c++ fstream not writing to object properly - c++

Question

I have the following code:
#define _CRT_SECURE_NO_WARNINGS
#define _SCL_SECURE_NO_WARNINGS
#include <iostream>
#include <fstream>
#include "Person.h"
using namespace std;
int main()
{
// ========================================================================================
// part a - initialiaze 100 records, lastName = "unassigned", firstName = "", age = 0
ofstream outPerson_0("nameage.dat", ios::out | ios::binary);
// exit program if ofstream could not open file
if (!outPerson_0)
{
cerr << "File could not be opened." << endl;
exit(EXIT_FAILURE);
} // end if
Person blankPerson("unassigned", "", 0); // constructor zeros out each data member
// output 100 blank records to file
for (int i = 0; i < 100; ++i)
outPerson_0.write(reinterpret_cast< const char * >(&blankPerson), sizeof(Person));
// ========================================================================================
// part b - input 10 first names and ages and write them to file
string firstName;
string lastName;
int age;
fstream outPerson_1("nameage.dat", ios::in | ios::out | ios::binary);
// exit program if fstream cannot open file
if (!outPerson_1)
{
cerr << "File could not be opened." << endl;
exit(EXIT_FAILURE);
} // end if
// iterate 10 times to get first names and ages
for (int i = 0; i < 10; ++i)
{
// get user input
cout << "Enter last name, first name and age (space separated): ";
// set dummy values in object
std::string s = std::to_string(i);
blankPerson.setLastName(s);
blankPerson.setFirstName(s);
blankPerson.setAge(i);
// seek position in file of user-specified record, using i
auto pos = (i) * sizeof(Person);
outPerson_1.seekp(pos);
// write user-specified information in file
outPerson_1.write(reinterpret_cast<const char *>(&blankPerson), sizeof(Person));
}
// ========================================================================================
// part c - update record with no info
}
Part a populated 100 objects with dummy values, and writes to file. Part b updates the first 10 objects with dummy values generated by iterator i. For some strange reason, part b does not work properly. Could someone tell me why? The result is exactly the same as when I run part a alone.

Answer 1

Normally object is not suggested to write to file directly, but just by case, your code has other problem:
the ofstream and fstream instances have its own internal buffer, the data will be written into internal buffer before flushing/syncing to underlying file.
Before quitting the program,
fstream outPerson_1 will destroyed
its internal buffer will be flushed to file "nameage.dat"
ofstream outPerson_0 is destroyed
its internal buffer will be flushed to file "nameage.dat" again
(as for why, it is another topic, don't talk here)
Since the data in outPerson_0 is more than the data in outPerson_1, so the outPerson_1 data is overwritten, and then you can't see it, if you write more data into outPerson_1, you will observe it.
There may be 2 options for your code:
Option#1:
flush the data after writing the first 100 person data
// output 100 blank records to file
for (int i = 0; i < 100; ++i)
outPerson_0.write(reinterpret_cast< const char * >(&blankPerson), sizeof(Person));
outPerson_0.flush();
Option#2:
Add ios::app flag to let ofstream/fstream to append the data to the file
ofstream outPerson_0("nameage.dat", ios::out | ios::binary | ios::app);
fstream outPerson_1("nameage.dat", ios::in | ios::out | ios::binary | ios::app);