Doubly Linked Lists

[Swerve]

Hi,

I am trying to create a doubly linked list and was hoping for some advice on if my classes are correctly declared.

class Car
{
public:
	AddaNewDieselHardTop();//All functions for adding new nodes declared here in Car. This is OK because they are Friend Classes?
        AddaNewDiesleSoftTop();
private:
	CarNode *head;
};

////////////////////////////////

class Diesel
{
	friend class Car;
};

///////////////////////////////


class DieselHardTop
{

	friend class Diesel;
	friend class Car;
private:
	int MPG;
	DieselHardTop *llink, *rlink;
};

class DieselSoftTop
{
	friend class Diesel;
	friend class Car;
private:
	int MPG;
	DieselSoftTop *llink, *rlink;
};

My classes are '3 deep' as shown in the attached image. I want to hold all the objects from all the classes within a single doubly linked list.

The user will be able to add new instances of DieselHardTop and DieselSoftTop, and I'm wondering if I can declare the 'AddNewDieselHardTop()' and 'AddNewDieselSoftTop' functions just once within the Car class declaration, or if I would need to declare these functions in their own respective classes.

The user will select 'Add a new car' from a menu, and I was going to use a Switch statement, so if the user chose to 'Add a new DieselSoftTop', I would then call that function, which if correct, would be declared within Car, and not the DieselSoftTop class.

Hope that makes sense!

Many thanks!!!

[anon]

This design doesn't seem to make much sense. Base classes shouldn't know about classes derived from them. Also there doesn't seem any reason in C++ to implement your own doubly-linked list.

Without all in-house linked list implementation non-sense:

#include <iostream>
#include <list>
#include <algorithm>
#include <functional>

class Car
{
public:
    virtual ~Car() {}
    virtual void print() const = 0;
};

class Diesel: public Car
{
};

class Petrol: public Car
{
public:
    void print() const { std::cout << "Petrol\n"; }
};

class DieselHardTop: public Diesel
{
public:
    void print() const { std::cout << "DieselHardTop\n"; }
};

class DieselSoftTop: public Diesel
{
public:
    void print() const { std::cout << "DieselSoftTop\n"; }
};

template <class T>
void Delete(const T* t)
{
    delete t;
}

int main()
{
    enum { petrol = 1, diesel_hardtop, diesel_softtop, max_car_type };
    std::list<Car*> cars;
    int choice;
    while (std::cout << "Enter " << petrol << " for Petrol, " << diesel_hardtop << " for DieselHardTop, "
        << diesel_softtop << " for DieselSoftTop or other to quit: "
        && std::cin >> choice && choice > 0 && choice < max_car_type) {
        switch (choice) {
            case petrol:
                cars.push_back(new Petrol);
                break;
            case diesel_hardtop:
                cars.push_back(new DieselHardTop);
                break;
            case diesel_softtop:
                cars.push_back(new DieselSoftTop);
                break;
        }
    }
    std::for_each(cars.begin(), cars.end(), std::mem_fun(&Car::print));
    std::for_each(cars.begin(), cars.end(), Delete<Car>);
}

[Swerve]

Many thanks anon!

That has thrown a 'spanner in the works' ! hehe.

For the last couple of days I've been reading about linked lists and vectors etc and the examples in the books have shown them this way.

I was planning declaring the different functions within the base class (add a car, delete a car etc) as virtual functions, and then specify the different implementations of the functions within the seperate inherited classes. This would give me a single interface from where I could add all the objects into the linked list.

[medievalelks]

Re: the design, I'd use composition instead of inheritance. A "car has-a soft top" makes more sense to me than a "car is-a soft top".

[Swerve]

Many thanks medievalelks

I had never heard of composition and it isn't something we've done in class yet either, but the reading on it I've just done suggests both ways would work, but perhaps composition is the better option, but for now I feel I will just stick with inheritance until I have a better understanding of the differences.

again, many thanks



EDIT - If I declare a virtual function inside of the Diesel class, and call it from within the DieselHardTop class, would the base class be the Diesel class? even though Diesel class is inherited from the Car class?

Or put another way, is the base class of a virtual function always where the function is first declared, even if the class inwhich it is declared in may be an inherited class it'self?

[medievalelks]

DieselHardTop is both a Diesel and a Car, but if you define new virtual functions at the Diesel level, they can't be called via the Car interface (without downcasting).

i.e., you can't do this:

Car * car = new DieselHardTop;

car->DieselOnlyMemberFunction();

But of course, you can do this:

Diesel * car = new DieselHardTop;

car->DieselOnlyMemberFunction();

[Swerve]

Thanks again medievalelks