c++ - 从并发析构函数停止 boost::asio::io_service::run()

Question

谁能解释一下为什么这个程序没有终止(见评论)？

#include <boost/asio/io_service.hpp>
#include <boost/asio.hpp>
#include <memory>
#include <cstdio>
#include <iostream>
#include <future>

class Service {
public:
    ~Service() {
        std::cout << "Destroying...\n";
        io_service.post([this]() {
            std::cout << "clean and stop\n"; // does not get called
            // do some cleanup
            // ...
            io_service.stop();
            std::cout << "Bye!\n";
        });
        std::cout << "...destroyed\n"; // last printed line, blocks
    }

    void operator()() {
        io_service.run();
        std::cout << "run completed\n";
    }

private:
    boost::asio::io_service io_service;
    boost::asio::io_service::work work{io_service};
};

struct Test {
    void start() {
        f = std::async(std::launch::async, [this]() { service(); std::cout << "exiting thread\n";});
    }
    std::future<void> f;
    Service service;
};

int main(int argc, char* argv[]) {
    {
        Test test;
        test.start();

        std::string exit;
        std::cin >> exit;
    }

    std::cout << "exiting program\n"; // never printed
}

Answer 1

真正的问题是破坏io_service是(显然)不是线程安全的。

只需重置工作并加入线程即可。 (可选)设置一个标志，以便您的 IO 操作知道正在关闭。

您的测试类和服务类试图分担 IO 服务的责任，但这是行不通的。这里有很多简化，合并类并删除未使用的 future 。

Live On Coliru

诀窍是制作 work对象 optional<> :

#include <boost/asio.hpp>
#include <boost/optional.hpp>
#include <iostream>
#include <thread>

struct Service {
    ~Service() {
        std::cout << "clean and stop\n";
        io_service.post([this]() {
            work.reset(); // let io_service run out of work
        });

        if (worker.joinable())
            worker.join();
    }

    void start() {
        assert(!worker.joinable());
        worker = std::thread([this] { io_service.run(); std::cout << "exiting thread\n";});
    }

private:
    boost::asio::io_service io_service;
    std::thread worker;
    boost::optional<boost::asio::io_service::work> work{io_service};
};

int main() {
    {
        Service test;
        test.start();

        std::cin.ignore(1024, '\n');
        std::cout << "Start shutdown\n";
    }

    std::cout << "exiting program\n"; // never printed
}

打印

Start shutdown
clean and stop
exiting thread
exiting program