c++ - 以下代码是单个文件中的文件读/写。但是此代码无法创建文件

Question

我正在编写一个程序，用于读取和写入学生使用类(class)的记录。该程序不打开文件。所以我无法从文件中读取数据。

class Student
{
   private:
      unsigned roll ;
      char name[30];
      float perc;

   public:
      void getvalue()
      {
         cout<<"enter rollno , name and percentage :\n";
         cin>>roll;
         cin.ignore(); 
         cin>>name>>perc;
      }

      void  display()
      {
         cout << "\nRoll No : " << roll << "\nName : " << name
            << endl << "percentage : " << perc << endl;
      }
};

int main()
{
   char choice;
   Student st ;
   fstream file1;

   file1.open("stud_rec1.bin", ios::binary|ios::in|ios::out );
   do
   {
      cout<<"\n Detail of student  :\n";
      st.getvalue();

      file1.write((char*)(&st) , sizeof(st));

      cout<<"\nwant to input more record(y/n) : ";

      cin>>choice;

   } while(tolower(choice) == 'y');

   file1.seekg(0,ios::beg);

   while(file1.read((char*)(&st) , sizeof(st))   )
   {
      cout<<"1";
      st.display();
   }

   file1.close();

   getch();
}

Answer 1

当您调用 fstream::open() 并将模式设置为 ios::out|ios::in 时，只有当文件存在时才能打开该文件.如果文件不存在，fstream::open() 失败。参见 http://en.cppreference.com/w/cpp/io/basic_fstream/open和相关的http://en.cppreference.com/w/cpp/io/basic_filebuf/open .

改变

file1.open("stud_rec1.bin", ios::binary|ios::in|ios::out );

到

file1.open("stud_rec1.bin", ios::binary|ios::in|ios::out);
if ( !file1.is_open() )
{
   file1.clear();
   file1.open("stud_rec1.bin", ios::out); //Create file.
   file1.close();
   file1.open("stud_rec1.bin", ios::binary|ios::in|ios::out);

   // If the file still cannot be opened, there may be permission
   // issues on disk.
   if ( !file1.is_open() )
   {
      std::cerr << "Unable to open file " << "stud_rec1.bin" << std::endl;
      exit(1);
   } 
}