Redesign inheritance from Message (#62)

... so that the code is more idiomatic, clear and expressive about its intended use

include/sdbus-c++/ConvenienceApiClasses.inl

@@ -253,10 +253,10 @@ namespace sdbus {
         if (propertySignature_.empty())
             propertySignature_ = signature_of_function_output_arguments<_Function>::str();
 
-        getter_ = [callback = std::forward<_Function>(callback)](Message& msg)
+        getter_ = [callback = std::forward<_Function>(callback)](PropertyGetReply& reply)
         {
-            // Get the propety value and serialize it into the message
-            msg << callback();
+            // Get the propety value and serialize it into the pre-constructed reply message
+            reply << callback();
         };
 
         return *this;
@@ -271,14 +271,14 @@ namespace sdbus {
         if (propertySignature_.empty())
             propertySignature_ = signature_of_function_input_arguments<_Function>::str();
 
-        setter_ = [callback = std::forward<_Function>(callback)](Message& msg)
+        setter_ = [callback = std::forward<_Function>(callback)](PropertySetCall& call)
         {
             // Default-construct property value
             using property_type = function_argument_t<_Function, 0>;
             std::decay_t<property_type> property;
 
-            // Deserialize property value from the message
-            msg >> property;
+            // Deserialize property value from the incoming call message
+            call >> property;
 
             // Invoke setter with the value
             callback(property);

include/sdbus-c++/Message.h

@@ -77,16 +77,6 @@ namespace sdbus {
     class Message
     {
     public:
-        Message() = default;
-        Message(internal::ISdBus* sdbus) noexcept;
-        Message(void *msg, internal::ISdBus* sdbus) noexcept;
-        Message(void *msg, internal::ISdBus* sdbus, adopt_message_t) noexcept;
-        Message(const Message&) noexcept;
-        Message& operator=(const Message&) noexcept;
-        Message(Message&& other) noexcept;
-        Message& operator=(Message&& other) noexcept;
-        ~Message();
-
         Message& operator<<(bool item);
         Message& operator<<(int16_t item);
         Message& operator<<(int32_t item);
@@ -150,6 +140,23 @@ namespace sdbus {
         void seal();
         void rewind(bool complete);
 
+        class Factory;
+
+    protected:
+        Message() = default;
+        Message(internal::ISdBus* sdbus) noexcept;
+        Message(void *msg, internal::ISdBus* sdbus) noexcept;
+        Message(void *msg, internal::ISdBus* sdbus, adopt_message_t) noexcept;
+
+        Message(const Message&) noexcept;
+        Message& operator=(const Message&) noexcept;
+        Message(Message&& other) noexcept;
+        Message& operator=(Message&& other) noexcept;
+
+        ~Message();
+
+        friend Factory;
+
     protected:
         void* msg_{};
         internal::ISdBus* sdbus_{};
@@ -158,8 +165,10 @@ namespace sdbus {
 
     class MethodCall : public Message
     {
-    public:
         using Message::Message;
+
+    public:
+        MethodCall() = default;
         MethodReply send() const;
         MethodReply createReply() const;
         MethodReply createErrorReply(const sdbus::Error& error) const;
@@ -173,29 +182,58 @@ namespace sdbus {
 
     class AsyncMethodCall : public Message
     {
+        using Message::Message;
+
     public:
         using Slot = std::unique_ptr<void, std::function<void(void*)>>;
 
-        using Message::Message;
-        AsyncMethodCall() = default; // Fixes gcc 6.3 error (default c-tor is not imported in above using declaration)
+        AsyncMethodCall() = default;
         AsyncMethodCall(MethodCall&& call) noexcept;
         Slot send(void* callback, void* userData) const;
     };
 
     class MethodReply : public Message
     {
-    public:
         using Message::Message;
+
+    public:
+        MethodReply() = default;
         void send() const;
     };
 
     class Signal : public Message
     {
-    public:
         using Message::Message;
+
+    public:
+        Signal() = default;
         void send() const;
     };
 
+    class PropertySetCall : public Message
+    {
+        using Message::Message;
+
+    public:
+        PropertySetCall() = default;
+    };
+
+    class PropertyGetReply : public Message
+    {
+        using Message::Message;
+
+    public:
+        PropertyGetReply() = default;
+    };
+
+    class PlainMessage : public Message
+    {
+        using Message::Message;
+
+    public:
+        PlainMessage() = default;
+    };
+
     template <typename _Element>
     inline Message& operator<<(Message& msg, const std::vector<_Element>& items)
     {

include/sdbus-c++/TypeTraits.h

@@ -41,10 +41,11 @@ namespace sdbus {
     class ObjectPath;
     class Signature;
     struct UnixFd;
-    class Message;
     class MethodCall;
     class MethodReply;
     class Signal;
+    class PropertySetCall;
+    class PropertyGetReply;
     template <typename... _Results> class Result;
     class Error;
 }
@@ -54,8 +55,8 @@ namespace sdbus {
     using method_callback = std::function<void(MethodCall msg)>;
     using async_reply_handler = std::function<void(MethodReply& reply, const Error* error)>;
     using signal_handler = std::function<void(Signal& signal)>;
-    using property_set_callback = std::function<void(Message& msg)>;
-    using property_get_callback = std::function<void(Message& reply)>;
+    using property_set_callback = std::function<void(PropertySetCall& msg)>;
+    using property_get_callback = std::function<void(PropertyGetReply& reply)>;
 
     template <typename _T>
     struct signature_of

include/sdbus-c++/Types.h

@@ -94,7 +94,7 @@ namespace sdbus {
         std::string peekValueType() const;
 
     private:
-        mutable Message msg_{};
+        mutable PlainMessage msg_{};
     };
 
     /********************************************//**