如何使用多态使复合对象全局化到派生类？

Question

I have written a program that allows users to take out and return vehicles. For example, a user decides to take a car out for personal use, the amount of vehicles left would then be decremented from 2 to 1. The problem is that whenever a new object is created, the amount of vehicles in the composite vehicle class would always be defaulted 2 and when you check out a vehicle, it would only decrement the amount of vehicles for that object created. I am not so good at explaining so maybe this would help.

int main(){
int a = 0;
int vn;
int counter = 0;
VMS *tracking[20];
while (a != 6){
    cout << "(1) Company (2) Personal (3) Carpool (4) Cargo (5) Return a vehicle (6) Exit" << endl << "Select the appropriate application: ";
    cin >> a;
    switch (a){
    case 1: tracking[counter] = new company(); cout << "YOU ARE USER #" << counter << " PLEASE REMEMBER YOUR USER NUMBER"; counter++; break;
    case 2: tracking[counter] = new personal(); cout << "YOU ARE USER #" << counter << " PLEASE REMEMBER YOUR USER NUMBER"; counter++; break;
    case 3: tracking[counter] = new carpool(); cout << "YOU ARE USER  #" << counter << " PLEASE REMEMBER YOUR USER NUMBER"; counter++; break;
    case 4: tracking[counter] = new cargo(); cout << "YOU ARE USER #" << counter << " PLEASE REMEMBER YOUR USER NUMBER"; counter++; break;
    case 5: cout << "Please enter your User #: "; cin >> vn; tracking[vn]->returned(); break;
    case 6: exit(1); break;//exit
    }

So every time 1 - 4 has been selected a new object will be created and every time there will be a new instance of the composite vehicle class that defaults 2 to the amount of vehicles remaining. What i am looking to do is somehow make the amount of vehicles in class vehicle decrement globally for all objects created so that when the amount of vehicles = 0, a prompt will display such. Is there any way to accomplish this by still having a vehicle class composition in the base class? I am not so sure what code to include that will help so i think these may be helpful.
BASE CLASS:

class VMS{
public:
    VMS(){}
    virtual void set();
    virtual void print();
    virtual void returned();
    virtual void setV();
private:
    string purpose;
    string dName;
    int emplidNum;
    string date;
    string Time;
    string rtime;
    vehicle vehicle;//Composition of vehicle class
};
#endif

VEHICLE CLASS: (COMPOSITION)

class vehicle{
public:
vehicle() { sedan = 2, pickup = 2, minivan = 2; }
bool empty1(){
    if (sedan == 0){
        cout << "All Vehicles of this type are in use, please select a different vehicle" << endl;
        return true;
    }
    else
        return false;
}
bool empty2(){
    if (pickup == 0){
        cout << "All Vehicles of this type are in use, please select a different vehicle" << endl;
        return true;
    }
    else
        return false;
}
bool empty3(){
    if (minivan == 0){
        cout << "All Vehicles of this type are in use, please select a different vehicle" << endl;
        return true;
    }
    else
        return false;
}
//the vehicle objects
int sedan;
int pickup;
int minivan;
};
#endif

Sorry for the length of the post. Any help would be appreciated.

Answer 1

To make the values independent of any instance of the class you can use static variables. You simply declare the variables static in your class and they will be the same variable for all instances of the class. If you change your declarations of sedan , pickup and minivan to

static int sedan;
static int pickup;
static int minivan;

the variables will be global.

You will not be able to initialize these in the constructor of the class as doing that will reset the values each time a new instance of vehicle is created. You will have to have an initializer method to initialize the values. So if you create a method like this

static void vehicle::init()
{
    sedan = 2;
    pickup = 2;
    minivan = 2;
}

and you call it at the start of your program the values will be initialized as in your code.

You can read more about static here and storage duration specifiers here .

---

As dwcanillas pointed out, a more oop way of accomplishing this would be to have a garage class that keeps track of some vehicles. So you could have a class like this

class garage
{
public:
   vehicle getVehicle();
   void returnVehicle();

   garage(int numVehicles);

private:
   int numVehicles;

};

which keeps track of all vehicles, or only keeps track of a specific subset of the vehicles, so you might have one garage for sedans, one for minivans and one for pickups.