Can anyone suggest a way to create an integer key out of a class?

I am new to computer science and I guess this has something to do with hash function.

I have a class in cpp:

class Car{
    public:
    int fleetId;
    OwnershipEnum owner;
    int buildDate;
    bool hasFourDoors;

    ... with many other members
};

Now, my problem is, how to 'scientifically' generate an integer key based on the 4 members I list?

I can think of a simple way, 10000*int(fleetId) + 1000 * int(owner) + 100×int(buildDate) + int(hasFourDoors)

I think ideally the key is continuous, so I can store all the Car objects in an array and use this generated key to DIRECTLY access the car object.

***** as per the comments: the cars are all different, there is no identical cars ********

***** these four members are static: they won't change after being created *****

Can anyone point me to the right solution here?

Thanks