Измени код:
-Добавь в код протокол peer-to-peer
-Добавь список друзей где просто будет список друзей их name для выбора кому написать.
-Убери ввод port после ввода id

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <stdexcept>
#include <set>

using namespace std;

//структура для хранения информации о пользователях
struct User {
	int id;
	string name;
	string masked_ip;
	int port;
};

//генерация случайного id пользователя
int generate_id(set<int>& used_ids) {
	int id;
	do {
		id = rand() % 1000000000;
	} while (used_ids.find(id) != used_ids.end());
	used_ids.insert(id);
	return id;
}

//генерация маскированного ip-адреса пользователя
string generate_masked_ip(set<string>& used_ips) {
	int number;
	stringstream ss;
	for (int i = 0; i < 9; i++) {
		number = rand() % 10;
		ss << number;
	}
	string masked_ip = ss.str();
	while (used_ips.find(masked_ip) != used_ips.end()) {
		masked_ip = generate_masked_ip(used_ips);
	}
	used_ips.insert(masked_ip);
	return masked_ip;
}

//функция для сохранения информации о пользователе в файл
void save_user_info(const vector<User>& users) {
	ofstream file("C: / users.txt");
	if (!file.is_open()) {
		throw runtime_error("Cannot open file for writing");
	}
	for (const User& user : users) {
		file << user.id << ", " << user.name << ", " << user.masked_ip << ", " << user.port << endl;
	}
	file.close();
}

//функция для загрузки информации о пользователях из файла
void load_user_info(vector<User>& users, set<int>& used_ids, set<string>& used_ips) {
	ifstream file("C: / users.txt");
	if (!file.is_open()) {
		ofstream new_file("C: / users.txt");
		new_file.close();
	}
	else {
		string line;
		while (getline(file, line)) {
			User user;
			int pos;
			pos = line.find(", ");
				user.id = stoi(line.substr(0, pos));
			if (used_ids.find(user.id) != used_ids.end()) {
				throw runtime_error("Duplicated user id : " + to_string(user.id));
			}
			used_ids.insert(user.id);
			line.erase(0, pos + 1);
			pos = line.find(", ");
			user.name = line.substr(1, pos - 2);
			line.erase(0, pos + 1);
			pos = line.find(", ");
				user.masked_ip = line.substr(1, pos - 2);
			if (used_ips.find(user.masked_ip) != used_ips.end()) {
				throw runtime_error("Duplicated user ip: " + user.masked_ip);
			}
			used_ips.insert(user.masked_ip);
			line.erase(0, pos + 1);
			user.port = stoi(line);
			users.push_back(user);
		}
		file.close();
	}
}

//функция для поиска пользователя по id
User find_user_by_id(const vector<User>& users, int id) {
	for (const User& user : users) {
		if (user.id == id) {
			return user;
		}
	}
	User user;
	user.id = -1;
	return user;
}

//функция для поиска пользователя по маскированному ip
User find_user_by_masked_ip(const vector<User>& users, const string& masked_ip) {
	for (const User& user : users) {
		if (user.masked_ip == masked_ip) {
			return user;
		}
	}
	User user;
	user.id = -1;
	return user;
}

//функция для проверки корректности порта
bool is_port_correct(int port) {
	if (port < 1024 || port > 65535) {
		return false;
	}
	return true;
}

//функция для отправки сообщения от пользователя sender пользователю recipient
void send_message(const vector<User>& users, int sender_id, const string& recipient_ip, const string& message) {
	User sender = find_user_by_id(users, sender_id);
	if (sender.id == -1) {
		throw runtime_error("User with this id was not found");
	}
	User recipient = find_user_by_masked_ip(users, recipient_ip);
	if (recipient.id == -1) {
		throw runtime_error("User with this ip was not found");
	}
	cout << "Message from " << sender.name << " to " << recipient.name << ": " << message << endl;
}

//функция для добавления нового пользователя
void add_new_user(vector<User>& users, set<int>& used_ids, set<string>& used_ips, int my_id) {
	User user;
	user.id = generate_id(used_ids);
	cout << "ID user: " << user.id << endl;
	cout << "Enter username: ";
	cin >> user.name;
	user.masked_ip = generate_masked_ip(used_ips);

	while (true) {
		cout << "Enter user port: ";
		cin >> user.port;
		if (is_port_correct(user.port)) {
			break;
		}
		else {
			cout << "Incorrect port.Please try again." << endl;
		}
	}

	if (user.id == my_id) {
		cout << "Do you want to add this user ? " << endl;
		cout << "1.Yes" << endl;
		cout << "2.No" << endl;
		int choice;
		cin >> choice;
		if (choice == 1) {
			users.push_back(user);
			save_user_info(users);
			cout << "User added successfully" << endl;
		}
		else {
			cout << "User not added." << endl;
		}
	}
	else {
		users.push_back(user);
		save_user_info(users);
		cout << "User added successfully" << endl;
	}
}

//главный метод программы
int main() {
	vector<User> users;
	set<int> used_ids;
	set<string> used_ips;
	int my_id = generate_id(used_ids);
	cout << "Your id: " << my_id << endl;
	load_user_info(users, used_ids, used_ips);

	int choice;
	cout << "1.Find user by id" << endl;
	cout << "2.Add new user" << endl;
	cout << "3.Send message" << endl;
	cout << "Your choice: ";
	cin >> choice;

	try {
		if (choice == 1) {
			int id;
			cout << "Enter user id: ";
			cin >> id;
			User found_user = find_user_by_id(users, id);
			if (found_user.id == -1) {
				cout << "User with this id was not found" << endl;
			}
			else {
				cout << "User found: " << endl;
				cout << "ID: " << found_user.id << endl;
				cout << "Name: " << found_user.name << endl;
				cout << "IP: " << found_user.masked_ip << endl;
				cout << "Port: " << found_user.port << endl;
			}
		}
		else if (choice == 2) {
			add_new_user(users, used_ids, used_ips, my_id);
		}
		else if (choice == 3) {
			int sender_id;
			cout << "Enter sender id: ";
			cin >> sender_id;
			if (sender_id == my_id) {
				cout << "You cannot send messages to yourself." << endl;
			}
			else {
				string recipient_ip;
				cout << "Enter recipient masked ip: ";
				cin >> recipient_ip;
				string message;
				cout << "Enter message: ";
				cin.ignore();
				getline(cin, message);
				send_message(users, sender_id, recipient_ip, message);
			}
		}
		else {
			throw invalid_argument("Invalid choice: " + to_string(choice));
		}
	}
	catch (const exception& e) {
		cerr << "Error: " << e.what() << endl;
		exit(1);
	}

	return 0;
}