ClassVersion.h

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#pragma once
 
namespace hdtree {
 
namespace class_version_impl {
 
template <class T, class R = void>
struct enable_if_type {
  using type = R;
};
 
template <class T, class Enable = void>
struct deducer {
  using version = std::integral_constant<int, 0>;
};
 
template <class T>
struct deducer<T, typename enable_if_type<typename T::version>::type> {
  using version = typename T::version;
};
 
}  // namespace class_version_impl
 
template <typename T>
inline constexpr int class_version =
    class_version_impl::deducer<T>::version::value;
 
}  // namespace hdtree
 
#define hdtree_class_version(VERS) \
  using version = std::integral_constant<int, VERS>