has_type template returns true for struct type {};

Question

There are a number of ways to implement a has_type<T> template that deduces if T has a nested class or typedef named type. ie

namespace detail {
    template<typename> struct tovoid { typedef void type; };
}

template<typename T, typename = void> struct has_type
    : std::false_type { };
// this one will only be selected if C::type is valid
template<typename C> struct has_type<C, typename detail::tovoid<typename C::type>::type>
    : std::true_type { };

Or

template <typename C> char test_for_type(...) { return '0'; }
template <typename C> double test_for_type(typename C::type const *) { return 0.0; }

template <typename T> struct has_type
{
    static const bool value = sizeof(test_for_type<T>(0)) == sizeof(double);
};

however in either case, has_type<type>::value is true for this class:

struct type
{
};

Now the above type doesn't have another type nested within it, but it does have a constructor type::type().

But should that constructor 'trigger' the checks for the nested type? Or is it a compiler bug? (I would like to think that typename type::type didn't apply to a constructor and/or that you couldn't take a pointer to a constructor, such as what would be produced by the second test method: typename type::type const *.

?

Answer 1

The name of a class is "injected" into the scope of the class, so type::type really is the name of a type, and it's the same type as ::type.