当我试图这样写自己的decay_t
时:
#include <utility>
template <class T>
struct auto_decay {
auto operator () () noexcept {
return std::declval<T>();
}
};
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );
并使用以下命令进行测试:
#include <type_traits>
int main() {
static_assert(is_same<decay_t<int ()&>, int (*)()>{}());
}
我遇到以下错误:
In file included from test_decay.cc:1:
In file included from ./../src/decay.hpp:4:
In file included from ./../src/utility.hpp:3:
./../src/add_reference.hpp:17:14: error: reference to function type 'int () &' cannot have '&' qualifier
typedef T&& type;
^
./../src/add_reference.hpp:20:1: note: in instantiation of template class 'nxwheels::concept_check::add_rvalue_reference<int () &>' requested here
using add_rvalue_reference_t = typename add_rvalue_reference<T>::type;
^
./../src/utility.hpp:14:1: note: in instantiation of template type alias 'add_rvalue_reference_t' requested here
auto declval() noexcept -> add_rvalue_reference_t<T>;
^
./../src/decay.hpp:10:16: note: while substituting explicitly-specified template arguments into function template 'declval'
return declval<T>();
^
./../src/decay.hpp:16:46: note: in instantiation of member function 'nxwheels::concept_check::decay_impl::auto_decay<int () &>::operator()' requested here
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );
^
test_decay.cc:17:17: note: in instantiation of template type alias 'decay_t' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
In file included from test_decay.cc:1:
./../src/decay.hpp:10:16: error: call to function 'declval' that is neither visible in the template definition nor found by argument-dependent lookup
return declval<T>();
^
./../src/decay.hpp:16:46: note: in instantiation of member function 'nxwheels::concept_check::decay_impl::auto_decay<int () &>::operator()' requested here
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );
^
test_decay.cc:17:17: note: in instantiation of template type alias 'decay_t' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
./../src/utility.hpp:14:6: note: 'declval' should be declared prior to the call site
auto declval() noexcept -> add_rvalue_reference_t<T>;
^
In file included from test_decay.cc:2:
./../src/concepts/is_same.hpp:18:5: error: static_assert failed
static_assert(is_same_v<T1, T2>);
^ ~~~~~~~~~~~~~~~~~
test_decay.cc:17:5: note: in instantiation of function template specialization 'nxwheels::concept_check::assert_same<int, int (*)()>' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
3 errors generated.
为什么不能将&&
添加到Ret (Args...) &
?
答案 0 :(得分:5)
int () &
并不是您(似乎)认为的那样。
它不是对函数的引用,而是对ref-qualified函数的引用。
如果要引用功能,则需要使用int (&)()
。
为什么我不能将
&&
添加到Ret (Args...) &
似乎合格的(ref-或cv合格的)函数类型只能在非常特定的地方使用:
[dcl.typedef]/6
具有 cv-qualifier-seq 或 ref-qualifier 的函数类型(包括以typedef-name(
[dcl.typedef]
,{{ 1}})) 仅显示为:-非静态成员的函数类型 功能
-指向成员的指针所指向的函数类型,
-函数typedef声明的顶级函数类型或 别名声明
- type-parameter 的默认参数中的 type-id ,或
- type-parameter (
[temp.param]
)的 template-argument 的 type-id 。>