使用std :: bind与std :: reference_wrapper :: get

Question

我正在尝试使用std::bind调用std::reference_wrapper::get但是我无法对其进行编译。 我确定我忽略了一些明显的问题，但是编译器错误并没有帮助我。 这段代码是人为设计的，并不代表我的实际用例：

#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：这可以用clang编译。

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>'

Answer 1

获取标准库成员函数的地址是非法的，因为实现可能会增加重载并随意破坏其签名 。 因此，标准库，尤其是bind和friends使用的INVOKE协议 ，是基于以下假设设计的：它不会传递指向std::reference_wrapper成员的指针。 使用reference_wrapper r调用指向成员pm的指针等效于使用r.get()调用pm ； 也就是说，该引用是无条件展开的。

另请参见LWG问题2219 ，该问题在INVOKE添加了reference_wrapper处理。