Binary file I/O in C++

Question

I've written a code to store student info to a binary file and also get the informations as required. But I'm not getting the desired output. When I'm trying to read a student info from the student.bin file the name field is always showing NULL and the roll field is the last entered roll number. What's going wrong here ?

#include <bits/stdc++.h>
using namespace std;

class student{
public:
    string name;
    int roll;
};


int main(){
    fstream file("student.bin", ios::in | ios::out | ios::binary);
    int written = 0;
    while(1){
        int choice;
        student a;
        int i;
        cout << "0. Exit\n1. Write\n2. Read\nEnter Choice : ";
        cin >> choice;
        switch(choice){
            case 1:
                cout << "Enter name & roll : ";
                cin >> a.name >> a.roll;
                cout << "You entered " << a.name << " " << a.roll << endl;
                cout << "Enter the index : ";
                cin >> i;
                if(i > written){
                    cout << "This index doesnt exist !" << endl;
                    break;
                }
                file.seekp(i * sizeof(student), ios::beg);
                file.write((char*) &a, sizeof(student));
                written++;
                break;
            case 2:
                cout << "Enter index : ";
                cin >> i;
                file.seekg(i * sizeof(student), ios::beg);
                file.read((char*) &a, sizeof(student));
                cout << "Name : " << a.name << " Roll : " << a.roll << endl;
                break;
            case 0:
                exit(0);
            default:
                cout << "Wrong Choice !" << endl;
                break;
        }
    }
    file.close();
}

Answer 1

file.write((char*) &a, sizeof(student));

I'm sure that when you were taught sizeof () in class, you were told that it returns the size of the structure, which is a constant value.

Your student structure contains a std::string , which is a text string of unlimited length. Ask yourself how would it be possible for sizeof () to return a relatively small value for this short structure, on the order of a dozen bytes, or so, whether it contains a short string of a few characters, or a few megabytes.

A std::string , or any other class, is a binary structure. The actual text isn't a part of the string. The std::string class dynamically allocates memory for the text string it holds, and maintains internal pointers to the dynamically allocated memory.

Writing the binary values of those pointers, here, to a file accomplishes nothing useful. When read in subsequently, chances are that they would be pointers to memory that no longer exists.