write_obj_to_file.cpp

#include<iostream>
#include<fstream>
using namespace std;
// define a structure to store student data
struct student {
	int sapid;
	char name[30];
	float marks;
	void getData();		// get student data from user
	void displayData(); // display data
};
void student::getData() {
	cout << "\nEnter SAPId. : ";
	cin >> sapid;
	cin.ignore(); // ignore the newline char inserted when you press enter
	cout << "Enter Name : ";
	cin.getline(name, 30);
	cout << "Enter Marks : ";
	cin >> marks;
}

void student::displayData() {
	cout << "SAP Id. : " << sapid << endl;
	cout << "Name : " << name << endl;
	cout << "Marks : " << marks << endl;
}
void menu();
void writeData();
void readData();
void displayData();
void deleteData();

int maxSize = 3;
int no_of_records = 0;
student s[3]; // array of 3 students
ofstream wfile;
ifstream rfile;
//to count to number of students after deletion

int main() {
	int choice;
	while (true)
	{
		menu();
		cin >> choice;
		switch (choice)
		{
		case 1:
			writeData();
			break;
		case 2:
			readData();
			break;
		case 3:
			displayData();
			break;
		case 4:
			deleteData();
			break;
		case 5:
			exit(0);
		default:
			break;
		}//end switch()
	}//end while()
	return 0;
}//end main()
void menu()
{
	cout << "Press 1 to write to the file" << endl;
	cout << "Press 2 to read from file" << endl;
	cout << "Press 3 to dispaly student data" << endl;
	cout << "Press 4 to delete a student data" << endl;
	cout << "Press 5 to exit" << endl;
}

void writeData()
{
	wfile.open("students.txt", ios::app); // open file for writing
	for (int i = 0; i < maxSize; i++)
	{
		s[i].getData();
		wfile.write((char *)&s[i], sizeof(s[i]));	//(char*)&s - type casting &s into a char pointer.
													//(char *)&s[i] Pointer to an array of at least n characters.
	}
	wfile.close(); // close the file
}
void readData()
{
	no_of_records = 0;
	rfile.open("students.txt");// , ios::in); // open file for reading
	for (int i = 0; i < maxSize; i++)
	{
		//(char *)&s[i] Pointer to an array where the extracted
		//characters are stored.
		rfile.read((char *)&s[i], sizeof(s[i])); // read an object from a file
		no_of_records++;
	}
	rfile.close(); // close the file
}

void displayData()
{
	for (int i = 0; i < no_of_records; i++) {
		s[i].displayData();
	}
}
void deleteData()
{
	int sapid, count = 0;
	wfile.open("students.txt");
	cout << "enter student SAPId to delete it:";
	cin >> sapid;

	for (int i = 0; i < maxSize; i++)
	{
		if (s[i].sapid != sapid)
		{
			wfile.write((char *)&s[i], sizeof(s[i]));
			count++;
		}
	}
	wfile.close();
	maxSize = count;
	readData();
}