Merge pull request #251 from qicosmos/fix_variant

iguana/json_reader.hpp

@@ -499,13 +499,45 @@ IGUANA_INLINE void skip_object_value(It &&it, It &&end) {
   }
 }
 
+template <typename value_type, typename U, typename It>
+IGUANA_INLINE bool from_json_variant_impl(U &value, It it, It end, It &temp_it,
+                                          It &temp_end) {
+  try {
+    value_type val;
+    from_json_impl(val, it, end);
+    value = val;
+    temp_it = it;
+    temp_end = end;
+    return true;
+  } catch (std::exception &ex) {
+    return false;
+  }
+}
+
+template <size_t Idx, typename T>
+using variant_element_t = std::remove_reference_t<decltype(std::get<Idx>(
+    std::declval<std::remove_reference_t<T>>()))>;
+
+template <typename U, typename It, size_t... Idx>
+IGUANA_INLINE void from_json_variant(U &value, It &it, It &end,
+                                     std::index_sequence<Idx...>) {
+  bool r = false;
+  It temp_it = it;
+  It temp_end = end;
+  ((void)(!r && (r = from_json_variant_impl<
+                     variant_element_t<Idx, std::remove_reference_t<U>>>(
+                     value, it, end, temp_it, temp_end),
+                 true)),
+   ...);
+  it = temp_it;
+  end = temp_end;
+}
+
 template <typename U, typename It, std::enable_if_t<variant_v<U>, int> = 0>
 IGUANA_INLINE void from_json_impl(U &value, It &&it, It &&end) {
-  std::visit(
-      [&](auto &val) {
-        from_json_impl(val, it, end);
-      },
-      value);
+  from_json_variant(value, it, end,
+                    std::make_index_sequence<
+                        std::variant_size_v<std::remove_reference_t<U>>>{});
 }
 }  // namespace detail
 

test/test.cpp

@@ -175,9 +175,10 @@ REFLECTION(inner_struct, x, y, z);
 struct nest_t {
   std::string name;
   my_space::inner_struct value;
-  std::variant<int, std::string> var;
+  std::variant<int, std::string, double> var;
+  std::variant<int, std::string, my_space::inner_struct> var2;
 };
-REFLECTION(nest_t, name, value, var);
+REFLECTION(nest_t, name, value, var, var2);
 
 struct point_t1 {
   int x;
@@ -199,14 +200,16 @@ TEST_CASE("test double to int") {
 }
 
 TEST_CASE("test variant") {
-  std::variant<int, std::string> var;
-  var = 1;
-  nest_t v{"Hi", {1, 2, 3}, var}, v2;
+  std::variant<int, std::string, double> var;
+  var = "test";
+  nest_t v{"Hi", {1, 2, 3}, var, my_space::inner_struct{2, 4, 6}};
   std::string s;
   iguana::to_json(v, s);
   std::cout << s << std::endl;
+
+  nest_t v2;
   iguana::from_json(v2, s);
-  CHECK(v.name == v2.name);
+  CHECK(v.var == v2.var);
 }
 
 TEST_CASE("test from issues") {