refactor: use optional for passing potential call errors (#396)

This switches from a raw pointer to std::optional type to pass prospective call errors to the client (using std::optional was not possible years back when sdbus-c++ was based on C++14). This makes the API a little clearer, safer, idiomatically more expressive, and removes potential confusion associated with raw pointers (like ownership, lifetime questions, etc.).

tests/stresstests/concatenator-proxy.h

@@ -39,12 +39,12 @@ protected:
 
     virtual void onConcatenatedSignal(const std::string& concatenatedString) = 0;
 
-    virtual void onConcatenateReply(const std::string& result, const sdbus::Error* error) = 0;
+    virtual void onConcatenateReply(const std::string& result, std::optional<sdbus::Error> error) = 0;
 
 public:
     sdbus::PendingAsyncCall concatenate(const std::map<std::string, sdbus::Variant>& params)
     {
-        return proxy_->callMethodAsync("concatenate").onInterface(INTERFACE_NAME).withArguments(params).uponReplyInvoke([this](const sdbus::Error* error, const std::string& result){ this->onConcatenateReply(result, error); });
+        return proxy_->callMethodAsync("concatenate").onInterface(INTERFACE_NAME).withArguments(params).uponReplyInvoke([this](std::optional<sdbus::Error> error, const std::string& result){ this->onConcatenateReply(result, std::move(error)); });
     }
 
 private:

tests/stresstests/sdbus-c++-stress-tests.cpp

@@ -309,9 +309,9 @@ public:
     }
 
 private:
-    virtual void onConcatenateReply(const std::string& result, [[maybe_unused]] const sdbus::Error* error) override
+    virtual void onConcatenateReply(const std::string& result, [[maybe_unused]] std::optional<sdbus::Error> error) override
     {
-        assert(error == nullptr);
+        assert(error == std::nullopt);
 
         std::stringstream str(result);
         std::string aString;