C ++ oops创建头文件undefine引用

Question

我正在尝试为类编写头文件，但每当我使用命令g++ test.cpp Date.c链接文件时，我得到的输出为1

以下是日期类

的头文件和.cpp文件的代码

Date.h

#include<string.h>
using namespace std;
#ifndef DATE_H
#define DATE_H

       /*   string temp[]={ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
         // vector<string> months{ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
          vector<string> months(temp,temp+sizeof(temp)/sizeof(temp[0]));
       */   
           string intToString(const int& n);

          class  Date{
            //Private members
            string month;
            int day;
            int year;

            bool isValidMonth() const;
            int daysInMonth(const string& m) const;
            bool isLeapYear() const;

            public:

            friend istream& operator >>(istream& is, Date& d);
            friend ostream& operator <<(ostream&os ,const Date& d);
            //public constructor
            Date(const string& m="January",const int& d=1,const int& y=2000);

            void setMonth(const string& m);
            void setDay(const int& d);
            void setYear(const int& y);

            string getMonth()const;
            int getDay() const;
            int getYear() const;
            void Month();
            bool isValidDate() const;
            string toString();
          };




#endif       

Date.cpp

      #include<iostream>
      #include<vector>
      #include<string.h>
      #include<ctype.h>
      #include<algorithm>

      #include "Date.h"
      using namespace std;

      string temp[]={ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
     // vector<string> months{ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
      vector<string> months(temp,temp+sizeof(temp)/sizeof(temp[0]));

       string intToString(const int& n){
        return to_string(n);           //inbuilt function
        //return string(itoa(n));
      }


        bool Date::isValidMonth() const{
          if(day>=1 && day<=daysInMonth(month)){
            return true;
          }
          else{
            return false;
          }
        }

        int Date::daysInMonth(const string& m) const{
          vector<string>::iterator m_index;
         // while(find(months.begin(),months.end(),m)!=months.end()){
            m_index=find(months.begin(),months.end(),m);
         // }
          int month_index=distance(months.begin(),m_index)+1;
          switch(month_index)
          {
            case 2: return isLeapYear()?29:28;

            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                   return 31;

            default:
                   return 30;
          }  
        }


        bool Date::isLeapYear() const{
          if(year%4==0){
            if(year%100==0 && year%400==0){       //2000 is leap year but 1900 is not
                return true;
              }
            else{
              return false;
            }
          }
          else{
            return false;
          }
        }


        Date::Date(const string& m,const int& d,const int&y){
          month=m;
          day=d;
          year=y;
        }

        void Date::setMonth(const string& m){
            month=m;
        }
        void Date::setDay(const int& d){
            day=d;
        }
        void Date::setYear(const int& y){
            year=y;
        }

        string Date::getMonth()const{
            return month;
        }
        int Date::getDay() const{
          return day;
        }
        int Date::getYear() const{
            return year;
        }
        void Date::Month(){
            if(!month.empty())
            {
                month[0]=toupper( month[0] );
                for(int i=1;i<month.length();++i)
                    month[i]=tolower(month[i]);
            }
        }
        bool Date::isValidDate() const{
            //calls isValidMonth() to check if days match 
            if(isValidMonth() ){
              return true;
            }
            return false;
        }

      string Date::toString(){
            string result_date;
            result_date=intToString(day)+"-"+month+"-"+intToString(year);
            return result_date;
        }

      istream & operator >>(istream & is, Date& d){
        //overloads the >>operator, which reads 
        cout<<"\n Enter the month of the date \n";
        cin>>d.month;
        cout<<"\n Enter the day of  the date \n";
        cin>>d.day;
        cout<<"\n Enter the year of the date \n";
        cin>>d.year;
        return is;
      }

      ostream & operator <<(ostream & os ,const Date& d){
        os<<d.month<<" "<<d.day<<","<<d.year;
        return os;
      }

当我从.h文件中删除string.h时，我得到string does not name a type error
以及如何申报朋友职能

        istream & operator >>(istream & is, Date& d){
        //overloads the >>operator, which reads 
        cout<<"\n Enter the month of the date \n";
        cin>>d.month;
        cout<<"\n Enter the day of  the date \n";
        cin>>d.day;
        cout<<"\n Enter the year of the date \n";
        cin>>d.year;
        return is;
        }           

和const函数bool isValidMonth() const;