std :: bind with std :: reference_wrapper :: get

Question

std::bind을 호출하여 std::reference_wrapper::get을 호출하려고하는데 컴파일 할 수 없습니다. 나는 명백한 것을 간과하고 있지만 컴파일러 에러가 나를 도와주지 않는다고 확신한다. 이 코드는 인위적이고 내 실제 사용 사례 나타내지 않는 :

#include <functional> 
#include <iostream> 

struct A 
{ 
    void p() {std::cout << this << '\n';}; 
}; 

using ARef = std::reference_wrapper<A>; 

int main() 
{ 
    A a; 
    a.p(); 
    auto p = std::bind (&A::p, std::placeholders::_1); 
    p (a); // ok 

    ARef ar (a); 
    p (ar.get()); // ok 
    auto get = std::bind (&ARef::get, std::placeholders::_1); 
    p (get (ar)); // error 
} 

편집 : 이것은 그 소리 괜찮 컴파일합니다.

GCC 6.3.0 출력 :http://coliru.stacked-crooked.com/a/00bffc7549193cb8

main.cpp: In function 'int main()': 
main.cpp:21:19: error: no match for call to '(std::_Bind<std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>(std::_Placeholder<1>)>) (ARef&)' 
     p (get (ar)); // error 
       ^
In file included from main.cpp:1:0: 
/usr/local/include/c++/6.3.0/functional:989:2: note: candidate: template<class ... _Args, class _Result> _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) [with _Args = {_Args ...}; _Result = _Result; _Functor = std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>; _Bound_args = {std::_Placeholder<1>}] 
    operator()(_Args&&... __args) 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:989:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional:985:39: error: no match for call to '(std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>) (std::reference_wrapper<A>&)' 
    = decltype(std::declval<_Functor&>()(
       ~~~~~~~~~~~~~~~~~~~~~~~~~^ 
     _Mu<_Bound_args>()(std::declval<_Bound_args&>(), 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
     std::declval<tuple<_Args...>&>())...))> 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: candidate: template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _MemFunPtr = A& (std::reference_wrapper<A>::*)() const noexcept; bool __is_mem_fn = true] 
    operator()(_Args&&... __args) const 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:985:39: required from here 
/usr/local/include/c++/6.3.0/functional:603:27: error: no matching function for call to '__invoke(A& (std::reference_wrapper<A>::* const&)() const noexcept, std::reference_wrapper<A>&)' 
    -> decltype(std::__invoke(_M_pmf, std::forward<_Args>(__args)...)) 
       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: candidate: template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) 
    __invoke(_Callable&& __fn, _Args&&... __args) 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = A& (std::reference_wrapper<A>::* const&)() const noexcept; _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:603:27: required by substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]' 
/usr/local/include/c++/6.3.0/functional:985:39: required from here 
/usr/local/include/c++/6.3.0/functional:245:5: error: no type named 'type' in 'class std::result_of<A& (std::reference_wrapper<A>::* const&(std::reference_wrapper<A>&))() const noexcept>' 
/usr/local/include/c++/6.3.0/functional:1003:2: note: candidate: template<class ... _Args, class _Result> _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _Result = _Result; _Functor = std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>; _Bound_args = {std::_Placeholder<1>}] 
    operator()(_Args&&... __args) const 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:1003:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional:999:53: error: no match for call to '(const std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>) (std::reference_wrapper<A>&)' 
    = decltype(std::declval<typename enable_if<(sizeof...(_Args) >= 0), 
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
      typename add_const<_Functor>::type&>::type>()(
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: candidate: template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _MemFunPtr = A& (std::reference_wrapper<A>::*)() const noexcept; bool __is_mem_fn = true] 
    operator()(_Args&&... __args) const 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:999:53: required from here 
/usr/local/include/c++/6.3.0/functional:603:27: error: no matching function for call to '__invoke(A& (std::reference_wrapper<A>::* const&)() const noexcept, std::reference_wrapper<A>&)' 
    -> decltype(std::__invoke(_M_pmf, std::forward<_Args>(__args)...)) 
       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: candidate: template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) 
    __invoke(_Callable&& __fn, _Args&&... __args) 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = A& (std::reference_wrapper<A>::* const&)() const noexcept; _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:603:27: required by substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]' 
/usr/local/include/c++/6.3.0/functional:999:53: required from here 
/usr/local/include/c++/6.3.0/functional:245:5: error: no type named 'type' in 'class std::result_of<A& (std::reference_wrapper<A>::* const&(std::reference_wrapper<A>&))() const noexcept>' 
