Why is the data not getting properly written/read in this file? - c++

Question

The following code just accepts some data from the user in an object and writes it in a binary file. The first object gets written fine but after that it gets messed up.
Below is the output on reading the file after writing the first object onto it.
ROLL NO. NAME MARKS
234 Sansa 67
All fine and just as I expect. But when I append another object in the file, then this happens.
ROLL NO. NAME MARKS
234 SansSnow 78
564 Jon Snow 78
The roll numbers remain intact however names of the previous objects gets mixed up with the last object's and their marks get overwritten by the marks of the last object.
Why is it happening?
CODE:
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
fstream file;
class Student
{
protected:
int rollno;
char name[30];
float marks;
public:
void getData()
{
cout<<"ENTER THE DATA..."<<endl<<endl;
cout<<"Roll Number : ";
cin>>rollno;
fflush(stdin);
cout<<"Name : ";
cin.getline(name,30);
cout<<"Marks : ";
cin>>marks;
}
void displayData()
{
cout<<"\n"<<rollno;
cout<<setw(17)<<name;
cout<<setw(13)<<marks;
}
void writeData()
{
file.open("StudentData.dat", ios::app|ios::binary);
file.write((char*)this, sizeof(this));
file.close();
}
void readData()
{
file.open("StudentData.dat",ios::in|ios::binary);
while(file.read((char*)this, sizeof(this)))
{
displayData();
}
file.close();
}
};
Student S1;
int main()
{
cout<<"\nMAIN MENU"<<endl<<endl;
cout<<"1. Enter data and write to file\n";
cout<<"2. Read data from file and display\n";
cout<<"3. Exit\n\n";
cout<<"Enter your choice : ";
int choice;
cin>>choice;
switch(choice)
{
case 1:{
S1.getData();
S1.writeData();
cout<<endl<<endl;
cout<<"Data written to file successfully!"<<endl;
main();
break;
}
case 2:{
cout<<"ROLL NO."<<setw(10)<<"NAME"<<setw(15)<<"MARKS";
S1.readData();
cout<<endl;
main();
break;
}
case 3:{
exitop:
char confirmExit;
cout<<"\nAre you sure you want to exit? (Y/N) : ";
cin>>confirmExit;
if(confirmExit=='y' or confirmExit=='Y')
exit(0);
else if(confirmExit=='n' or confirmExit=='N')
{
cout<<"Exit Aborted.\n\n";
getchar();
main();
}
else{
cout<<"Invalid Input!";
goto exitop;
}
}
default:{
cout<<"Invalid Input!";
break;
}
}
return 0;
}

Answer 1

You’re only reading and writing 8 bytes. this is a pointer so sizeof(this) is 8 (on a 64 bit machine). What you actually want to do is get the size of the object instead so use sizeof(*this) or sizeof(Student).