C ++如何实现std :: is_class

Question

Here is the implementation of std::is_class in mac. I don't understand what is the parameter of "int _Tp::*". Can someone explain this? what is the type of parameter and what is the name of parameter? Thanks.

And another question is the ellipsis. In C it needs at least one parameter before ellipsis, but in c++ it can be just the ellipsis. Where can I find the standard about this part?

namespace __is_class_imp
{
template <class _Tp> char  __test(int _Tp::*);
template <class _Tp> __two __test(...);
}

template <class _Tp> struct _LIBCPP_TYPE_VIS_ONLY is_class
    : public integral_constant<bool, sizeof(__is_class_imp::__test<_Tp>(0)) == 1 && !is_union<_Tp>::value> {};

Answer 1

To answer one of your questions, the __test(...) function accepts any number of any kind of arguments. But they can't be used. The va_start macro needs a previous argument to work, without it one can not get any arguments.

The reason for it is so that __test can be used with any kind of arguments, but won't actually be implemented. This works because the function is only used ad compile-time, no actual calling of will happen. And since it's overloaded the compiler will do different things depending on the types used.