/usr/local/include/c++/6.3.0/functional:1017:2: note: candidate: template<class ... _Args, class _Result> _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) volatile [with _Args = {_Args ...}; _Result = _Result; _Functor = std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>; _Bound_args = {std::_Placeholder<1>}] 
    operator()(_Args&&... __args) volatile 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:1017:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional:1013:70: error: no match for call to '(volatile std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>) (std::reference_wrapper<A>&)' 
    = decltype(std::declval<typename enable_if<(sizeof...(_Args) >= 0), 
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
         typename add_volatile<_Functor>::type&>::type>()(
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: candidate: template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _MemFunPtr = A& (std::reference_wrapper<A>::*)() const noexcept; bool __is_mem_fn = true] 
    operator()(_Args&&... __args) const 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:1013:70: required from here 
/usr/local/include/c++/6.3.0/functional:603:27: error: no matching function for call to '__invoke(A& (std::reference_wrapper<A>::* const&)() const noexcept, std::reference_wrapper<A>&)' 
    -> decltype(std::__invoke(_M_pmf, std::forward<_Args>(__args)...)) 
       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: candidate: template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) 
    __invoke(_Callable&& __fn, _Args&&... __args) 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = A& (std::reference_wrapper<A>::* const&)() const noexcept; _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:603:27: required by substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]' 
/usr/local/include/c++/6.3.0/functional:1013:70: required from here 
/usr/local/include/c++/6.3.0/functional:245:5: error: no type named 'type' in 'class std::result_of<A& (std::reference_wrapper<A>::* const&(std::reference_wrapper<A>&))() const noexcept>' 
/usr/local/include/c++/6.3.0/functional:1031:2: note: candidate: template<class ... _Args, class _Result> _Result std::_Bind<_Functor(_Bound_args ...)>::operator()(_Args&& ...) const volatile [with _Args = {_Args ...}; _Result = _Result; _Functor = std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>; _Bound_args = {std::_Placeholder<1>}] 
    operator()(_Args&&... __args) const volatile 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:1031:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional:1027:64: error: no match for call to '(const volatile std::_Mem_fn<A& (std::reference_wrapper<A>::*)() const noexcept>) (std::reference_wrapper<A>&)' 
    = decltype(std::declval<typename enable_if<(sizeof...(_Args) >= 0), 
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
         typename add_cv<_Functor>::type&>::type>()(
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: candidate: template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {_Args ...}; _MemFunPtr = A& (std::reference_wrapper<A>::*)() const noexcept; bool __is_mem_fn = true] 
    operator()(_Args&&... __args) const 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:600:2: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:1027:64: required from here 
/usr/local/include/c++/6.3.0/functional:603:27: error: no matching function for call to '__invoke(A& (std::reference_wrapper<A>::* const&)() const noexcept, std::reference_wrapper<A>&)' 
    -> decltype(std::__invoke(_M_pmf, std::forward<_Args>(__args)...)) 
       ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: candidate: template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) 
    __invoke(_Callable&& __fn, _Args&&... __args) 
    ^~~~~~~~ 
/usr/local/include/c++/6.3.0/functional:245:5: note: template argument deduction/substitution failed: 
/usr/local/include/c++/6.3.0/functional: In substitution of 'template<class _Callable, class ... _Args> typename std::result_of<_Callable&&(_Args&& ...)>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = A& (std::reference_wrapper<A>::* const&)() const noexcept; _Args = {std::reference_wrapper<A>&}]': 
/usr/local/include/c++/6.3.0/functional:603:27: required by substitution of 'template<class ... _Args> decltype (std::__invoke(((const std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>*)this)->std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::_M_pmf, (forward<_Args>)(std::_Mem_fn_base::operator()::__args)...)) std::_Mem_fn_base<_MemFunPtr, __is_mem_fn>::operator()(_Args&& ...) const [with _Args = {std::reference_wrapper<A>&}]' 
/usr/local/include/c++/6.3.0/functional:1027:64: required from here 
/usr/local/include/c++/6.3.0/functional:245:5: error: no type named 'type' in 'class std::result_of<A& (std::reference_wrapper<A>::* const&(std::reference_wrapper<A>&))() const noexcept>' 

Answer 1

표준 라이브러리의 멤버 함수의 주소를 가지고하는 것은 불법이다, implementations can add overloads and mutilate their signatures at will 때문이다. 따라서 표준 라이브러리, 특히 the INVOKE protocol used by bind and friends은 std::reference_wrapper의 멤버에 대한 포인터가 전달되지 않는다는 가정하에 설계되었습니다. reference_wrapperr과 함께 구성원 pm에 대한 포인터를 호출하는 것은 pm을 r.get()으로 호출하는 것과 동일합니다. 즉, 기준은 무조건적으로 언 래핑된다.

도 참조하십시오. reference_wrapper 처리를 INVOKE에 추가 한 LWG issue 2219도 참조하십시오.