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我正在尝试了解 WebSocket++ 库的广播服务器示例。
我有两个问题：

  1. 为什么m_connections需要锁？
  2. 为什么不直接插入/删除connection_hdlinon_open和on_close回调，而不是先将它们添加到队列中，然后将它们添加到m_connectionsin process_messages？

这是代码中最重要的部分：

class broadcast_server {
public:
    broadcast_server() {
        // Initialize Asio Transport
        m_server.init_asio();

        // Register handler callbacks
        m_server.set_open_handler(bind(&broadcast_server::on_open,this,::_1));
        m_server.set_close_handler(bind(&broadcast_server::on_close,this,::_1));
        m_server.set_message_handler(bind(&broadcast_server::on_message,this,::_1,::_2));
    }

    void run(uint16_t port) {
        // listen on specified port
        m_server.listen(port);

        // Start the server accept loop
        m_server.start_accept();

        // Start the ASIO io_service run loop
        try {
            m_server.run();
        } catch (const std::exception & e) {
            std::cout << e.what() << std::endl;
        }
    }

    void on_open(connection_hdl hdl) {
        {
            lock_guard<mutex> guard(m_action_lock);
            //std::cout << "on_open" << std::endl;
            m_actions.push(action(SUBSCRIBE,hdl));
        }
        m_action_cond.notify_one();
    }

    void on_close(connection_hdl hdl) {
        {
            lock_guard<mutex> guard(m_action_lock);
            //std::cout << "on_close" << std::endl;
            m_actions.push(action(UNSUBSCRIBE,hdl));
        }
        m_action_cond.notify_one();
    }

    void on_message(connection_hdl hdl, server::message_ptr msg) {
        // queue message up for sending by processing thread
        {
            lock_guard<mutex> guard(m_action_lock);
            //std::cout << "on_message" << std::endl;
            m_actions.push(action(MESSAGE,hdl,msg));
        }
        m_action_cond.notify_one();
    }

    void process_messages() {
        while(1) {
            unique_lock<mutex> lock(m_action_lock);

            while(m_actions.empty()) {
                m_action_cond.wait(lock);
            }

            action a = m_actions.front();
            m_actions.pop();

            lock.unlock();

            if (a.type == SUBSCRIBE) {
                lock_guard<mutex> guard(m_connection_lock);
                m_connections.insert(a.hdl);
            } else if (a.type == UNSUBSCRIBE) {
                lock_guard<mutex> guard(m_connection_lock);
                m_connections.erase(a.hdl);
            } else if (a.type == MESSAGE) {
                lock_guard<mutex> guard(m_connection_lock);

                con_list::iterator it;
                for (it = m_connections.begin(); it != m_connections.end(); ++it) {
                    m_server.send(*it,a.msg);
                }
            } else {
                // undefined.
            }
        }
    }
private:
    typedef std::set<connection_hdl,std::owner_less<connection_hdl> > con_list;

    server m_server;
    con_list m_connections;
    std::queue<action> m_actions;

    mutex m_action_lock;
    mutex m_connection_lock;
    condition_variable m_action_cond;
};
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