Add emplace-unmarshaling (experimental)

code/json/marshaling.hxx

@@ -260,6 +260,46 @@ namespace code::json
 
   };
 
+  template<member_name_t Name, typename Type>
+  struct emplace_member_t
+  {
+    using T = Type;
+
+    static constexpr bool is_optional{
+      is_optional_v<T>
+    };
+
+    static
+    T
+    unmarshal(variant const& v, marshaling_context_t* context)
+    {
+      if (!v.contains(Name.str())) {
+        if constexpr (is_optional) {
+          return std::nullopt;
+        }
+
+        throw std::runtime_error{"missing field '" + Name.str() + "'"};
+      }
+
+      return marshaling_traits<T>::unmarshal(v.get(Name), context);
+    }
+
+  };
+
+  template<typename T, typename... Members>
+  struct emplace_t
+  {
+    static
+    T
+    unmarshal(variant const& v, marshaling_context_t* context)
+    {
+      return T{
+        Members::unmarshal(v, context)...
+      };
+    }
+
+  };
+
   template<typename T>
   struct marshaling_traits
   {