我`std :: packaged_task`是否需要一個CopyConstructible構造函數參數?
#include <future>
int main()
{
auto intTask = std::packaged_task<int()>([]()->int{ return 5; });
std::packaged_task<void()> voidTask{ std::move(intTask) };
}
爲什麼不把它編譯(海合會4.8.1)的代碼,這個最小的不工作的例子嗎?我懷疑,原因是,std::packaged_task
將lambda內部存儲在std::function
之內,該參數需要CopyConstructible
參數。但是,std::packaged_task
是僅移動的。這是一個錯誤?標準對此有何評論?在我看來,std::packaged_task
不應該需要CopyConstructible
參數,但MoveConstructible
論據應該足夠了。
順便說一下,當我用std::packaged_task<void()>
替換std::packaged_task<int()>
時,一切都很好編譯。
GCC 4.8.1是給我這個錯誤消息:
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
更新:我寫了下面的測試程序。它似乎支持原因缺失CopyConstructability
的假設。同樣,可以構造std::packaged_task
的對象類型的要求是什麼?
#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{});
}
我修改我的答案。你正在徘徊在'explicit'構造函數中。 –
你爲什麼試圖將std :: packaged_task轉換成std :: packaged_task ? –
其實我想在'std :: packaged_task'的隊列中爲任何類型'T'推送'std :: packaged_task '。我需要一個通用類型的隊列元素。目標是實現一個線程池,它在內部保存一個併發的任務隊列。 'std :: function'將不夠用,因爲我想爲計算結果返回'std :: futures'。 –