Serialize empty tuple into '[]' instead of null (#4594)

Signed-off-by: Michael Valladolid <[email protected]>

Author: codenut

include/nlohmann/detail/conversions/from_json.hpp

@@ -448,6 +448,12 @@ std::tuple<Args...> from_json_tuple_impl_base(BasicJsonType&& j, index_sequence<
     return std::make_tuple(std::forward<BasicJsonType>(j).at(Idx).template get<Args>()...);
 }
 
+template<typename BasicJsonType>
+std::tuple<> from_json_tuple_impl_base(BasicJsonType& /*unused*/, index_sequence<> /*unused*/)
+{
+    return {};
+}
+
 template < typename BasicJsonType, class A1, class A2 >
 std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair<A1, A2>> /*unused*/, priority_tag<0> /*unused*/)
 {

include/nlohmann/detail/conversions/to_json.hpp

@@ -426,6 +426,13 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
+template<typename BasicJsonType, typename Tuple>
+inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)
+{
+    using array_t = typename BasicJsonType::array_t;
+    j = array_t();
+}
+
 template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
 inline void to_json(BasicJsonType& j, const T& t)
 {

single_include/nlohmann/json.hpp

@@ -5190,6 +5190,12 @@ std::tuple<Args...> from_json_tuple_impl_base(BasicJsonType&& j, index_sequence<
     return std::make_tuple(std::forward<BasicJsonType>(j).at(Idx).template get<Args>()...);
 }
 
+template<typename BasicJsonType>
+std::tuple<> from_json_tuple_impl_base(BasicJsonType& /*unused*/, index_sequence<> /*unused*/)
+{
+    return {};
+}
+
 template < typename BasicJsonType, class A1, class A2 >
 std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair<A1, A2>> /*unused*/, priority_tag<0> /*unused*/)
 {
@@ -6019,6 +6025,13 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
     j = { std::get<Idx>(t)... };
 }
 
+template<typename BasicJsonType, typename Tuple>
+inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)
+{
+    using array_t = typename BasicJsonType::array_t;
+    j = array_t();
+}
+
 template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
 inline void to_json(BasicJsonType& j, const T& t)
 {

tests/src/unit-regression2.cpp

@@ -814,6 +814,15 @@ TEST_CASE("regression tests 2")
         }
     }
 
+    SECTION("issue #4530 - Serialization of empty tuple")
+    {
+        const auto source_tuple = std::tuple<>();
+        const nlohmann::json j = source_tuple;
+
+        CHECK(j.get<decltype(source_tuple)>() == source_tuple);
+        CHECK("[]" == j.dump());
+    }
+
     SECTION("issue #2865 - ASAN detects memory leaks")
     {
         // the code below is expected to not leak memory