c++ - 使用use_future的多播Boost Asio async_recieve_与async_send_to不能同时使用

Question

ITNOA
我想编写同时发送和接收多播的程序，如下所示

int main()
{
    const string group_address = "235.127.1.1";
    constexpr uint16_t PORT_NUMBER = 8765;

    boost::asio::io_context io_context;
    auto work_guard = boost::asio::make_work_guard(io_context);

    BoostMessageSender message_sender(group_address, "eth1", PORT_NUMBER, io_context);
    BoostMessageReceiver message_receiver(group_address, "eth1", PORT_NUMBER, io_context);

    std::thread t1([&io_context]()
        {
            cout << __LINE__ << endl;
            io_context.run();
        });

    this_thread::sleep_for(1s);

    std::array<char, 20> data = {};
    auto result = std::async(std::launch::async, [&message_receiver, &data]()
        {
                message_receiver.receive(gw::buffer(data));
                cout << " receive finished" << endl; // does not reach this line :(((
        });

    this_thread::sleep_for(1s);
    message_sender.send("Any body there!");

    result.wait(); // Raise exception what(): std::future_error: No associated state
    cout << "receive message: ";
    std::copy(std::begin(data),
          std::end(data),
          std::ostream_iterator<char>(std::cout, ""));

    io_context.stop();
    t1.join();
    return 0;
}

我的问题是为什么result.wait()引发异常？
可能重要的一点是，当我删除 message_sender.send行时，该程序不会引发任何异常，并且 result.wait()块行直到结束。但是当添加 message_sender.send行时，此程序引发异常，并且无法正常工作。
我的message_sender等于 this代码。
我的message_receiver如下所示

BoostMessageReceiver::BoostMessageReceiver(const std::string& group_address, const std::string& interface_name, uint16_t port_number, ba::io_context& io_context)
    : AbstractMessageReceiver(group_address, interface_name, port_number)
    , io_context(io_context)
    , listen_endpoint(ba::ip::address_v4::any(), port_number)
    , socket(io_context)
{
    socket.open(listen_endpoint.protocol());
    socket.set_option(ba::ip::udp::socket::reuse_address(true));
    socket.bind(listen_endpoint);

    // Join to multicast group
    socket.set_option(ba::ip::multicast::enable_loopback(true));
    socket.set_option(ba::ip::multicast::join_group(ba::ip::address::from_string(group_address)));
}

std::pair<bool, std::size_t> BoostMessageReceiver::receive(const AbstractMessageReceiver::MutableBuffer& buffer) noexcept
{
    ba::ip::udp::endpoint sender_endpoint;

    std::future<std::size_t> result = socket.async_receive_from(ba::buffer(buffer.data(), buffer.size()), sender_endpoint, ba::use_future);

    return std::make_pair(result.get() != 0, result.get());
}

我哪里错了？
我在以下平台上测试这些代码

Visual Studio 2019 16.7.4
Windows 10 1909 latest update
Boost 1.73.0

Ubuntu 20.04.1 LTS
GCC 9.3.0
Boost 1.71.0

Answer 1

您正在获取no_state，因为get上的future只能被调用一次。
下面

std::make_pair(result.get() != 0, result.get());

get被调用两次。
参见 reference:

Notes The implementations are encouraged to detect the case whenvalid() is false before the call and throw a std::future_error with anerror condition of std::future_errc::no_state.

首次调用 get后， valid返回 false。