程序功能：

• 客户端用名字登录服务端
• 连接由客户端驱动，客户端请求，服务端应答
• 所有的请求和应答都以\n结尾
• 当客户端在5秒内没有发ping请求，服务端就关闭连接
• 客户端可以请求所有已经建立连接的用户列表
• 客户端可以发ping请求，服务端应答 ping ok或者ping client_list_chaned，客户端收到 ping client_list_chaned后会重新请求用户列表

让程序变得有意思，给程序加点难度。

• 客户端有六个用户进行登录，用户名是John, James,Lucy, Tracy, Frank, Abby
• 每个连接给服务端发ping消息的时间是0到7秒的随机数。（超过5秒服务端就会断开连接）

客户端程序

#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;

#define MEM_FN(x) boost::bind(&self_type::x, shared_from_this())
#define MEM_FN1(x,y) boost::bind(&self_type::x, shared_from_this(),y)
#define MEM_FN2(x,y,z) boost::bind(&self_type::x, shared_from_this(),y,z)

class talk_to_svr : public boost::enable_shared_from_this<talk_to_svr>
    , boost::noncopyable {
    typedef talk_to_svr self_type;
    talk_to_svr(const std::string & username)
        : sock_(service), started_(true), username_(username), timer_(service) {}
    void start(ip::tcp::endpoint ep) {
        sock_.async_connect(ep, MEM_FN1(on_connect, _1));
    }
public:
    typedef boost::system::error_code error_code;
    typedef boost::shared_ptr<talk_to_svr> ptr;

    static ptr start(ip::tcp::endpoint ep, const std::string & username) {
        ptr new_(new talk_to_svr(username));
        new_->start(ep);
        return new_;
    }
    void stop() {
        if (!started_) return;
        std::cout << "stopping " << username_ << std::endl;
        started_ = false;
        sock_.close();
    }
    bool started() { return started_; }
private:
    void on_connect(const error_code & err) {
        if (!err)      do_write("login " + username_ + "\n");
        else            stop();
    }
    void on_read(const error_code & err, size_t bytes) {
        if (err) stop();
        if (!started()) return;
        // process the msg
        std::string msg(read_buffer_, bytes);
        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();
        else postpone_ping();
    }
    void on_clients(const std::string & msg) {
        std::string clients = msg.substr(8);
        std::cout << username_ << ", new client list:" << clients;
        postpone_ping();
    }

    void do_ping() {
        do_write("ping\n");
    }
    void postpone_ping() {
        // note: even though the server wants a ping every 5 secs, we randomly 
        // don't ping that fast - so that the server will randomly disconnect us
        int millis = rand() % 7000;
        std::cout << username_ << " postponing ping " << millis
            << " millis" << std::endl;
        timer_.expires_from_now(boost::posix_time::millisec(millis));
        timer_.async_wait(MEM_FN(do_ping));
    }
    void do_ask_clients() {
        do_write("ask_clients\n");
    }

    void on_write(const error_code & err, size_t bytes) {
        do_read();
    }
    void do_read() {
        async_read(sock_, buffer(read_buffer_),
            MEM_FN2(read_complete, _1, _2), MEM_FN2(on_read, _1, _2));
    }
    void do_write(const std::string & msg) {
        if (!started()) return;
        std::copy(msg.begin(), msg.end(), write_buffer_);
        sock_.async_write_some(buffer(write_buffer_, msg.size()),
            MEM_FN2(on_write, _1, _2));
    }
    size_t read_complete(const boost::system::error_code & err, size_t bytes) {
        if (err) return 0;
        bool found = std::find(read_buffer_, read_buffer_ + bytes, '\n') < read_buffer_ + bytes;
        // we read one-by-one until we get to enter, no buffering
        return found ? 0 : 1;
    }

private:
    ip::tcp::socket sock_;
    enum { max_msg = 1024 };
    char read_buffer_[max_msg];
    char write_buffer_[max_msg];
    bool started_;
    std::string username_;
    deadline_timer timer_;
};

int main(int argc, char* argv[]) {
    // connect several clients
    ip::tcp::endpoint ep(ip::address::from_string("127.0.0.1"), 8001);
    char* names[] = { "John", "James", "Lucy", "Tracy", "Frank", "Abby", 0 };
    for (char ** name = names; *name; ++name) {
        talk_to_svr::start(ep, *name);
        boost::this_thread::sleep(boost::posix_time::millisec(100));
    }

    service.run();
}

服务端程序

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

class talk_to_client;
typedef boost::shared_ptr<talk_to_client> client_ptr;
typedef std::vector<client_ptr> array;
array clients;

#define MEM_FN(x) boost::bind(&self_type::x, shared_from_this())
#define MEM_FN1(x,y) boost::bind(&self_type::x, shared_from_this(),y)
#define MEM_FN2(x,y,z) boost::bind(&self_type::x, shared_from_this(),y,z)


void update_clients_changed();

