c++ - 调用 join 时如何修复 `terminate called without an active exception`

Question

我编写了一个简单的线程池实现，但出现terminate called without an active exception 错误。我检查了线程是否可连接，然后尝试调试调用析构函数的位置，但没有成功。

workerpool.hpp

#ifndef WORKERPOOL_H
#define WORKERPOOL_H

#include <condition_variable>
#include <iostream>
#include <memory>
#include <mutex>
#include <optional>
#include <queue>
#include <thread>
#include <vector>

class task {
  public:
    virtual void operator()() = 0;
    virtual ~task() = default;
};

class WorkerPool {
  private:
    class Worker {
      private:
        std::shared_ptr<WorkerPool> wp;
        long id;

      public:
        Worker(std::shared_ptr<WorkerPool> _wp, long _id) : wp(_wp), id(_id) {
            std::cout << "Worker " << id << " created" << std::endl;
        };
        ~Worker() { std::cout << "Worker " << id << " destroyed" << std::endl; }

        void operator()() {
            while (!wp->stop) {
                auto t = wp->fetch_task();
                if (!t.has_value())
                    continue;
                else
                    t.value()->operator()();
            }
            std::cout << "thread " << id << " ended" << std::endl;
        };
    };

    std::vector<std::thread> workers;

    std::queue<std::unique_ptr<task>> q;
    std::condition_variable cv;
    std::mutex mx;

    std::optional<std::unique_ptr<task>> fetch_task() {
        std::unique_lock l(mx);
        cv.wait(l, [&] { return !q.empty() || stop; });
        if (stop)
            return {};
        auto res = std::move(q.front());
        q.pop();
        return std::move(res);
    };

  public:
    WorkerPool() {
        std::cout << "worker pool created" << std::endl;
        for (long i = 0; i < std::thread::hardware_concurrency(); i++) {
            workers.push_back(
                std::thread(Worker(std::shared_ptr<WorkerPool>(this), i)));
        }
    }

    ~WorkerPool() {
        std::cout << "worker pool destroyed" << std::endl;
        terminate();
        for (size_t i = 0; i < workers.capacity(); i++) {
            if (workers[i].joinable())
                workers[i].join();
        }
    }

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

    bool stop;

    void submit(std::unique_ptr<task> t) {
        std::lock_guard l(mx);
        q.push(std::move(t));
        cv.notify_one();
    }

    void terminate() {
        stop = true;
        cv.notify_all();
    }
};

#endif // WORKERPOOL_H

main.cpp

#include <workerpool.hpp>

#include <condition_variable>
#include <iostream>
#include <mutex>

using namespace std;

class foo : public task {
public:
    void operator()() override { cout << "test" << endl; }
};

int main(int argc, char *argv[]) {

    WorkerPool wp;

    wp.submit(make_unique<foo>());

    wp.terminate();

    cout << "program ended" << endl;
    return 0; 
}

控制台输出:

> make run
rm -f bin/*
clang++ -std=c++17 -fuse-ld=lld  -pthread -Iinclude -Llib -O2 src/main.cpp -o bin/main 
./bin/main
worker pool created
Worker 0 created
Worker 0 destroyed
Worker 0 destroyed
Worker 1 created
Worker 1 destroyed
Worker 1 destroyed
Worker 2 created
Worker 2 destroyed
Worker 2 destroyed
Worker 3 created
Worker 3 destroyed
Worker 3 destroyed
thread 0 ended
Worker 0 destroyed
worker pool destroyed
program endedthread 2 ended
Worker 2 destroyed
worker pool destroyed

worker pool destroyed
terminate called without an active exception
make: *** [Makefile:32: run] Aborted (core dumped)

Answer 1

std::shared_ptr<WorkerPool>(this)

你不能只是那样做。您告诉 shared_ptr 它拥有该对象，但它没有。它的生命周期已经在别处管理(在本例中，main 中的“堆栈上”)。

因此，您显然以双重删除结束(注意 2 × “worker pool destroyed”)。形式上，结果是未定义的，特别是当您为每个 工作线程创建更多不相关的 shared_ptr 时:如果程序没有崩溃，我打赌您会看到4×那条线。

有很多方法可以做到这一点，using enable_shared_from_this , 虽然that cannot work in the constructor (因为“原始”shared_ptr 还没有完成取得所有权)。