Add index variable template to get index of member pointer

include/boost/pfr/core.hpp

@@ -129,6 +129,9 @@ using tuple_element = detail::sequence_tuple::tuple_element<I, decltype( ::boost
 template <std::size_t I, class T>
 using tuple_element_t = typename tuple_element<I, T>::type;
 
+template <auto ptr>
+  requires std::is_member_object_pointer_v<decltype(ptr)>
+constexpr auto index = detail::index_impl<decltype(detail::class_of(ptr)), ptr>::value;
 
 /// \brief Creates a `std::tuple` from fields of an \aggregate `val`.
 ///

include/boost/pfr/detail/core_index.hpp

@@ -0,0 +1,56 @@
+// Copyright (c) 2024 Uy Ha
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BOOST_PFR_DETAIL_CORE_INDEX_HPP
+#define BOOST_PFR_DETAIL_CORE_INDEX_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+#include <boost/pfr/detail/for_each_field_impl.hpp>
+
+#include <type_traits>
+
+namespace boost {
+namespace pfr {
+namespace detail {
+template <typename T, typename TMember>
+auto class_of(TMember T::*) -> T;
+
+template <typename T, auto ptr, std::size_t index>
+constexpr auto index_fn();
+
+template <class T, auto ptr, std::size_t index = 0, bool out_of_bound = index >= tuple_size_v<T>>
+  requires std::is_member_object_pointer_v<decltype(ptr)>
+struct index_impl {};
+
+template <class T, auto ptr, std::size_t index>
+  requires std::is_member_object_pointer_v<decltype(ptr)>
+struct index_impl<T, ptr, index, false> : decltype(index_fn<T, ptr, index>()) {};
+
+template <typename T, auto ptr, std::size_t index>
+constexpr auto index_fn() {
+  if constexpr (index < tuple_size_v<T>) {
+    using Class                = decltype(class_of(ptr));
+    constexpr auto pfr_address = std::addressof(boost::pfr::get<index>(fake_object<Class>));
+    constexpr auto mem_address = std::addressof(fake_object<Class>.*ptr);
+
+    if constexpr (std::is_same_v<decltype(pfr_address), decltype(mem_address)>) {
+      if constexpr (pfr_address == mem_address) {
+        return std::integral_constant<std::size_t, index>{};
+      } else {
+        return index_impl<T, ptr, index + 1>{};
+      }
+    } else {
+      return index_impl<T, ptr, index + 1>{};
+    }
+  } else {
+    return index_impl<T, ptr, index + 1>{};
+  }
+}
+} // namespace pfr
+} // namespace boost
+
+#endif // BOOST_PFR_DETAIL_CORE_INDEX_HPP