Question

C++ How to create subclass in class with array

Hi guys, i still learning C++ and face some issues here.

Basically i got a parent class

lets call this parent class as

Vehicle

It got 2 sub class, lets assume they are

Car and Motorcycle

I will create a vehicle object assume vehicle size is 20

Vehicle veh[20]

I will do the following

string vType;

cout << "Please enter your vehicle Type:";
cin >> vType;

so i do a comparision if (vType==”Car”)

it will return 4 wheels from the sub class, but how do i declare its 4 wheels at Car and 2 wheels at Motorcycle , i know i need create 2 additional cpp file which is

class Car : public Vehicle
{
private:
int noOfWheels;
public:
computePrice();
}

But how do i set noOfWheels specially to Car as 4 and Motorcycle as 2.

The next is the tricky part.. after knowing how many wheel it is

i will need store a array for each wheel

string wheel[4];

since i know there 4 wheel in cars.

How do i prompt 4 type and store it in an array, and all of this in an object call Vehicle.

I can use a for loop and thats not the issue, the part i am stuck on is how do i create a string array and store the 4 prompt and then into this Vehicle[0]

wheel 1:
wheel 2:
wheel 3:
wheel 4:

When user want to print data it will be

Vehicle[0]
Type: Car
Wheel: 4
Wheel[0] = Fine condition
Wheel[1] = Need to check again
Wheel[2] = Fine condition
Wheel[3] = Might need get repair

Thanks for all help.

Answer 1

Firstly the declaration for your array is wrong. Since you are dealing with polymorphic classes you need to use pointers.

Vehicle* veh[20];

Otherwise you will have what is called object slicing. Which means that even if you create a Car or a Motorcycle they will be converted into Vehicles when you assign them to your array.

‘how do i set noOfWheels specially to Car as 4 and Motorcycle as 2.’

In the constructor

class Car : public Vehicle
{
public:
    Car() : noOfWheels(4) { ... }
private:
    int noOfWheels;
    ...
};

class Motorcycle : public Vehicle
{
public:
    Motorcycle() : noOfWheels(2) { ... }
private:
    int noOfWheels;
    ...
};

But personally I don’t think you need a noOfWheels data member at all. Since the number of wheels is fixed for each type of Vehicle it’s a waste of space, instead you need a virtual function

class  Vehicle
{
public:
    virtual int noOfWheels() const = 0;
    ...
};

class Car : public Vehicle
{
public:
    virtual int noOfWheels() const { return 4; }
    ...
};

class Motorcycle  : public Vehicle
{
public:
    virtual int noOfWheels() const { return 2; }
    ...
};

‘how do i create a string array and store the 4 prompt and then into this Vehicle[0]’

Again I would use the constructor to initialize the car wheel names.

class Car : public Vehicle
{
public:
    Car(const std::string* w)
    { wheel[0] = w[0]; wheel[1] = w[1]; wheel[2] = w[2]; wheel[3] = w[3]; }
    virtual int noOfWheels() const { return 4; }
private:
    std::string wheel[4];
    ...
};

Use constructors to initialize classes. That’s what they are for.