multithreading - 使用 Promise 时出现 C2248 错误

Question

下面的代码触发错误：

错误 1 ​​错误 C2248：“std::promise<_Ty>::promise”：无法访问在类“std::promise<_Ty>”中声明的私有成员

我该如何解决？

非常感谢 ！

#define _parallel_qick_sort
#ifdef _parallel_qick_sort

#include <boost/shared_ptr.hpp>
#include <thread>
#include <vector>
#include <list>
#include <future>
#include <atomic>
#include "ThreadSafeStack.hpp"

using namespace std;

template<typename T>
struct sorter
{
    struct chunk_to_sort
    {
        std::list<T> data_m;
        std::promise<std::list<T> > promise_m;
    };

    threadsafe_stack<chunk_to_sort> chunks_m;
    std::vector<std::thread> threads_m;
    unsigned const max_thread_count_m;
    std::atomic<bool> end_of_data_m;
    sorter():
        max_thread_count_m(std::thread::hardware_concurrency()-1),
        end_of_data_m(false)
    {}
    ~sorter()
    {
        end_of_data_m=true;
        for(unsigned i=0;i<threads_m.size();++i)
        {
            threads_m[i].join();
        }
    }
    void try_sort_chunk()
    {
        boost::shared_ptr<chunk_to_sort > chunk=chunks.pop();
        if(chunk)
        {
            sort_chunk(chunk);
        }
    }
    std::list<T> do_sort(std::list<T>& chunk_data)
    {
        if(chunk_data.empty())
        {
            return chunk_data;
        }
        std::list<T> result;
        result.splice(result.begin(),chunk_data,chunk_data.begin());
        T const& partition_val=*result.begin();
        typename std::list<T>::iterator divide_point = std::partition(chunk_data.begin(),chunk_data.end(),[&](T const& val){return val<partition_val;});
        chunk_to_sort new_lower_chunk;
        new_lower_chunk.data_m.splice(new_lower_chunk.data_m.end(),chunk_data,chunk_data.begin(),divide_point);
        std::future<std::list<T> > new_lower = new_lower_chunk.promise_m.get_future();
        chunks_m.push(std::move(new_lower_chunk));
        if(threads_m.size()<max_thread_count_m)
        {
            threads_m.push_back(std::thread(&sorter<T>::sort_thread,this));
        }
        std::list<T> new_higher(do_sort(chunk_data));
        result.splice(result.end(),new_higher);
        while(new_lower.wait_for(std::chrono::seconds(0)) !=
            std::future_status::ready)
        {
            try_sort_chunk();
        }
        result.splice(result.begin(),new_lower.get());
        return result;
    }
    void sort_chunk(boost::shared_ptr<chunk_to_sort > const& chunk)
    {
        chunk->promise_m.set_value(do_sort(chunk->data));
    }
    void sort_thread()
    {
        while(!end_of_data)
        {
            try_sort_chunk();
            std::this_thread::yield();
        }
    }
};

template<typename T>
std::list<T> parallel_quick_sort(std::list<T> input)
{
    if(input.empty())
    {
        return input;
    }
    sorter<T> s;
    return s.do_sort(input);
}

int main()
{
    list<int> l;
    l.push_back(4);
    l.push_back(3);
    l.push_back(1);
    l.push_back(2);

    parallel_quick_sort(l);

    return 0;
}

#endif

下面是 threadsafe_stack 类：

#ifndef __ThreadSafeStack_hpp__
#define __ThreadSafeStack_hpp__

#include "stdafx.h"
#include <exception>
#include <memory>
#include <mutex>
#include <stack>
#include <thread>
#include <iostream>
#include <functional>
#include <future>
using namespace std;

struct empty_stack : std::exception
{
    const char* what() const throw()
    {
        return "empty stack";
    }
};

template <typename T>
class threadsafe_stack
{
    std::stack<T> data_m;
    mutable std::mutex mutex_m;
    threadsafe_stack& operator = (const threadsafe_stack&);

public:
    typedef void (threadsafe_stack<T>:: * ext_push) (T);
    typedef shared_ptr<T> (threadsafe_stack<T>:: *ext_pop)();
    typedef void (threadsafe_stack<T>:: *ext_pop_void)(T& );
    typedef bool (threadsafe_stack<T>:: *ext_empty) () const;
    threadsafe_stack()
    {
        int i=0;
    }
    threadsafe_stack(const threadsafe_stack& other)
    {
        std::lock_guard<std::mutex> lock(other.mutex_m);
        for(int i=0; i<3; ++i)
            cout << "threadsafe_stack ctor "<< i << endl;
        data_m = other.data_m;
    }

    void push(T new_value)
    {
        std::lock_guard<std::mutex> lock(mutex_m);
        for(int i=0; i<3; ++i)
            cout << "push "<< i << endl;
        data_m.push(new_value);
    }
    std::shared_ptr<T> pop()
    {
        std::lock_guard<std::mutex> lock(mutex_m);
        for(int i=0; i<3; ++i)
            cout << "pop "<< i << endl;
        if(data_m.empty())
            throw empty_stack();
        std::shared_ptr<T> const res(std::make_shared<T>(data_m.top()));
        data_m.pop();
        return res;
    }
    void pop(T& value)
    {
        std::lock_guard<std::mutex> lock(mutex_m);
        for(int i=0; i<3; ++i)
            cout << "pop "<< i << endl;

        if(data_m.empty())
            throw empty_stack();
        value = data_m.top();
        data_m.pop();
    }
    bool empty() const
    {
        //std::lock_guard<std::mutex> lock(mutex_m);
        for(int i=0; i<3; ++i)
            cout << "empty "<< i << endl;
        return data_m.empty();
    }
};

#endif __ThreadSafeStack_hpp__