一个奇怪的错误错误4error LNK1120: 3 个无法解析的外部命令G:\visual studio2010\visual studio 2010
错误1error LNK2019: 无法解析的外部符号 "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Sales_item const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABVSales_item@@@Z)，该符号在函数 _main 中被引用G:\visual studio2010\visual studio 2010 编程实例\lianxi2\lianxi2\sales_item.objlianxi2
错误2error LNK2019: 无法解析的外部符号 "class Sales_item __cdecl operator+(class Sales_item const &,class Sales_item const &)" (??H@YA?AVSales_item@@ABV0@0@Z)，该符号在函数 _main 中被引用G:\visual studio2010\visual studio 2010 编程实例\lianxi2\lianxi2\sales_item.objlianxi2
错误3error LNK2019: 无法解析的外部符号 "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class Sales_item &)" (??5@YAAAV?$basic_istream@DU?$char_traits@D@std@@@std@@AAV01@AAVSales_item@@@Z)，该符号在函数 _main 中被引用G:\visual studio2010\visual studio 2010 编程实例\lianxi2\lianxi2\sales_item.objlianxi2

C/C++ code

#include "Sales_item.h"#include <iostream>#include <string>using namespace std;inline    Sales_item operator+(const Sales_item& lhs, const Sales_item& rhs) {    Sales_item ret(lhs);  // copy lhs into a local object that we'll return    ret += rhs;           // add in the contents of rhs     return ret;           // return ret by value}inline    bool operator==(const Sales_item &lhs, const Sales_item &rhs){    // must be made a friend of Sales_item    return lhs.units_sold == rhs.units_sold &&        lhs.revenue == rhs.revenue &&        lhs.same_isbn(rhs);}inline bool     operator!=(const Sales_item &lhs, const Sales_item &rhs){    return !(lhs == rhs); // != defined in terms of operator==}using std::istream; using std::ostream;// assumes that both objects refer to the same isbninline    Sales_item& Sales_item::operator+=(const Sales_item& rhs) {    units_sold += rhs.units_sold;     revenue += rhs.revenue;     return *this;}// assumes that both objects refer to the same isbninline    istream& operator>>(istream& in, Sales_item& s){    double price;    in >> s.isbn >> s.units_sold >> price;    // check that the inputs succeeded    if (in)        s.revenue = s.units_sold * price;    else         s = Sales_item();  // input failed: reset object to default state    return in;}inline    ostream& operator<<(ostream& out, const Sales_item& s){    out << s.isbn << "\t" << s.units_sold << "\t"         << s.revenue << "\t" <<  s.avg_price();    return out;}inline    double Sales_item::avg_price() const{    if (units_sold)         return revenue/units_sold;     else         return 0;}

感觉莫名其妙呀，代码都没写错呀！谢谢大家了

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