Write a program to manage inventory of goods. The specific functions are: (1) Enter the quantity of products and the name, number, and inventory quantity of each product through the keyboard. (2) Save the inventory quantity of the product to a text file. (3) Read the document and print product information in descending order of inventory for the product.use c++

Sure, here's an example C++ program that implements the requirements you've specified:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

struct Product {
    string name;
    int number;
    int quantity;
};

vector<Product> inventory;

void addProduct() {
    Product p;
    cout << "Enter product name: ";
    cin >> p.name;
    cout << "Enter product number: ";
    cin >> p.number;
    cout << "Enter product quantity: ";
    cin >> p.quantity;
    inventory.push_back(p);
}

void saveInventory() {
    ofstream fout("inventory.txt");
    if (fout) {
        for (const auto& p : inventory) {
            fout << p.name << "," << p.number << "," << p.quantity << endl;
        }
        fout.close();
        cout << "Inventory saved to file." << endl;
    } else {
        cout << "Error: could not open file." << endl;
    }
}

void readInventory() {
    ifstream fin("inventory.txt");
    if (fin) {
        inventory.clear();
        string line;
        while (getline(fin, line)) {
            Product p;
            size_t pos = line.find(",");
            p.name = line.substr(0, pos);
            line = line.substr(pos + 1);
            pos = line.find(",");
            p.number = stoi(line.substr(0, pos));
            p.quantity = stoi(line.substr(pos + 1));
            inventory.push_back(p);
        }
        fin.close();
        cout << "Inventory loaded from file." << endl;
    } else {
        cout << "Error: could not open file." << endl;
    }
}

void printInventory() {
    sort(inventory.begin(), inventory.end(), [](const Product& p1, const Product& p2) {
        return p1.quantity > p2.quantity;
    });
    cout << "Inventory:" << endl;
    for (const auto& p : inventory) {
        cout << p.name << " (" << p.number << "): " << p.quantity << endl;
    }
}

int main() {
    char choice;
    do {
        cout << "Select an option:" << endl;
        cout << "1. Add product" << endl;
        cout << "2. Save inventory to file" << endl;
        cout << "3. Load inventory from file" << endl;
        cout << "4. Print inventory" << endl;
        cout << "5. Exit" << endl;
        cout << "> ";
        cin >> choice;
        switch (choice) {
            case '1':
                addProduct();
                break;
            case '2':
                saveInventory();
                break;
            case '3':
                readInventory();
                break;
            case '4':
                printInventory();
                break;
            case '5':
                cout << "Exiting program." << endl;
                break;
            default:
                cout << "Invalid choice." << endl;
                break;
        }
    } while (choice != '5');
    return 0;
}

This program defines a `Product` struct to hold the name, number, and quantity of each product. It uses a `vector` to store the inventory of products. The program provides a menu with options to add a product, save the inventory to a file, load the inventory from a file, print the inventory, and exit the program. The program uses `fstream` to read from and write to the `inventory.txt` file. The `printInventory` function sorts the products in descending order of their inventory quantity before printing them to the console.