@hek said: You might need to declare them extern so the compiler won't complain. This IS the solution. Thank you for this. To be honest i never used (or knew) extern keyword exists. As a Java-dev, i see this against I believe in : ) Thank you! You made me see the light in the tunnel. (I'm still in the hope its not the train : ) To share the actual solution: In the class, where I was using MyGateway *gw before, now I have this (in the .cpp file): extern bool send(MyMessage &message, bool enableAck) ...and i replaced gw->send(..) with send(..) The imports for the MyMessage class had to be changed to core/MyMessage.h. I'm using visual micro + visual studio for this development.