Quand j'essayais d'écrire mon propre decay_t
comme ça :
#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 >>())() );
Et testez-le en utilisant :
#include <type_traits>
int main() {
static_assert(is_same<decay_t<int ()&>, int (*)()>{}());
}
J'ai obtenu les erreurs suivantes :
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.
Mais quand j'ai changé le test en :
#include <type_traits>
int main() {
static_assert(is_same<decay_t<int (&)()>, int (*)()>{}());
}
Puis le test est passé, et je n'arrive pas à comprendre quelle est la différence entre int (&)()
y int () &
. J'ai vérifié std::is_funcion y Déclarer des fonctions qui dit à la fois int () &
pas int (&)()
.
Edita:
Et BTW, dans le possible implementation
section de std::is_funcion , Ret (Args...) &
est une spécialisation de std::is_function. Que vont-ils faire ?