From c5424dc35136c00b2d6549704346c872c25d8685 Mon Sep 17 00:00:00 2001
From: Christian Parpart <christian@parpart.family>
Date: Sat, 8 Feb 2025 16:36:11 +0100
Subject: [PATCH] Add MemberClassType<P> and MemberIndexOf<P> to get the class
 type and member-index of P.

Signed-off-by: Christian Parpart <christian@parpart.family>
---
 include/reflection-cpp/reflection.hpp | 36 ++++++++++++++++++++++++++-
 test-reflection-cpp.cpp               |  7 ++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/include/reflection-cpp/reflection.hpp b/include/reflection-cpp/reflection.hpp
index 00265f2..d529d6d 100644
--- a/include/reflection-cpp/reflection.hpp
+++ b/include/reflection-cpp/reflection.hpp
@@ -535,6 +535,24 @@ constexpr auto TypeNameOf = [] {
 #endif
 }();
 
+namespace detail
+{
+    template <typename T>
+    struct MemberClassTypeHelper;
+
+    template <typename M, typename T>
+    struct MemberClassTypeHelper<M T::*>
+    {
+        using type = std::remove_cvref_t<T>;
+    };
+} // namespace detail
+
+/// Represents the class type of a member pointer
+///
+/// @tparam P The member pointer, e.g. &MyClass::member
+template <auto P>
+using MemberClassType = typename detail::MemberClassTypeHelper<decltype(P)>::type;
+
 namespace detail
 {
     template <class T, size_t... I>
@@ -641,7 +659,6 @@ constexpr void template_for(F&& f)
 
 namespace detail
 {
-
     template <auto P>
         requires(std::is_member_pointer_v<decltype(P)>)
     consteval std::string_view GetName()
@@ -691,6 +708,23 @@ namespace detail
 template <auto V>
 constexpr std::string_view NameOf = detail::GetName<V>();
 
+namespace detail
+{
+    // private helper for implementing MemberIndexOf<P>
+    template <auto P, size_t... I>
+    consteval size_t MemberIndexHelperImpl(std::index_sequence<I...>)
+    {
+        return (((NameOf<P> == MemberNames<MemberClassType<P>>[I]) ? I : 0) + ...);
+    }
+} // namespace detail
+
+/// Gets the index of a member pointer in the member list of its class
+///
+/// @tparam P The member pointer, e.g. &MyClass::member
+template <auto P>
+constexpr size_t MemberIndexOf =
+    detail::MemberIndexHelperImpl<P>(std::make_index_sequence<CountMembers<MemberClassType<P>>> {});
+
 /// Calls a callable on members of an object specified with ElementMask sequence with the index of the member as the
 /// first argument. and the member's default-constructed value as the second argument.
 template <typename ElementMask, typename Object, typename Callable>
diff --git a/test-reflection-cpp.cpp b/test-reflection-cpp.cpp
index 385f64b..4bab11e 100644
--- a/test-reflection-cpp.cpp
+++ b/test-reflection-cpp.cpp
@@ -35,6 +35,13 @@ struct SingleValueRecord
     int value;
 };
 
+TEST_CASE("MemberIndex", "[reflection]")
+{
+    static_assert(Reflection::MemberIndexOf<&Person::name> == 0);
+    static_assert(Reflection::MemberIndexOf<&Person::email> == 1);
+    static_assert(Reflection::MemberIndexOf<&Person::age> == 2);
+}
+
 TEST_CASE("TypeNameOf", "[reflection]")
 {
     CHECK(Reflection::TypeNameOf<int> == "int");