我尝试使用std::bind
来调用std::reference_wrapper::get
但我无法进行编译。我确定我忽略了一些显而易见的事情,但编译错误并没有帮助我。此代码是人为设计的,并不能代表我的实际用例:
#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
}
编辑:使用clang编译好。
gcc 6.3.0输出: 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>'
答案 0 :(得分:5)
获取标准库成员函数的地址是非法的,因为implementations can add overloads and mutilate their signatures at will。因此,标准库,特别是the INVOKE
protocol used by bind
and friends,是围绕着它不会传递指向std::reference_wrapper
成员的指针而设计的。使用pm
reference_wrapper
调用指向成员r
的指针等同于使用pm
调用r.get()
;即,无条件打开参考文献。
另请参阅LWG issue 2219,其中将reference_wrapper
处理添加到INVOKE
。