Refactor this whole C++ program Using classes

#include

#include

#include

using namespace std;

//Global Variables

string inv_no;

string inv_date;

string cust_name;

string cust_address;

float total_amt;

float tax_amt;

float discount;

float advance;

//Function Definitions

void main_menu();

void new_invoice();

void new_company();

void new_customer();

void new_order();

void get_item();

void update();

void print();

int main()

{

//Main Menu

main_menu();

return 0;

}

//Main Menu

void main_menu()

{

//Variables

int choice;

//Display Main Menu

cout << "Choose an option:" << endl;

cout << "1. New Invoice" << endl;

cout << "2. New Company" << endl;

cout << "3. New Customer" << endl;

cout << "4. New Order" << endl;

cout << "5. Others" << endl;

cout << "6. Exit" << endl;

//Input choice

cin >> choice;

//Switch statement

switch(choice)

{

case 1:

new_invoice();

break;

case 2:

new_company();

break;

case 3:

new_customer();

break;

case 4:

new_order();

break;

case 5:

//Other sub-menu

cout << "1. Get Item" << endl;

cout << "2. Update" << endl;

cout << "3. Print" << endl;

//Input choice

cin >> choice;

//Switch statement

switch(choice)

{

case 1:

get_item();

break;

case 2:

update();

break;

case 3:

print();

break;

}

break;

case 6:

//Exit

exit(0);

break;

}

}

//New Invoice

void new_invoice()

{

//Input invoice number, date and customer name

cout << "Enter invoice number: ";

cin >> inv_no;

cout << "Enter invoice date: ";

cin >> inv_date;

cout << "Enter customer name: ";

cin >> cust_name;

//Open file

ofstream inv_file;

inv_file.open("Invoice.txt", ios::out | ios::app);

//Write to file

inv_file << inv_no << " " << inv_date << " " << cust_name << endl;

//Close file

inv_file.close();

//Return to main menu

main_menu();

}

//New Company

void new_company()

{

//Input company name, address and layout

string comp_name;

string comp_address;

string layout;

cout << "Enter company name: ";

cin >> comp_name;

cout << "Enter company address: ";

cin >> comp_address;

cout << "Enter invoice layout: ";

cin >> layout;

//Open file

ofstream comp_file;

comp_file.open("Company.txt", ios::out | ios::app);

//Write to file

comp_file << comp_name << " " << comp_address << " " << layout << endl;

//Close file

comp_file.close();

//Return to main menu

main_menu();

}

//New Customer

void new_customer()

{

//Input customer name, address and contact number

string cust_name;

string cust_address;

string contact_no;

cout << "Enter customer name: ";

cin >> cust_name;

cout << "Enter customer address: ";

cin >> cust_address;

cout << "Enter customer contact number: ";

cin >> contact_no;

//Open file

ofstream cust_file;

cust_file.open("Customers.txt", ios::out | ios::app);

//Write to file

cust_file << cust_name << " " << cust_address << " " << contact_no << endl;

//Close file

cust_file.close();

//Return to main menu

main_menu();

}

//New Order

void new_order()

{

//Input order details

string order_no;

string order_date;

string item;

float quantity;

float unit_price;

cout << "Enter order number: ";

cin >> order_no;

cout << "Enter order date: ";

cin >> order_date;

cout << "Enter item: ";

cin >> item;

cout << "Enter quantity: ";

cin >> quantity;

cout << "Enter unit price: ";

cin >> unit_price;

//Open file

ofstream order_file;

order_file.open("Orders.txt", ios::out | ios::app);

//Write to file

order_file << order_no << " " << order_date << " " << item << " " << quantity << " " << unit_price << endl;

//Close file

order_file.close();

//Return to main menu

main_menu();

}

//Get Item

void get_item()

{

//Input item details

string item_no;

string item_name;

float quantity;

float unit_price;

cout << "Enter item number: ";

cin >> item_no;

cout << "Enter item name: ";

cin >> item_name;

cout << "Enter quantity: ";

cin >> quantity;

cout << "Enter unit price: ";

cin >> unit_price;

//Open file

ofstream item_file;

item_file.open("Items.txt", ios::out | ios::app);

//Write to file

item_file << item_no << " " << item_name << " " << quantity << " " << unit_price << endl;

//Close file

item_file.close();

//Return to main menu

main_menu();

}

//Update

void update()

{

//Input invoice details

float total_amt;

float tax_amt;

float discount;

float advance;

cout << "Enter total amount: ";

cin >> total_amt;

cout << "Enter tax amount: ";

cin >> tax_amt;

cout << "Enter discount: ";

cin >> discount;

cout << "Enter advance: ";

cin >> advance;

//Open file

ofstream inv_file;

inv_file.open("Invoice.txt", ios::out | ios::app);

//Write to file

inv_file << total_amt << " " << tax_amt << " " << discount << " " << advance << endl;

//Close file

inv_file.close();

//Return to main menu

main_menu();

}

//Print

void print()

{

//Print invoice

cout << "Invoice Number: " << inv_no << endl;

cout << "Invoice Date: " << inv_date << endl;

cout << "Customer Name: " << cust_name << endl;

cout << "Customer Address: " << cust_address << endl;

cout << "Total Amount: " << total_amt << endl;

cout << "Tax Amount: " << tax_amt << endl;

cout << "Discount: " << discount << endl;

cout << "Advance: " << advance << endl;

//Return to main menu

main_menu();