> item.bookNo >> item.uints_sold >> price;
item.revenue = price * item.uints_sold ;
return is;

}
std::ostream &print(ostream &os, const Sales_data &item)
{

os << item.isbn() << " " << item.uints_sold << " " << item.revenue << " ";
return os;

}

endif // SALES_DATA_H_INCLUDED

---

请在 #include <string>后面加上一句using namespace std;

贴代码的时候不要直接贴，使用添加代码的功能。

#ifndef SALES_DATA_H_INCLUDED
#define SALES_DATA_H_INCLUDED
#include <iostream>
#include <string>

using namespace std;

struct Sales_data {
    std::string isbn() const
    {
        return(bookNo);
    }
    Sales_data & combine( const Sales_data &rhs );
    std::string    bookNo;
    unsigned    uints_sold    = 0;
    double        revenue        = 0.0;
};
Sales_data & Sales_data::combine( const Sales_data &rhs )
{
    uints_sold    += rhs.uints_sold;
    revenue        += rhs.revenue;
    return(*this);
}

std::istream &read(istream &is, Sales_data &item )
{
    double price = 0;
    is >> item.bookNo >> item.uints_sold >> price;
    item.revenue = price * item.uints_sold;
    return(is);
}

std::ostream &print( ostream &os, const Sales_data &item )
{
    os << item.isbn() << " " << item.uints_sold << " " << item.revenue << " ";
    return(os);
}

#endif /* SALES_DATA_H_INCLUDED */