Fixes problem of working on structs with only a single element

Signed-off-by: Christian Parpart <[email protected]>

Author: christianparpart

include/reflection-cpp/reflection.hpp

@@ -146,7 +146,7 @@ namespace detail
 
 template <class T, class... Args>
     requires(std::is_aggregate_v<std::remove_cvref_t<T>>)
-inline constexpr auto CountMembers = [] {
+inline constexpr auto CountMembers = []() constexpr {
     using V = std::remove_cvref_t<T>;
     if constexpr (requires { V { Args {}..., detail::any_t {} }; })
     {
@@ -463,8 +463,20 @@ constexpr decltype(auto) GetMemberAt(T&& t)
 }
 
 /// Represents the type of the member at index I of type Object
+// template <auto I, typename Object>
+// using MemberTypeOf = std::remove_cvref_t<decltype(std::get<I>(ToTuple(Object {})))>;
+
 template <auto I, typename Object>
-using MemberTypeOf = std::remove_cvref_t<decltype(std::get<I>(ToTuple(Object {})))>;
+using MemberTypeOf = decltype([]() constexpr {
+    if constexpr (CountMembers<Object> == 1)
+    {
+        // Work around the problem of trying to construct a get<0>(tuple) with only a single field member.
+        auto const [p0] = Object {};
+        return p0;
+    }
+    else
+        return std::get<I>(ToTuple(Object {}));
+}());
 
 template <class T>
 struct WrappedPointer final

test-reflection-cpp.cpp

@@ -30,6 +30,11 @@ enum Color
     Blue
 };
 
+struct SingleValueRecord
+{
+    int value;
+};
+
 TEST_CASE("GetName", "[reflection]")
 {
     auto const enumValue = Reflection::GetName<Color::Red>();
@@ -38,13 +43,34 @@ TEST_CASE("GetName", "[reflection]")
     auto const enumValue2 = Reflection::GetName<Color::Green>();
     CHECK(enumValue2 == "Green");
 
-    auto const person = Person { "John Doe", "[email protected]", 42 };
     auto const memberName1 = Reflection::GetName<&Person::email>();
     CHECK(memberName1 == "email");
+
+    auto const singleValueField = Reflection::GetName<&SingleValueRecord::value>();
+    CHECK(singleValueField == "value");
+}
+
+TEST_CASE("single value record", "[reflection]")
+{
+    static_assert(Reflection::CountMembers<SingleValueRecord> == 1);
+
+    auto const s = SingleValueRecord { 42 };
+    auto const t = Reflection::ToTuple(s);
+
+    CHECK(std::get<0>(t) == 42);
+    CHECK(Reflection::GetMemberAt<0>(s) == 42);
+
+    Reflection::CallOnMembers(s, [](auto&& name, auto&& value) {
+        CHECK(name == "value");
+        CHECK(value == 42);
+    });
 }
 
 TEST_CASE("core", "[reflection]")
 {
+    auto s = SingleValueRecord { 42 };
+    CHECK(Reflection::Inspect(s) == "value=42");
+
     auto p = Person { "John Doe", "[email protected]", 42 };
     auto const result = Reflection::Inspect(p);
     CHECK(result == R"(name="John Doe" email="[email protected]" age=42)");