entity: get field data indirectly via a specialization

so now classes will be able to #ifdef including `entity/metadata.hpp`
and specifying the fields without incurring an ODR violation.

2 files changed, 33 insertions, 19 deletions

diff --git a/entity/metadata.hpp b/entity/metadata.hpp
index 935f1792..0014f63c 100644
--- a/entity/metadata.hpp
+++ b/entity/metadata.hpp
@@ -15,6 +15,12 @@
 #include <compat/function2.hpp>
 #include <Corrade/Containers/StringView.h>
 
+namespace floormat::entities {
+
+template<typename T> struct entity_accessors;
+
+} // namespace floormat::entities
+
 namespace floormat::entities::detail {
 
 template<typename F, typename Tuple, std::size_t N>
@@ -46,7 +52,7 @@ constexpr CORRADE_ALWAYS_INLINE bool find_in_tuple(F&& fun, Tuple&& tuple)
 template<typename T> struct decay_tuple_;
 template<typename... Ts> struct decay_tuple_<std::tuple<Ts...>> { using type = std::tuple<std::decay_t<Ts>...>; };
 template<typename T> using decay_tuple = typename decay_tuple_<T>::type;
-template<typename T> struct accessors_for_ { using type = decay_tuple<std::decay_t<decltype(T::accessors())>>; };
+template<typename T> struct accessors_for_ { using type = decay_tuple<std::decay_t<decltype(entity_accessors<T>::accessors())>>; };
 template<typename T> using accessors_for = typename accessors_for_<T>::type;
 
 template<typename Obj, typename Type, typename Default, std::size_t I, typename... Fs> struct find_reader;
@@ -269,7 +275,7 @@ class entity_metadata final {
 public:
     static constexpr StringView class_name = name_of<T>;
     static constexpr std::size_t size = std::tuple_size_v<entities::detail::accessors_for<T>>;
-    static constexpr entities::detail::accessors_for<T> accessors = T::accessors();
+    static constexpr entities::detail::accessors_for<T> accessors = entities::entity_accessors<T>::accessors();
     static constexpr auto erased_accessors = erased_helper(accessors);
 };
 
diff --git a/test/entity.cpp b/test/entity.cpp
index a10cbb98..ce7af58b 100644
--- a/test/entity.cpp
+++ b/test/entity.cpp
@@ -15,23 +15,31 @@ struct TestAccessors {
     int foo;
     int _bar;
     int _baz;
+};
+
+} // namespace
 
-    static constexpr auto accessors() noexcept;
+namespace floormat::entities {
+
+template<> struct entity_accessors<TestAccessors> {
+    static constexpr auto accessors()
+    {
+        using entity = Entity<TestAccessors>;
+        constexpr auto r_baz = [](const TestAccessors& x) { return x._baz; };
+        constexpr auto w_baz = [](TestAccessors& x, int v) { x._baz = v; };
+
+        constexpr auto tuple = std::make_tuple(
+            entity::type<int>::field{"foo"_s, &TestAccessors::foo, &TestAccessors::foo},
+            entity::type<int>::field{"bar"_s, &TestAccessors::bar, &TestAccessors::set_bar},
+            entity::type<int>::field("baz"_s, r_baz, w_baz)
+        );
+        return tuple;
+    }
 };
 
-constexpr auto TestAccessors::accessors() noexcept
-{
-    using entity = Entity<TestAccessors>;
-    constexpr auto r_baz = [](const TestAccessors& x) { return x._baz; };
-    constexpr auto w_baz = [](TestAccessors& x, int v) { x._baz = v; };
-
-    constexpr auto tuple = std::make_tuple(
-        entity::type<int>::field{"foo"_s, &TestAccessors::foo, &TestAccessors::foo},
-        entity::type<int>::field{"bar"_s, &TestAccessors::bar, &TestAccessors::set_bar},
-        entity::type<int>::field("baz"_s, r_baz, w_baz)
-    );
-    return tuple;
-}
+} // namespace floormat::entities
+
+namespace {
 
 using entity = Entity<TestAccessors>;
 constexpr auto m_foo = entity::type<int>::field{"foo"_s, &TestAccessors::foo, &TestAccessors::foo};
@@ -151,7 +159,7 @@ void test_type_name()
     constexpr auto foo = entity::type<int>::field{"foo"_s, &TestAccessors::foo, nullptr};
     static_assert(foo.writer == nullptr);
     static_assert(!foo.can_write);
-    static_assert(std::get<0>(TestAccessors::accessors()).can_write);
+    static_assert(std::get<0>(entity_accessors<TestAccessors>::accessors()).can_write);
 }
 
 void test_predicate()
@@ -189,7 +197,7 @@ constexpr bool test_names()
 constexpr void test_constraints()
 {
     constexpr auto x = TestAccessors{};
-    constexpr auto foo = entity::type<int>::field{
+    constexpr auto foo = entity::type<int>::field {
         "foo"_s, &TestAccessors::foo, &TestAccessors::foo,
         constantly(constraints::max_length{42}),
         constantly(constraints::range<int>{37, 42}),
@@ -221,8 +229,8 @@ void test_erased_constraints()
         constantly(constraints::range<int>{37, 42}),
         constantly(constraints::group{"foo"_s})
     };
+    static constexpr auto erased = foo.erased();
     const auto x = TestAccessors{};
-    const auto erased = foo.erased();
 
     fm_assert(erased.get_range(x) == constraints::range<int>{37, 42});
     fm_assert(erased.get_max_length(x) == 42);