C++ ofstream seekg() move file pointer

#include <fstream>
#include <iostream>
using namespace std;
class person/*www  .ja  va 2 s  . c  om*/
{
   protected:
   char name[40];
   int age;
   public:
   void getData(void)         //get person's data
   {
      cout << "\n   Enter last name: "; cin >> name;
      cout << "   Enter age: "; cin >> age;
   }
   void showData(void)        //display person's data
   {
      cout << "\n   Name: " << name;
      cout << "\n   Age: " << age;
   }
   void diskIn(int);          //read from file
   void diskOut();            //write to file
   static int diskCount();    //return number of persons in file
};
void person::diskIn(int pn)      //read person number pn from file
{
   ifstream infile;                           //make stream
      infile.open("PERSFILE.DAT", ios::binary);  //open it
   infile.seekg( pn*sizeof(person) );         //move file ptr
   infile.read( (char*)this, sizeof(*this) ); //read one person
}
void person::diskOut()           //write person to end of file
{
   ofstream outfile;             //make stream, open it
   outfile.open("PERSFILE.DAT", ios::app | ios::binary);
   outfile.write( (char*)this, sizeof(*this) ); //write to it
}
int person::diskCount()          //return number of persons
{                             //in file
    ifstream infile;
    infile.open("PERSFILE.DAT", ios::binary);
    infile.seekg(0, ios::end);    //go to 0 bytes from end
    //calculate number of persons
    return (int)infile.tellg() / sizeof(person);
}
int main()
{
    person p;
    char ch;
    do {
       cout << "Enter data for person:";
       p.getData();               //get data
       p.diskOut();               //write to disk
       cout << "Do another (y/n)? ";
       cin >> ch;
    }  while(ch=='y');         //until user enters 'n'
    int n = person::diskCount();  //how many persons in file?
    cout << "There are " << n << " persons in file\n";
    for(int j=0; j<n; j++)        //for each one,
    {
       cout << "\nPerson " << j;
       p.diskIn(j);               //read person from disk
       p.showData();
    }
    cout << endl;
    return 0;
}