i don't understand why this demo is ok.... why have lots of ~Test()....

[583654109]

this demo res is:

test running
Test(), str: abc
Test(), str: bcd
~Test(), str: bcd
~Test(), str: abc
~Test(), str: abc
~Test(), str: bcd
~Test(), str: abc
~Test(), str: bcd
~Test(), str: abc
this res is: abc
~Test(), str: bcd
this res is: bcd
end mark

#include <string>
#include <algorithm>
#include <iostream>
#ifndef THREAD_POOL_H
#define THREAD_POOL_H

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

class ThreadPool {
public:
    ThreadPool(size_t);
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args) 
        -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();
private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;

    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads)
    :   stop(false)
{
    for(size_t i = 0;i<threads;++i)
        workers.emplace_back(
            [this]
            {
                for(;;)
                {
                    std::function<void()> task;

                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        this->condition.wait(lock,
                            [this]{ return this->stop || !this->tasks.empty(); });
                        if(this->stop && this->tasks.empty())
                            return;
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                    }

                    task();
                }
            }
        );
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    using return_type = typename std::result_of<F(Args...)>::type;

    auto task = std::make_shared< std::packaged_task<return_type()> >(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );

    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);

        // don't allow enqueueing after stopping the pool
        if(stop)
            throw std::runtime_error("enqueue on stopped ThreadPool");

        tasks.emplace([task](){ (*task)(); });
    }
    condition.notify_one();
    return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();
    for(std::thread &worker: workers)
        worker.join();
}

#endif

class Test {
public:
    Test(const std::string& str) {
        std::cout << "Test(), str: " << str << std::endl;
        this->str_ = str;
    }
    ~Test() {
        std::cout << "~Test(), str: " << str_ << std::endl;
    }
    std::string getstr() const {
        return str_;
    }

private:
    std::string str_;
};

int main() {
    std::cout << "test running" << std::endl;

    {
        ThreadPool tp(1);
        std::vector<std::future<std::string>> vec_res;
        {
            std::vector<Test> vec_src = { Test("abc"), Test("bcd") };
            for (int i = 0; i < vec_src.size(); ++i) {
                Test& one = vec_src[i];
                vec_res.emplace_back(
                    tp.enqueue(
                        [](Test& one) {
                            return one.getstr();
                        }, one
                    )
                );
            }
        }
        for (int i = 0; i < vec_res.size(); ++i) {
            auto res = vec_res[i].get();
            std::cout << "this res is: "<< res << std::endl;
        }
    }
    std::cout << "end mark" << std::endl;

    return 0;
}

[583654109]

this demo i use local variable：Test why running is ok..... i'm a litte don't understand

[583654109]

i'm fool