從本篇開始，我們會深入學習怎樣使用Boost.Asio建立更加複雜的客戶端和服務端應用。你可以執行並測試它們，而且在理解之後，你可以把它們做為框架來構造自己的應用。
在接下來的例子中：
1.客戶端使用一個使用者名稱（無密碼）登入到服務端
2.所有的連線由客戶端建立，當客戶端請求時服務端迴應
3.所有的請求和回覆都以換行符結尾（’\n’）
4.對於5秒鐘沒有ping操作的客戶端，服務端會自動斷開其連線
客戶端可以傳送如下請求：
1.獲得所有已連線客戶端的列表
2.客戶端可以ping，當它ping時，服務端返回ping ok或者pingclient_list_chaned
為了更有趣一點，我們增加了一些難度：
1.每個客戶端登入6個使用者連線，比如Johon,James,Lucy,Tracy,Frank和Abby

2.每個客戶端連線隨機地ping服務端（隨機7秒；這樣的話，服務端會時不時關閉一個連線）

基於TCP的同步客戶端

1.流程圖

2.實現

#ifdef WIN32
#define _WIN32_WINNT 0x0501
#include <stdio.h>
#endif

#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>
using namespace boost::asio;
io_service service;

/** simple connection to server:
- logs in just with username (no password)
- all connections are initiated by the client: client asks, server answers
- server disconnects any client that hasn't pinged for 5 seconds

Possible requests:
- gets a list of all connected clients
- ping: the server answers either with "ping ok" or "ping client_list_changed"
*/
struct talk_to_svr {
	talk_to_svr(const std::string & username)
		: sock_(service), started_(true), username_(username) {}
	void connect(ip::tcp::endpoint ep) {
		sock_.connect(ep);
	}
	void loop() {
		// read answer to our login
		write("login " + username_ + "\n");
		read_answer();
		while (started_) {
			// 迴圈傳送ping請求
			write_request();
			read_answer();
			int millis = rand() % 7000;
			std::cout << username_ << " postpone ping "
				<< millis << " ms" << std::endl;
			boost::this_thread::sleep(boost::posix_time::millisec(millis));
		}
	}
	std::string username() const { return username_; }
private:
	void write_request() {
		write("ping\n");
	}
	void read_answer() {
		already_read_ = 0;
		read(sock_, buffer(buff_),
			boost::bind(&talk_to_svr::read_complete, this, _1, _2));
		process_msg();
	}
	void process_msg() {
		std::string msg(buff_, already_read_);
		if (msg.find("login ") == 0) on_login();
		else if (msg.find("ping") == 0) on_ping(msg);
		else if (msg.find("clients ") == 0) on_clients(msg);
		else std::cerr << "invalid msg " << msg << std::endl;
	}

	void on_login() {
		std::cout << username_ << " logged in" << std::endl;
		do_ask_clients();
	}
	void on_ping(const std::string & msg) {
		std::istringstream in(msg);
		std::string answer;
		in >> answer >> answer;
		if (answer == "client_list_changed")
			do_ask_clients();
	}
	void on_clients(const std::string & msg) {
		std::string clients = msg.substr(8);
		std::cout << username_ << ", new client list:" << clients;
	}
	// 獲得所有已連線客戶端的列表
	void do_ask_clients() {
		write("ask_clients\n");
		read_answer();
	}

	void write(const std::string & msg) {
		sock_.write_some(buffer(msg));
	}
	size_t read_complete(const boost::system::error_code & err, size_t bytes) {
		if (err) return 0;
		already_read_ = bytes;
		bool found = std::find(buff_, buff_ + bytes, '\n') < buff_ + bytes;
		return found ? 0 : 1;
	}

private:
	ip::tcp::socket sock_;
	enum { max_msg = 1024 };
	int already_read_;
	char buff_[max_msg];
	bool started_;
	std::string username_;
};

ip::tcp::endpoint ep(ip::address::from_string("127.0.0.1"), 8001);
void run_client(const std::string & client_name) {
	talk_to_svr client(client_name);
	try {
		client.connect(ep);
		client.loop();
	}
	catch (boost::system::system_error & err) {
		// 捕獲socket斷開原因
		std::cout << "client terminated " << client.username()
			<< "——" << err.what() << std::endl;
	}
}

int main(int argc, char* argv[]) {
	boost::thread_group threads;
	char* names[] = { "John", "James", "Lucy", "Tracy", "Frank", "Abby", 0 };
	for (char ** name = names; *name; ++name) {
		threads.create_thread(boost::bind(run_client, *name));
		boost::this_thread::sleep(boost::posix_time::millisec(100));
	}
	threads.join_all();
	system("pause");
}