Read code

jni.hpp/include/jni/tagging.hpp

template<class Tag, class = int>
struct SuperTag
{
    using Type = ObjectTag;
};

template<class Tag>
struct SuperTag<Tag, decltype(std::declval<typename Tag::SuperTag>(), 0)>
{
    using Type = typename Tag::SuperTag;
};

上述实现:

1、class SFINAE

2、detection idiom: expression SFINAE: decltype+ std::declval

3、decltype(std::declval<typename Tag::SuperTag>(), 0) 中的 comma 是 "built-in comma operator"，它的return value是 0，所以 decltype(std::declval<typename Tag::SuperTag>(), 0) 取的是 0 的 type，即 int

jni.hpp/include/jni/traits.hpp

#include<type_traits>
template<class ... > struct Conjunction: std::true_type
{
};
template<class B1> struct Conjunction<B1> : B1
{
};
template<class B1, class ... Bn> struct Conjunction<B1, Bn...> : std::conditional_t<bool(B1::value), Conjunction<Bn...>, B1>
{
};

上述实现:

1、recursive class template