Fix serialization macros with no member variables

cmake/clang_flags.cmake

@@ -19,4 +19,5 @@ set(CLANG_CXXFLAGS
     -Wno-covered-switch-default
     -Wno-unsafe-buffer-usage
     -Wno-missing-noreturn
+    -Wno-gnu-zero-variadic-macro-arguments
 )

include/nlohmann/detail/macro_scope.hpp

@@ -544,6 +544,28 @@
     template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
     void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
 
+/*!
+@brief macro
+@def NLOHMANN_DEFINE_EMPTY_TYPE_INTRUSIVE
+@since version 3.12.0
+*/
+#define NLOHMANN_DEFINE_EMPTY_TYPE_INTRUSIVE(Type)  \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    friend void to_json(BasicJsonType& nlohmann_json_j, const Type&) { nlohmann_json_j = BasicJsonType::object(); } \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    friend void from_json(const BasicJsonType&, Type&) noexcept { }
+
+/*!
+@brief macro
+@def NLOHMANN_DEFINE_EMPTY_TYPE_NON_INTRUSIVE
+@since version 3.12.0
+*/
+#define NLOHMANN_DEFINE_EMPTY_TYPE_NON_INTRUSIVE(Type)  \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    void to_json(BasicJsonType& nlohmann_json_j, const Type&) { nlohmann_json_j = BasicJsonType::object(); } \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    void from_json(const BasicJsonType&, Type&) noexcept { }
+
 // inspired from https://stackoverflow.com/a/26745591
 // allows calling any std function as if (e.g., with begin):
 // using std::begin; begin(x);

single_include/nlohmann/json.hpp

@@ -2908,6 +2908,28 @@ JSON_HEDLEY_DIAGNOSTIC_POP
     template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
     void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
 
+/*!
+@brief macro
+@def NLOHMANN_DEFINE_EMPTY_TYPE_INTRUSIVE
+@since version 3.12.0
+*/
+#define NLOHMANN_DEFINE_EMPTY_TYPE_INTRUSIVE(Type)  \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    friend void to_json(BasicJsonType& nlohmann_json_j, const Type&) { nlohmann_json_j = BasicJsonType::object(); } \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    friend void from_json(const BasicJsonType&, Type&) noexcept { }
+
+/*!
+@brief macro
+@def NLOHMANN_DEFINE_EMPTY_TYPE_NON_INTRUSIVE
+@since version 3.12.0
+*/
+#define NLOHMANN_DEFINE_EMPTY_TYPE_NON_INTRUSIVE(Type)  \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    void to_json(BasicJsonType& nlohmann_json_j, const Type&) { nlohmann_json_j = BasicJsonType::object(); } \
+    template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
+    void from_json(const BasicJsonType&, Type&) noexcept { }
+
 // inspired from https://stackoverflow.com/a/26745591
 // allows calling any std function as if (e.g., with begin):
 // using std::begin; begin(x);

tests/src/unit-udt_macro.cpp

@@ -707,3 +707,36 @@ TEST_CASE_TEMPLATE("Serialization of non-default-constructible classes via NLOHM
                                            R"([{"age":1,"hair_color":"brown","name":"Erik"},{"age":2,"hair_color":"black","name":"Kyle"}])"));
     }
 }
+
+namespace emptys
+{
+class empty_intrusive
+{
+    NLOHMANN_DEFINE_EMPTY_TYPE_INTRUSIVE(empty_intrusive)
+};
+
+class empty_non_intrusive
+{
+};
+
+NLOHMANN_DEFINE_EMPTY_TYPE_NON_INTRUSIVE(empty_non_intrusive)
+
+} // namespace emptys
+
+TEST_CASE_TEMPLATE("Serialization/deserialization of classes with no member variables", T,
+                   emptys::empty_intrusive,
+                   emptys::empty_non_intrusive)
+{
+    SECTION("empty")
+    {
+        {
+            T empty;
+            std::string const s = json(empty).dump();
+            CHECK(s == "{}");
+
+            nlohmann::json const json_empty = nlohmann::json::parse(s);
+            T empty2;
+            json_empty.get_to(empty2);
+        }
+    }
+}