朋友功能无法访问私人会员

Question

我的ostream friend功能正常工作，但我的ofstream和ifstream正常工作，因为给出的错误表明不能通过ofstream或ifstream函数访问不同的私有成员。请帮忙。

class Module 
{ 
public: 
   Module() = default; 
   explicit Module(const string& purpose, const string& id)  
   {  
      this->purpose = purpose; 
      this->id = id; 
   } 
   explicit Module(const string& purpose) { this->purpose = purpose; } 
   ~Module() { } 

   virtual void setCrew( ) = 0; 
   virtual void display( ) = 0; 
   void addCrew(Crew c) { crew_list.push_back(c); } 
   void setPowerReq(double power) { this->power = power; }  
   void setMaxCrew(int max_crew) { this->max_crew = max_crew; } 

   string getPurpose( ) { return purpose; } 
   string getId( ) { return id; } 
   int getNumberOfCrew( ) { return number_of_crew; } 
   int getMaxCrew( ) { return max_crew; } 
   double getPower( ) { return power; }  

   friend ostream& operator << ( ostream& os, const Module& m ); 
   friend ofstream& operator << ( ofstream os, const Module& m ); 
   friend ifstream& operator >> ( ifstream is, Module& m ); 

private: 
   static string purpose; 
   string id = "unknown"; 
   int number_of_crew = 0; 
   int max_crew = 0; 
   double power = 0; 
   vector<Crew> crew_list;  
};

#include "module.h"

string Module::purpose = "Module";

ofstream& operator << ( ofstream& os, const Module& m )
{
  os << m.purpose << endl
      << m.id << endl
      << m.number_of_crew << endl
      << m.max_crew << endl
      << m.power << endl;
   for ( auto i : m.crew_list )
   {
      os << i << endl;
   }
   return os;
}

ifstream& operator >> ( ifstream& is, Module& m )
{
   getline( is, m.purpose );
   return is;
}

ostream& operator << ( ostream& os, const Module& m )
{
   os << m.purpose << endl
      << m.id << endl
      << m.number_of_crew << endl
      << m.max_crew << endl
      << m.power << endl;
   for ( auto i : m.crew_list )
   {
      os << i << endl;
   }
   return os;
}