/** 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 client requests: - gets a list of all connected clients - ping: the server answers either with "ping ok" or "ping client_list_changed" */
class talk_to_client : public boost::enable_shared_from_this<talk_to_client>
    , boost::noncopyable {
    typedef talk_to_client self_type;
    talk_to_client() : sock_(service), started_(false),
        timer_(service), clients_changed_(false) {
    }
public:
    typedef boost::system::error_code error_code;
    typedef boost::shared_ptr<talk_to_client> ptr;

    void start() {
        started_ = true;
        clients.push_back(shared_from_this());
        last_ping = boost::posix_time::microsec_clock::local_time();
        // first, we wait for client to login
        do_read();
    }
    static ptr new_() {
        ptr new_(new talk_to_client);
        return new_;
    }
    void stop() {
        if (!started_) return;
        started_ = false;
        sock_.close();

        ptr self = shared_from_this();
        array::iterator it = std::find(clients.begin(), clients.end(), self);
        clients.erase(it);
        update_clients_changed();
    }
    bool started() const { return started_; }
    ip::tcp::socket & sock() { return sock_; }
    std::string username() const { return username_; }
    void set_clients_changed() { clients_changed_ = true; }
private:
    void on_read(const error_code & err, size_t bytes) {
        if (err) stop();
        if (!started()) return;
        // process the msg
        std::string msg(read_buffer_, bytes);
        if (msg.find("login ") == 0) on_login(msg);
        else if (msg.find("ping") == 0) on_ping();
        else if (msg.find("ask_clients") == 0) on_clients();
        else std::cerr << "invalid msg " << msg << std::endl;
    }

    void on_login(const std::string & msg) {
        std::istringstream in(msg);
        in >> username_ >> username_;
        std::cout << username_ << " logged in" << std::endl;
        do_write("login ok\n");
        update_clients_changed();
    }
    void on_ping() {
        do_write(clients_changed_ ? "ping client_list_changed\n" : "ping ok\n");
        clients_changed_ = false;
    }
    void on_clients() {
        std::string msg;
        for (array::const_iterator b = clients.begin(), e = clients.end(); b != e; ++b)
            msg += (*b)->username() + " ";
        do_write("clients " + msg + "\n");
    }

    void do_ping() {
        do_write("ping\n");
    }
    void do_ask_clients() {
        do_write("ask_clients\n");
    }

    void on_check_ping() {
        boost::posix_time::ptime now = boost::posix_time::microsec_clock::local_time();
        if ((now - last_ping).total_milliseconds() > 5000) {
            std::cout << "stopping " << username_ << " - no ping in time" << std::endl;
            stop();
        }
        last_ping = boost::posix_time::microsec_clock::local_time();
    }
    void post_check_ping() {
        timer_.expires_from_now(boost::posix_time::millisec(5000));
        timer_.async_wait(MEM_FN(on_check_ping));
    }


    void on_write(const error_code & err, size_t bytes) {
        do_read();
    }
    void do_read() {
        async_read(sock_, buffer(read_buffer_),
            MEM_FN2(read_complete, _1, _2), MEM_FN2(on_read, _1, _2));
        post_check_ping();
    }
    void do_write(const std::string & msg) {
        if (!started()) return;
        std::copy(msg.begin(), msg.end(), write_buffer_);
        sock_.async_write_some(buffer(write_buffer_, msg.size()),
            MEM_FN2(on_write, _1, _2));
    }
    size_t read_complete(const boost::system::error_code & err, size_t bytes) {
        if (err) return 0;
        bool found = std::find(read_buffer_, read_buffer_ + bytes, '\n') < read_buffer_ + bytes;
        // we read one-by-one until we get to enter, no buffering
        return found ? 0 : 1;
    }
private:
    ip::tcp::socket sock_;
    enum { max_msg = 1024 };
    char read_buffer_[max_msg];
    char write_buffer_[max_msg];
    bool started_;
    std::string username_;
    deadline_timer timer_;
    boost::posix_time::ptime last_ping;
    bool clients_changed_;
};

void update_clients_changed() {
    for (array::iterator b = clients.begin(), e = clients.end(); b != e; ++b)
        (*b)->set_clients_changed();
}

ip::tcp::acceptor acceptor(service, ip::tcp::endpoint(ip::tcp::v4(), 8001));

void handle_accept(talk_to_client::ptr client, const boost::system::error_code & err) {
    client->start();
    talk_to_client::ptr new_client = talk_to_client::new_();
    acceptor.async_accept(new_client->sock(), boost::bind(handle_accept, new_client, _1));
}


int main(int argc, char* argv[]) {
    talk_to_client::ptr client = talk_to_client::new_();
    acceptor.async_accept(client->sock(), boost::bind(handle_accept, client, _1));
    service.run();
}

运行结果

详细参考《Boost.Asio C++ Network Programming》