// Object I/O by Chung
// "this" pointer points to the object itself

/* Sample Run
Enter data for person:
   Enter name: chan
   Enter age: 1
Do another (y/n)? y

Enter data for person:
   Enter name: jin
   Enter age: 2
Do another (y/n)? y

Enter data for person:
   Enter name: chung
   Enter age: 3
Do another (y/n)? n
Number of person objects in the file = 3
Enter Person Number(start from 0): 1

   Name: jin
   Age: 2
Do another (y/n)? y

Enter Person Number(start from 0): 5
No such person id
Do another (y/n)?
*/

#include <fstream.h>

class person {
   protected:
      char name[40];
      int age;
   public:
      void getData(void) {
         cout << "\n   Enter name: "; cin >> name;
         cout << "   Enter age: "; cin >> age;
      }
      void showData(void) {
         cout << "\n   Name: " << name;
         cout << "\n   Age: " << age;
      }
      bool readFF(int);           // read from file
      void write2F();             // write to file
      static int howManyObj();    // return num of persons in file
};

bool person::readFF(int pn) {     // read person number pn from file
   if( pn > 0 && pn < howManyObj()) {
     ifstream infile;
     infile.open("PERSON.DAT", ios::binary);
     infile.seekg( pn*sizeof(person) );
     infile.read( (char*)this, sizeof(*this) ); // read one person
     return true;
   } else {
     cout << "No such person id!";
     return false;
   }
}

void person::write2F() {         // write person to end of file
   ofstream outfile;
   outfile.open("PERSON.DAT", ios::app | ios::binary);
   outfile.write( (char*)this, sizeof(*this) );
}

int person::howManyObj() {       // return num of persons in file
   ifstream infile;
   infile.open("PERSON.DAT", ios::binary);
   infile.seekg(0, ios::end);    // go to 0 bytes from end
   return (int)infile.tellg() / sizeof(person); // cal. num of persons
}

void main(void) {
   person p;                     // make an empty person
   char ch;

   do {                          // save persons to disk
      cout << "\nEnter data for person:";
      p.getData();               // get data
      p.write2F();               // write to disk
      cout << "Do another (y/n)? ";
      cin >> ch;
   } while(ch=='y');               // until user enters 'n'

   int n = person::howManyObj();  // how many persons in file?
   cout << "Number of person objects in the file = " << n;
   do {
      cout << "\nEnter Person Number(start from 0): ";
      cin >> n;
      if(p.readFF(n))
         p.showData();
      cout << "\nDo another (y/n)? ";
      cin >> ch;
   } while(ch=='y');               // until user enters 'n'
}