how can I check if an object exists in C++

I am trying to write a function that will check if an object exists:

bool UnloadingBay::isEmpty() {
    bool isEmpty = true;
    if(this->unloadingShip != NULL) {
        isEmpty = false;
    }
    return isEmpty;
}

I am pretty new to C++ and not sure if my Java background is confusing something, but the compiler gives an error:

UnloadingBay.cpp:36: error: no match for ‘operator!=’ in ‘((UnloadingBay*)this)->UnloadingBay::unloadingShip != 0’

I can't seem to figure out why it doesn't work.

Here is the declaration for class UnloadingBay:

class UnloadingBay {

    private:
        Ship unloadingShip;

    public:
        UnloadingBay();
        ~UnloadingBay();

        void unloadContainer(Container container);
        void loadContainer(Container container);
        void dockShip(Ship ship);
        void undockShip(Ship ship);
        bool isEmpty();

};