Child Class defined before its parent

Question

I am writing a program for a shopping cart. Everything is derived from the customer. Every customer has a shopping cart. Therefore I thought to put the shopping cart before the Customer class. When doing this I can't declare Shopping cart as a child of Customer. Any fixes?

class ShoppingCart : public Customer
{
    private:
    Item* cart[MAX_SIZE] = {NULL};  //problem  may occur
    int size;

    public:
    void addItem(char* type, char* title, char* description, double price);
    void showCart();
};

class Customer
{
    private:
    int ID, items;
    char* firstName, *lastName;
    ShoppingCart* Cart;

    public:
    Customer(char* userFirst, char* userLast, int userID, ShoppingCart* cart)
    {
        this->ID = userID;
        this->firstName = userFirst;
        this->lastName = userLast;
        this->Cart = cart;
    }
    friend void ShoppingCart::addItem(char* type, char* title, char* description, double price);
};

Look into `std::string`, shared pointers. And avoid friends. Perhaps `std::vector` — Ed Heal, Jan 31 '18 at 22:13
"Every customer has a shopping cart." that clearly shows that it is "Has-A" relationship and not "Is-A" — Slava, Jan 31 '18 at 22:13
Thats a good point but I want them to be related. I want the cart to have access to the name of the customer and his ID — MaxAttax, Jan 31 '18 at 22:14
@Slava so what could I use to show a has-a relationship. I'm kinda teaching myself this. — MaxAttax, Jan 31 '18 at 22:15
@George I have it as a friend function because during the additem function I want to increase the "items" variable — MaxAttax, Jan 31 '18 at 22:16
@DemarcusSales it is possible that one customer can have more than one shopping card, so inherit one from another is clearly wrong. — Slava, Jan 31 '18 at 22:18
"class ShoppingCart : public Customer" - Is a shopping cart a type of customer? i.e. shoppingcat isa customer — Ed Heal, Jan 31 '18 at 22:18
And a customer might not have a shopping cart. Might just be popping into the shop for a packet of luckies — Ed Heal, Jan 31 '18 at 22:19
There is inheritance, composition and aggregation. See [this SO post](https://stackoverflow.com/questions/2218937/has-a-is-a-terminology-in-object-oriented-language) for more information. — Ron, Jan 31 '18 at 22:22
You already have a "has a" relationship by having ShoppingCart as a member of Customer. You could then have a pointer or reference from the ShoppingCart to the Customer and get rid of the weird subclassing relationship. — Ian4264, Jan 31 '18 at 22:22

Arnav Borborah · Answer 1 · 2018-01-31T22:19:14.367

1

As for your problem, no, there is no solution to the problem other than declaring and defining the base class before the derived class. Forward declaration would not work here (As I naively thought), since the child class needs contextual information from the base class.

While the above may work, I would like to point out that you really aren't understanding inheritance. Inheritance is based off a is a relationship i.e Derived is a Base, not a has a relationship, which is what your code is trying to emulate. If you want to emulate a has a relationship, then you should make the class ShoppingCart a member of Customer, since that will show that each Customer has a shopping cart. Inheritance is not the right method to solve your problem here.

edited Jan 31 '18 at 22:19

answered Jan 31 '18 at 22:10

Arnav Borborah

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Is there any way I could make the shopping cart aware of the of the private member variables in customer – MaxAttax Jan 31 '18 at 22:32
1

Yes, make `ShoppingCart` a [`friend`](http://en.cppreference.com/w/cpp/language/friend) of `Customer`. – Arnav Borborah Jan 31 '18 at 22:34
I would highly advise against the use of `friend` here as that breaks encapsulation when setters and getters would work just fine. `friend` should be reserved for special cases like `operator<<` and when you need to write unit tests. – scohe001 Feb 01 '18 at 00:17
On the other hand, `public` accessor functions can globally weaken or break encapsulation, while `friend` restricts that break to only trusted classes and functions. In this case I agree though. Add `removeItem` and `checkout` methods and all the accessibility anyone should need is present in `ShoppingCart`. `ShoppingCart` should be looking after itself, not letting others poke around inside it. – user4581301 Feb 01 '18 at 00:35

Child Class defined before its parent

1 Answers1