Does `std::packaged_task` need a CopyConstructible constructor argument? - c++

Question

I have this minimal not-working example of code
#include <future>
int main()
{
auto intTask = std::packaged_task<int()>( []()->int{ return 5; } );
std::packaged_task<void()> voidTask{ std::move(intTask) };
}
Why doesn't it compile (on gcc 4.8.1)? I suspect, the reason is, that std::packaged_task stores the lambda internally inside an std::function which needs a CopyConstructible argument. However, std::packaged_task is move-only. Is this a bug? What does the standard say about it? In my opinion std::packaged_task should not need a CopyConstructible argument, but a MoveConstructible argument should be enough.
By the way, when I replace std::packaged_task<int()> by std::packaged_task<void()> everything compiles fine.
GCC 4.8.1 is giving me this error message:
In file included from /usr/include/c++/4.6/future:38:0,
from ../cpp11test/main.cpp:160:
/usr/include/c++/4.6/functional: In static member function 'static void std::_Function_base::_Base_manager<_Functor>::_M_clone(std::_Any_data&, const std::_Any_data&, std::false_type) [with _Functor = std::packaged_task<int()>, std::false_type = std::integral_constant<bool, false>]':
/usr/include/c++/4.6/functional:1652:8: instantiated from 'static bool std::_Function_base::_Base_manager<_Functor>::_M_manager(std::_Any_data&, const std::_Any_data&, std::_Manager_operation) [with _Functor = std::packaged_task<int()>]'
/usr/include/c++/4.6/functional:2149:6: instantiated from 'std::function<_Res(_ArgTypes ...)>::function(_Functor, typename std::enable_if<(! std::is_integral<_Functor>::value), std::function<_Res(_ArgTypes ...)>::_Useless>::type) [with _Functor = std::packaged_task<int()>, _Res = void, _ArgTypes = {}, typename std::enable_if<(! std::is_integral<_Functor>::value), std::function<_Res(_ArgTypes ...)>::_Useless>::type = std::function<void()>::_Useless]'
/usr/include/c++/4.6/bits/shared_ptr_base.h:410:4: instantiated from 'std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc, _Args&& ...) [with _Args = {std::packaged_task<int()>}, _Tp = std::__future_base::_Task_state<void()>, _Alloc = std::allocator<std::__future_base::_Task_state<void()> >, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]'
/usr/include/c++/4.6/bits/shared_ptr_base.h:518:8: instantiated from 'std::__shared_count<_Lp>::__shared_count(std::_Sp_make_shared_tag, _Tp*, const _Alloc&, _Args&& ...) [with _Tp = std::__future_base::_Task_state<void()>, _Alloc = std::allocator<std::__future_base::_Task_state<void()> >, _Args = {std::packaged_task<int()>}, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]'
/usr/include/c++/4.6/bits/shared_ptr_base.h:987:35: instantiated from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<std::__future_base::_Task_state<void()> >, _Args = {std::packaged_task<int()>}, _Tp = std::__future_base::_Task_state<void()>, __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]'
/usr/include/c++/4.6/bits/shared_ptr.h:317:64: instantiated from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<std::__future_base::_Task_state<void()> >, _Args = {std::packaged_task<int()>}, _Tp = std::__future_base::_Task_state<void()>]'
/usr/include/c++/4.6/bits/shared_ptr.h:535:39: instantiated from 'std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = std::__future_base::_Task_state<void()>, _Alloc = std::allocator<std::__future_base::_Task_state<void()> >, _Args = {std::packaged_task<int()>}]'
/usr/include/c++/4.6/bits/shared_ptr.h:551:42: instantiated from 'std::shared_ptr<_Tp1> std::make_shared(_Args&& ...) [with _Tp = std::__future_base::_Task_state<void()>, _Args = {std::packaged_task<int()>}]'
/usr/include/c++/4.6/future:1223:66: instantiated from 'std::packaged_task<_Res(_ArgTypes ...)>::packaged_task(_Fn&&) [with _Fn = std::packaged_task<int()>, _Res = void, _ArgTypes = {}]'
../cpp11test/main.cpp:165:61: instantiated from here
/usr/include/c++/4.6/functional:1616:4: error: use of deleted function 'std::packaged_task<_Res(_ArgTypes ...)>::packaged_task(std::packaged_task<_Res(_ArgTypes ...)>&) [with _Res = int, _ArgTypes = {}, std::packaged_task<_Res(_ArgTypes ...)> = std::packaged_task<int()>]'
/usr/include/c++/4.6/future:1244:7: error: declared here
UPDATE: I have written the following test program. It seems to support the assumption that the reason is missing CopyConstructability. Again, what are the requirements on the type of the object from which an std::packaged_task may be constructed?
#include <future>
struct Functor {
Functor() {}
Functor( const Functor & ) {} // without this line it doesn't compile
Functor( Functor && ) {}
int operator()(){ return 5; }
};
int main() {
auto intTask = std::packaged_task<int()>( Functor{} );
}

Answer 1

Indeed, packaged_task only has a moving constructor (30.6.9/2):
template <class F> explicit packaged_task(F&& f);
However, your problem is the explicit constructor. So write it like this:
std::packaged_task<int()> pt([]() -> int { return 1; });
Complete example:
#include <future>
#include <thread>
int main()
{
std::packaged_task<int()> intTask([]() -> int { return 5; } );
auto f = intTask.get_future();
std::thread(std::move(intTask)).detach();
return f.get();
}

Answer 2

No. You just can't move a packaged_task<int ()> into a packaged_task<void ()>. These types a unrelated and cannot be move-assigned or move-constructed from each other. If you for some reason really want to do that you can "swallow" the result of the int () like this

Answer 3

The standard (as of N3690) doesn't state anything explicitly about the requirements of the type F in
template <class R, class... ArgTypes>
template <class F>
packaged_task<R(ArgTypes...)>::packaged_task(F&& f);
(see 30.6.9.1) However, it states that
Invoking a copy of f shall behave the same as invoking f.
and that this call can throw
any exceptions thrown by the copy or move constructor of f, or std::bad_alloc if memory
for the internal data structures could not be allocated.
This implicitly implies that the type F must be at least MoveConstructible, or CopyConstructible, if an lvalue reference is handed to the function.
Hence, it's not a bug, it's just not specified that precisely. To solve the problem of putting a std::packaged_task<int()> into a std::packaged_task<void()> just wrap the first into a shared_ptr like this:
#include <future>
#include <memory>
int main()
{
auto intTask = std::make_shared<std::packaged_task<int()>>(
[]()->int{ return 5; } );
std::packaged_task<void()> voidTask{ [=]{ (*intTask)(); } };
}