编译时报错“ undefined reference to”

报错

写了一段小程序，报错“exception.cpp:(.text+0x28): undefined reference to `Sales_item::setPrice(double)“

程序就算c++ prime中得测试程序：

程序代码

//*********Sales_Item.h

#ifndef SALES_H
#define SALES_H

#include <string>
using namespace std;
class Sales_item
{
        public:
                Sales_item();
                Sales_item(double p,double q);
                inline double getPrice();
                inline double getQuatity();
                inline void setPrice(double p);
                void setQuatity(double q);
                double operator + (Sales_item& s2);
        private:
                string Id;
                double price;
                double quatity;
};

#endif

//********Sales_Item.cpp

 #include "Sales_item.h"
  2 #include<iostream>
  3 
  4 //using namespace std;
  5 
  6 Sales_item::Sales_item(){}