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Rebuild、reconnect promise-future channel

1、我的应用中，需要重复地登录、接收登录响应，它是Asynchronous IO + callback，为了处理方便我使用promise-future channel来实现"async to sync-blocking-等待"，简化处理；后来基于它开发了一个交互式的application，在这个application可能需要重复进行登录，此时会抛出:

std::future_errc promise_already_satisfied

2、后来经过查阅方才知道:

对于一个promise对象，std::promise::set_value 只能够被调用一次，第二次调用，可能会抛出上述exception；也就是 promise-future channel 只能够使用一次。

3、那如何实现我的重复使用的目的呢？方法是: "rebuild、reconnect promise-future channel"

经过实践，下面是我的最终程序:

#include <future>
#include <chrono>
#include <thread>

#include <sstream>
#include <iostream>

#define LOG(...) LogWrapper(__FILE__, __LINE__, __VA_ARGS__)

// Terminator
void Log_Recursive(const char *file, int line, std::ostringstream &msg)
{
    std::cout << file << "(" << line << "): " << msg.str() << std::endl;
}

// "Recursive" variadic function
template<typename T, typename ... Args>
void Log_Recursive(const char *file, int line, std::ostringstream &msg, T value, const Args &... args)
{
    msg << value;
    Log_Recursive(file, line, msg, args...);
}

// Log_Recursive wrapper that creates the ostringstream
template<typename ... Args>
void LogWrapper(const char *file, int line, const Args &... args)
{
    std::ostringstream msg;
    Log_Recursive(file, line, msg, args...);
}

class Test
{
    std::future<bool> m_ConnectResultFuture;
    std::promise<bool> m_ConnectResultPromise;
    bool m_ConnectedFlag { false };
    std::future<void> m_SendRspTask;
public:
    bool Init()
    {
        m_ConnectResultPromise = std::promise<bool>();
        m_ConnectResultFuture = m_ConnectResultPromise.get_future(); // 建立future-promise channel

        m_SendRspTask = std::async(std::launch::async, [this]
        {
            std::this_thread::sleep_for(std::chrono::seconds(2));
            this->m_ConnectResultPromise.set_value(true);
        }
        );
        std::future_status status = m_ConnectResultFuture.wait_for(std::chrono::seconds(3));
        if (status == std::future_status::deferred)
        {
            LOG("未知状态，将等待3S(可能会连接超时)");
            std::this_thread::sleep_for(std::chrono::seconds(3));
            m_ConnectedFlag = true;
        }
        else if (status == std::future_status::timeout)
        {
            LOG("连接超时了");
            m_ConnectedFlag = true;
        }
        else if (status == std::future_status::ready)
        {
            m_ConnectedFlag = m_ConnectResultFuture.get();
            if (m_ConnectedFlag)
            {
                LOG("连接成功了!");
            }
            else
            {
                LOG("连接失败了!");
            }

        }
        return m_ConnectedFlag;
    }
};

int main()
{
    Test t;
    t.Init();
    t.Init();
    return 0;
}
// g++ --std=c++11 test.cpp -Wall -Wextra -pedantic -lpthread

cppreference std::promise::set_value

NOTE:

1、这里面已经讨论了thread safety问题

stackoverflow How do I “clear” std::promise?

A

To reuse promises, simply reassign them.

std::promise<int> my_promise;
//use the promise
my_promise = std::promise<int>(); //now you have a new promise

NOTE:

1、然后需要重新建立promise-future channel

stackoverflow std::promise set_value and thread safety

NOTE:

1、非常好的topic

A

NOTE:

1、非常好的回答

github std::future_error Promise already satisfied #55