[英]Why does std::void_t not work in such a case?
#include <type_traits>
template<typename, typename = void>
struct IsIterator final : std::false_type
{};
template<typename T>
struct IsIterator<T,
std::void_t<std::enable_if_t<std::is_base_of_v<std::input_iterator_tag,
typename std::iterator_traits<T>::iterator_category>>>>
final : std::true_type
{};
int main()
{
return IsIterator<void*>::value;
}
clang 8.0 gives the following error messages: clang 8.0给出以下错误消息:
/usr/bin/../include/c++/v1/iterator:507:16: error: cannot form a reference to 'void'
typedef _Tp& reference;
^
main.cpp:20:23: note: in instantiation of template class 'std::__1::iterator_traits<void *>' requested
here
typename std::iterator_traits<T>::iterator_category>>>>
^
main.cpp:29:16: note: during template argument deduction for class template partial specialization
'IsIterator<T, std::void_t<std::enable_if_t<std::is_base_of_v<std::input_iterator_tag, typename
std::iterator_traits<T>::iterator_category> > > >' [with T = void *]
return IsIterator<void*>::value;
^
main.cpp:29:16: note: in instantiation of template class 'IsIterator<void *, void>' requested here
Why does std::void_t
not work in such a case? 为什么在这种情况下
std::void_t
不起作用?
std::iterator_traits<T>::iterator_category
forces instantiation of std::iterator_traits<T>
which is ill formed (hard error and not soft error for SFINAE) for void*
. std::iterator_traits<T>::iterator_category
强制实例化std::iterator_traits<T>
(对于void*
,格式不正确(对于SFINAE是硬错误而不是软错误))。
You have to handle void*
manually. 您必须手动处理
void*
。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.