C ++继承链表

Question

class LinkedList{
    public:
    int data;
    LinkedList *next;
};

class NewLinkedList: public LinkedList{
    public:
    int data2;
};

When I use NewLinkedList , its next is still a pointer to LinkedList instead of NewLinkedList , so that I cannot access newlinkedlist.next->data2 without type casts. ( newlinkedlist is an object of NewLinkedList .)

How can I design these two classes to avoid this problem?

Is there something like SELF_TYPE *next; and it becomes the type of derived class itself automatically when it is inherited?

Answer 1

You can use a template:

template <typename T>
class LinkedList{
public:
    int data;
    T * next;
};

class NewLinkedList: public LinkedList<NewLinkedList>{
public:
    int data2;
};

This technique is known as the Curiously recurring template pattern (CRTP)

To improve the design, your LinkedList class should not contain any data other than what is needed for the linked list mechanism : it should be a base class only. Then you subclass it as above with specialized classes.

Answer 2

I would suggest, make the parent class as abstract and make the data members private(in most of the cases, that should be done; unless you have some really good reason not to do so) and add abstract functions to access these members. That would be a good design and would solve your problem as well.

Answer 3

Lets suppose what you wish to achieve is possible.

Consider:

class AveryNewLinkedList: public LinkedList{
    public:
    int data2;
};

Will next be of a type AveryNewLinkedList or NewLinkedList ?

The compiler does not know - nor do you.

So inheritance means you get bits from your parents and add to them - but the parents do not get anything from you!