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Why is std :: packaged_task <void ()> not valid?

Using MSVC2012,

The following code will compile and execute as expected.

std::packaged_task< int() > task( []()->int{ std::cout << "hello world" << std::endl; return 0; } );
std::thread t( std::move(task) );
t.join();

while the following code will not be able to compile and run

std::packaged_task< void() > task( [](){ std::cout << "hello world" << std::endl; } );
std::thread t( std::move(task) );
t.join();

Why is this so?

Edit: As a workaround, you can use std :: prom to get std :: future for a function that returns void

std::promise<void> promise;
auto future = promise.get_future();
std::thread thread( [](std::promise<void> &p){ std::cout << "hello world" << std::endl; p.set_value(); }, std::move(promise) );
future.wait();

Please note that there is an error in the vs2012 library with std :: thread that forces you to pass the promise as an l-value link and transfer the promise, it will not compile if you pass the promise by value or by r-value link. This is presumably because the implementation uses std :: bind (), which does not behave as expected.

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3 answers

gcc 4.7.2:

#include <thread>
#include <future>
#include <iostream>

int main() {
  std::packaged_task< void() > task( [](){ std::cout << "hello world" << std::endl; } );
  std::thread t( std::move(task) );
  t.join();
  std::packaged_task< int() > task2( []()->int{ std::cout << "hello world" << std::endl; return 0; } );
  std::thread t2( std::move(task2) );
  t2.join();
}

@WhozCraig , , , MSVC2012.

struct Nothing {}; nullptr_t ?

+3

The problem still exists in MSVS 2013RC, but I made this temporary patch while MS fixes it. This is the specialization of packaged_task for void (...), so I suggest putting this in the header file and including it after the standard headers. Please note that make_ready_at_thread_exit () is not implemented, and some functions have not been fully tested, use your own risk.

namespace std {

template<class... _ArgTypes>
class packaged_task<void(_ArgTypes...)>
{
    promise<void> _my_promise;
    function<void(_ArgTypes...)> _my_func;

public:
    packaged_task() {
    }

    template<class _Fty2>
    explicit packaged_task(_Fty2&& _Fnarg)
      : _my_func(_Fnarg) {
    }

    packaged_task(packaged_task&& _Other)
      : _my_promise(move(_Other._my_promise)),
        _my_func(move(_Other._my_func)) {
    }

    packaged_task& operator=(packaged_task&& _Other) {
        _my_promise = move(_Other._my_promise);
        _my_func = move(_Other._my_func);
        return (*this);
    }

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

    ~packaged_task() {
    }

    void swap(packaged_task& _Other) {
        _my_promise.swap(_Other._my_promise);
        _my_func.swap(_Other._my_func);
    }

    explicit operator bool() const {
        return _my_func != false;
    }

    bool valid() const {
        return _my_func != false;
    }

    future<void> get_future() {
        return _my_promise.get_future();
    }

    void operator()(_ArgTypes... _Args) {
        _my_func(forward<_ArgTypes>(_Args)...);
        _my_promise.set_value();
    }

    void reset() {
        swap(packaged_task());
    }
};

}; // namespace std
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