refactor: improve type extensibility and its safety (#348)

include/sdbus-c++/Message.h

@@ -158,6 +158,9 @@ namespace sdbus {
         Message& enterStruct(const std::string& signature);
         Message& exitStruct();
 
+        Message& appendArray(char type, const void *ptr, size_t size);
+        Message& readArray(char type, const void **ptr, size_t *size);
+
         explicit operator bool() const;
         void clearFlags();
 
@@ -199,9 +202,6 @@ namespace sdbus {
         template <typename _Element, typename _Allocator>
         void deserializeArraySlow(std::vector<_Element, _Allocator>& items);
 
-        void appendArray(char type, const void *ptr, size_t size);
-        void readArray(char type, const void **ptr, size_t *size);
-
         template <typename _Dictionary>
         void serializeDictionary(const _Dictionary& items);
         template <typename _Dictionary>
@@ -529,7 +529,6 @@ namespace sdbus {
         while (true)
         {
             _Element elem;
-            // TODO: Is there a way to find D-Bus message container size upfront? We could reserve space in vector upfront.
             if (*this >> elem)
                 items.emplace_back(std::move(elem));
             else

include/sdbus-c++/Types.h

@@ -56,7 +56,7 @@ namespace sdbus {
         Variant();
 
         template <typename _ValueType>
-        Variant(const _ValueType& value)
+        explicit Variant(const _ValueType& value)
             : Variant()
         {
             msg_.openVariant(signature_of<_ValueType>::str());