Error - Going from Java to C++

[icu222much]

I have created a linked list in Java, and now I am trying to convert my program to C++.
When I try to compile, it gives me the error saying that 'next' uses 'CarNode' which is being defined. I am wondering if there is any way around this?

Java:

public class CarNode {
public Car carObject;
public CarNode next;

public CarNode (Car newCar, CarNode newNext) {
carObject = newCar;
next = newNext;
}
}


C++:

class CarNode: public Car {
public:
Car carObject;
CarNode next;

CarNode(Car newCar, CarNode newNext) {
carObject = newCar;
next = newNext;
}
}

[Lindley]

A "manual" linked list implementation in C++ must use pointers. A CarNode can't contain another CarNode directly, because that would imply sizeof(CarNode) > sizeof(CarNode), which is a contradiction. However, a CarNode can easily contain a pointer to another CarNode.

In Java, "everything is pointers," so the concept shouldn't be much trouble, just the syntax.

If this isn't for a class, though, I'd suggest saving yourself some trouble and just using a std::list<Car>, which is more or less the equivalent of Java's LinkedList class.

[Paul McKenzie]

I have created a linked list in Java, and now I am trying to convert my program to C++.

Doesn't Java already have a linked list class? C++ already has std::list, so I don't know why you need to create such a class.

When I try to compile, it gives me the error saying that 'next' uses 'CarNode' which is being defined. I am wondering if there is any way around this?

Java is not C++, and C++ is not Java. Just because the languages look the same doesn't mean that you can code them the same way.

The proper way to learn C++ in this case is to pretend Java does not even exist. If you try to code C++ as if you're coding Java, you will create more trouble for yourself.

For example:

CarNode(Car newCar, CarNode newNext) {

You are passing objects by value here. In C++, you should not pass these objects by value -- these parameters should be passed by reference or const reference, not by value. There is a huge difference between passing by value and by reference, but if you just copy what Java does, you never know the difference.

This is just one of the things that will get you in trouble if you write code in C++ trying to copy what Java syntax does.

Regards,

Paul McKenzie

[exterminator]

At the point where you are referring to CarNode as a member object CarNode is not fully defined (because you are still defining it). At that instant, CarNode is still incompletely defined. It has just been named in the scope where you declare the member object. And you can only refer to an incomplete type i.e. either have a pointer to a reference. In Java, you can get away with that because the same syntax there would mean declaring a reference member and not actually an object as a member. So, in C++ you can't have something like this (which is what you are essentially doing):

class A
{
     A a;
};

rather you can have this though:

class A
{
     A& a;
     //or
     //A* a;  that is, you can have a pointer or reference to an incomplete type
};

This what the standard says: [3.1/5] basic.def

A program is ill-formed if the definition of any object gives the object an incomplete type

Another ocassion where this might happen is if you had a forward declaration of a class and you tried to do the same as above.