In C++, I have the following definition inside the header file inside the class TransportInstructionBase:
    friend AUTOMATICLINECORE_SHARED_EXPORT QDataStream& operator<<(QDataStream& stream, const TransportInstructionBase& inst);

However, I am getting the following error by g++:
    error: undefined reference to 'operator<<(QDebug&, TransportInstructionBase const&)'

Why would that be?
