C++保证函数能在超时时间内返回

使用这两个接口就可以了

功能：保证某个操作能在超时时间内返回

说明：shared_from_this保证在退出作用域的时候智能指针不被释放，在线程退出的时候再自动销毁。

每执行一次execTimeout都会启动一个新线程，在线程退出后自动回收所有资源，注意T类型需要能被拷贝赋值

解决问题：某些操作会导致当前线程被阻塞（特别是网络操作），而这些接口又是第三方库提供的我们没办法修改

不适用：操作非常频繁的接口；可能会被永久阻塞的接口

demo:

 // 执行正常返回
int result1 = execTimeout<int>(3000, []() -> int {
    return 1 + 2;
});
std::cout << "result1:" << result1 << std::endl;

// 执行超时（执行函数需要花费4s而超时时间是3s），返回默认值
int result2 = execTimeout<int>(3000, 0, []() -> int {
    std::this_thread::sleep_for(std::chrono::seconds(4));
    return 3 + 4;
});
std::cout << "result2:" << result2 << std::endl;

#pragma once

#include <condition_variable>
#include <mutex>
#include <memory>
#include <thread>

template<typename T>
T execTimeout(int msTimeout, const std::function<T()> &cb)
{
    std::shared_ptr<TimeoutGuard<T>> a(new TimeoutGuard<T>(msTimeout));
    return a->exec(cb);
}

template<typename T>
T execTimeout(int msTimeout, const T &defaultValue, const std::function<T()> &cb)
{
    std::shared_ptr<TimeoutGuard<T>> a(new TimeoutGuard<T>(msTimeout, defaultValue));
    return a->exec(cb);
}

//////////////////////////////////////////////////////////////////////////
template<typename T>
class TimeoutGuard : public std::enable_shared_from_this<TimeoutGuard<T>>
{
public:
    TimeoutGuard(int msTimeout)
        : msTimeout_(msTimeout), isOk_(false)
    {
    }

    TimeoutGuard(int msTimeout, const T &defaultValue)
        : msTimeout_(msTimeout), result_(defaultValue), isOk_(false)
    {
    }

    TimeoutGuard(const TimeoutGuard&) = delete;
    TimeoutGuard& operator=(const TimeoutGuard&) = delete;

    T exec(const std::function<T()> &cb)
    {
        if (s_threadCount > 300) {
            std::cerr << "[TimeoutGuard] thread count overload:" << s_threadCount << std::endl;
            return result_;
        }

        cb_ = cb;
        long long startTime = getCurrentMillisecond();
        auto self(shared_from_this());
        thread_.reset(new std::thread([self]() {
            ++s_threadCount;
            std::cout << "[TimeoutGuard] thread start id:" << std::this_thread::get_id() << ", count:" << s_threadCount << std::endl;;
            T t = self->cb_();
            std::unique_lock<std::mutex> lock(self->mutex_);
            self->isOk_ = true;
            self->result_ = t;
            self->cond_.notify_one();
            --s_threadCount;
            std::cout << "[TimeoutGuard] thread end id:" << std::this_thread::get_id() << ", count:" << s_threadCount << std::endl;
        }));
        thread_->detach();

        std::unique_lock<std::mutex> lock(mutex_);
        while (!isOk_ && startTime + msTimeout_ > getCurrentMillisecond()) {
            cond_.wait_for(lock, std::chrono::milliseconds(100));
        }
        return result_;
    }

    long long getCurrentMillisecond()
    {
        auto duration = std::chrono::system_clock::now().time_since_epoch();
        auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();
        return millis;
    }

private:
    std::mutex mutex_;
    std::condition_variable cond_;
    std::unique_ptr<std::thread> thread_;
    std::function<T()> cb_;
    T result_;
    bool isOk_;
    long long msTimeout_;
    static int s_threadCount;
};

template<typename T>
int TimeoutGuard<T>::s_threadCount = 0;