why testing for member pointer determines the template type is a class?

[JUANDENT]

why does this code determine if the type is a class:

template<typename T>
class is_class
{
	template<typename C> static char test(int C::*);
	template<typename C> static int test(...);
public:
	static constexpr bool value = sizeof(test<T>(0) == 1);
};

since a class may have no members and especially no integral members

e.g.

struct A
{
};
std::cout << is_class<A>::value << std::endl;

A has no member variables and yet it reports that it is a class!

Regards,
Juan

[Duthomhas]

It isn’t testing whether there are any members, only if the type conforms to a class, which may have members.

I don’t know where you got that snippet from, but it usually appears in the context of SFINAE explanations. You should go reread it. (Or find a better read.)

[doug4]

Is the last closing parenthesis in the wrong place in line 7 of the initial post? It seems to me the line should be

static constexpr bool value = sizeof(test<T>(0)) == 1

If not, I'm confused why it's taking the size of a bool and then casting that size_t value back to another bool.

[Duthomhas]

Yes, it is in the wrong spot.

[JUANDENT]

Thanks!!