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

JUANDENT (411)
why does this code determine if the type is a class:

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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.
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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
Last edited on
Duthomhas (13206)
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 (1538)
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 (13206)
Yes, it is in the wrong spot.
JUANDENT (411)
Thanks!!
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