Error: cannot convert argument 1 from 'Packaged_Task::<class_name>' to 'std::nullptr_t'

Question

以下程序似乎是与显式构造函数相关的错误。但是，我无法找到答案。

使用 Visual Stduio 2017，构建时出现以下错误：

C:/data/msvc/14.28.29333/include\future(475): error C2664: 'std::function<_Ret (int,int)>::function(std::nullptr_t) noexcept': cannot convert argument 1 from 'PackagedTask::SumUp' to 'std::nullptr_t'
        with
        [
            _Ret=int
        ]
C:/data/msvc/14.28.29333/include\future(475): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
C:/data/msvc/14.28.29333/include\functional(1158): note: see declaration of 'std::function<_Ret (int,int)>::function'
        with
        [
            _Ret=int
        ]
C:/data/msvc/14.28.29333/include\future(1348): note: see reference to function template instantiation 'std::_Packaged_state<_Ret (int,int)>::_Packaged_state<PackagedTask::SumUp&>(_Fty2)' being compiled
        with
        [
            _Ret=int,
            _Fty2=PackagedTask::SumUp &
        ]
C:/data/msvc/14.28.29333/include\future(1348): note: see reference to function template instantiation 'std::_Packaged_state<_Ret (int,int)>::_Packaged_state<PackagedTask::SumUp&>(_Fty2)' being compiled
        with
        [
            _Ret=int,
            _Fty2=PackagedTask::SumUp &
        ]
<source>(30): note: see reference to function template instantiation 'std::packaged_task<int (int,int)>::packaged_task<PackagedTask::SumUp&,0>(_Fty2)' being compiled
        with
        [
            _Fty2=PackagedTask::SumUp &
        ]
<source>(30): note: see reference to function template instantiation 'std::packaged_task<int (int,int)>::packaged_task<PackagedTask::SumUp&,0>(_Fty2)' being compiled
        with
        [
            _Fty2=PackagedTask::SumUp &
        ]

#include <future>
#include <iostream>
#include <deque>

using namespace std;

class PackagedTask {
private:
    // returns sum of numbers [begin, end)
    class SumUp {
        int operator()(int begin, int end) {
            cout << "SumUp(" << begin << ", " << end << ") ..." << endl;

            long long int sum{ 0 };

            for (int i{ begin }; i < end; i++) {
                sum += i;
            }

            return sum;
        }
    };

public:
    // static main method invoked from client
    static void main() {
        SumUp s1, s2, s3, s4;

        // wrap the tasks
        packaged_task<int(int, int)> task1(s1);
        packaged_task<int(int, int)> task2(s2);
        packaged_task<int(int, int)> task3(s3);
        packaged_task<int(int, int)> task4(s4);

        // create the futures
        future<int> res1 = task1.get_future();
        future<int> res2 = task2.get_future();
        future<int> res3 = task3.get_future();
        future<int> res4 = task4.get_future();

        // push tasks to deque (packaged tasks are not copyable)
        deque<packaged_task<int(int, int)>> tasks;
        tasks.push_back(move(task1));
        tasks.push_back(move(task2));
        tasks.push_back(move(task3));
        tasks.push_back(move(task4));

        int begin{ 1 };
        int inc{ 2500 };
        int end{ begin + inc };

        while (!tasks.empty()) {
            packaged_task<int(int, int)> cur_task = move(tasks.front());

            tasks.pop_front();

            thread sum_thread(move(cur_task), begin, end);

            begin = end;
            end += inc;

            sum_thread.join();
        }

        int sum = res1.get() + res2.get() + res3.get() + res4.get();

        cout << "Sum of 0 .. 10000 = " << sum << endl;
    }
};

Answer 1

packaged_task 需要能够调用 SumUp::operator() 所以需要 public 不是私有的：

class SumUp {
    public:
    int operator()(int begin, int end) {
        cout << "SumUp(" << begin << ", " << end << ") ..." << endl;

        long long int sum{ 0 };

        for (int i{ begin }; i < end; i++) {
            sum += i;
        }

        return sum;
    }
};
class SumUp {
    int operator()(int begin, int end) {
        cout << "SumUp(" << begin << ", " << end << ") ..." << endl;

        long long int sum{ 0 };

        for (int i{ begin }; i < end; i++) {
            sum += i;
        }

        return sum;
    }
};

Error: cannot convert argument 1 from 'Packaged_Task::<class_name>' to 'std::nullptr_t'

Error: cannot convert argument 1 from 'Packaged_Task::<class_name>' to 'std::nullptr_t'

c++

compiler-errors

packaged-task