在开始之前，先来看一段代码　

void pop3::operator() ( const boost::system::error_code& ec, std::size_t length )
{
    using namespace boost::asio;

    ip::tcp::endpoint endpoint;
    std::string     status;
    std::string     maillength;
    std::istream    inbuffer ( m_streambuf.get() );
    std::string     msg;

    reenter ( this ) {
restart:
        m_socket.reset( new ip::tcp::socket(io_service) );

        do {
#ifndef DEBUG
            // 延时 60s
            _yield ::boost::delayedcallsec( io_service, 60, boost::bind(*this, ec, 0) );
#endif

            // dns 解析并连接.
            _yield boost::async_avconnect(
                boost::proxychain(io_service).add_proxy()(boost::proxy_tcp(*m_socket, ip::tcp::resolver::query(m_mailserver, "110"))),
                *this);

            // 失败了延时 10s
            if ( ec )
                _yield ::boost::delayedcallsec ( io_service, 10, boost::bind(*this, ec, 0) );
        } while ( ec ); // 尝试到连接成功为止!

        // 好了，连接上了.
        m_streambuf.reset ( new streambuf );
        // "+OK QQMail POP3 Server v1.0 Service Ready(QQMail v2.0)"
        _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
        inbuffer >> status;

        if ( status != "+OK" ) {
            // 失败，重试.
            goto restart;
        }

        // 发送用户名.
        _yield m_socket->async_write_some ( buffer ( std::string ( "user " ) + m_mailaddr + "\n" ), *this );
        if(ec) goto restart;
        // 接受返回状态.
        m_streambuf.reset ( new streambuf );
        _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
        inbuffer >> status;

        // 解析是不是　OK.
        if ( status != "+OK" ) {
            // 失败，重试.
            goto restart;
        }

        // 发送密码.
        _yield m_socket->async_write_some ( buffer ( std::string ( "pass " ) + m_passwd + "\n" ), *this );
        // 接受返回状态.
        m_streambuf.reset ( new streambuf );
        _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
        inbuffer >> status;

        // 解析是不是　OK.
        if ( status != "+OK" ) {
            // 失败，重试.
            goto restart;
        }

        // 完成登录. 开始接收邮件.

        // 发送　list 命令.
        _yield m_socket->async_write_some ( buffer ( std::string ( "list\n" ) ), *this );
        // 接受返回的邮件.
        m_streambuf.reset ( new streambuf );
        _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
        inbuffer >> status;

        // 解析是不是　OK.
        if ( status != "+OK" ) {
            // 失败，重试.
            goto restart;
        }

        // 开始进入循环处理邮件.
        maillist.clear();
        _yield  m_socket->async_read_some ( m_streambuf->prepare ( 8192 ), *this );
        m_streambuf->commit ( length );

        while ( status != "." ) {
            maillength.clear();
            status.clear();
            inbuffer >> status;
            inbuffer >> maillength;

            // 把邮件的编号push到容器里.
            if ( maillength.length() )
                maillist.push_back ( status );

            if ( inbuffer.eof() && status != "." )
                _yield  m_socket->async_read_some ( m_streambuf->prepare ( 8192 ), *this );
        }

        // 获取邮件.
        while ( !maillist.empty() ) {
            // 发送　retr #number 命令.
            msg = boost::str ( boost::format ( "retr %s\r\n" ) %  maillist[0] );
            _yield m_socket->async_write_some ( buffer ( msg ), *this );
            // 获得　+OK
            m_streambuf.reset ( new streambuf );
            _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
            inbuffer >> status;

            // 解析是不是　OK.
            if ( status != "+OK" ) {
                // 失败，重试.
                goto restart;
            }

            // 获取邮件内容，邮件一单行的 . 结束.
            _yield  async_read_until ( *m_socket, *m_streambuf, "\r\n.\r\n", *this );
            // 然后将邮件内容给处理.
            process_mail ( inbuffer );
            // 删除邮件啦.
            msg = boost::str ( boost::format ( "dele %s\r\n" ) %  maillist[0] );
            _yield m_socket->async_write_some ( buffer ( msg ), *this );

            maillist.erase ( maillist.begin() );
            // 获得　+OK
            m_streambuf.reset ( new streambuf );
            _yield  async_read_until ( *m_socket, *m_streambuf, "\n", *this );
            inbuffer >> status;

            // 解析是不是　OK.
            if ( status != "+OK" ) {
                // 失败，但是并不是啥大问题.
                std::cout << "deleting mail failed" << std::endl;
                // but 如果是连接出问题那还是要重启的.
                if(ec) goto restart;
            }
        }

        // 处理完毕.
        _yield async_write ( *m_socket, buffer ( "quit\n" ), *this );
        _yield ::boost::delayedcallsec ( io_service, 1, boost::bind ( *this, ec, 0 ) );
        if(m_socket->is_open())
            m_socket->shutdown ( ip::tcp::socket::shutdown_both );
        _yield ::boost::delayedcallsec ( io_service, 1, boost::bind ( *this, ec, 0 ) );
        m_socket.reset();
        std::cout << "邮件处理完毕" << std::endl;
        _yield ::boost::delayedcallsec ( io_service, 30, boost::bind ( *this, ec, 0 ) );
        goto restart;
    }
}

这个代码，乍一看就是同步代码嘛！而事实上它是异步的

在这个代码里，使用了　_yield 前缀再配合　async_* 异步函数，使用异步实现了同步的pop3登录算法。

这个神奇的代码，神奇之处就是 reenter(this) 和　_yield。这2个地方就是实现的全部的关键。

我在群课程里有简单的提到过协程，有兴趣的可以到　avplayer社区讲座：协程 围观。