×××××sales_data通过类型转换形成的临时对象不能转换成sales_data &(非常量引用)

文件   sales_data.h

#include <iostream>
#include <string>
using namespace std;
struct sales_data{
public:
  sales_data();            
  sales_data(const std::string &s,unsigned n,double p); 
  sales_data(std::string &s);
  sales_data(istream &is);
  friend istream &read(istream &is,sales_data &s);
  friend ostream &print(ostream &os,sales_data &s);
  string isbn()const;
  sales_data &combine(sales_data &);
  double avg_price()const;
  ostream & dis(ostream & os)const;
private:
  std::string bookno;
  unsigned sales_cnt ;
  double revenue  ;

};#include <iostrea