CODEWITHSUNDEEP
c++inheritance
Tysia Krepps
Tysia Krepps

Reputation: 39

Too few arguments inheritance c++

I'm working on learning inheritance by making a generic List class. List can be an Unordered list, an Ordered list, a Stack, or a Queue.

My List class looks like this:

class List
{
public:

    class Node
    {
    public:
        int data;
        Node * next;
        Node(int data);
    };

    int sizeOfList = 0;
    Node * head = nullptr;
    List::List();
    virtual void get(int pos);
    virtual void insert(int data);
    virtual void remove(int pos);
    virtual void list();
    virtual int size();
};

My Unordered list class looks like this:

class UnOrderedList : public List
{
public:
    UnOrderedList();

    void get();
    void insert(int data);
    virtual void remove(); //takes no parameters because first item is always removed
};

In main(), I create an array of Lists like this;

List * lists[8];

and make an Unordered list like this:

lists[0] = new UnOrderedList();

My question: lists[listNum]->get(); gives error

"too few arguments in function call"

because it thinks I am trying to call get() in the Listclass, but I want it to call the Unordered list's function get().

Upvotes: 0

Views: 206

Answers (3)

John Bargman
John Bargman

Reputation: 1262

I feel I must improve upon Sam Varshachik's answer - though he is totally correct.

class List
{
public:
    virtual void get(int pos);
};
class UnOrderedList : public List
{
public:
    void get();
};

Note that there are two problems here - not just one.

Firstly the signature of get(int) is different from get() - these are two different methods, and the compiler will treat them as such.
Further you have declared that the method List::get(int) be virtual, but you have not done-so for UnOrderedList::get() - Remember that the list object has no knowledge about it's children - Thus a list * cannot understand the details of UnOrderedList.

Consider this example:

class List
{
public:
    virtual void get(int pos);
};
class UnOrderedList : public List
{
public:
    virtual void get(); //STILL AN ERROR!
};

In this case, I have made UnOrderedList::get() virtual - However this won't help. As list still has no knowledge of this method.

The correct snippet is as follows:

class List
{
public:
    virtual void get(int pos);
    virtual void get();
};
class UnOrderedList : public List
{
public:
    virtual void get(); //We can use this now!
    virtual void get(int pos); //This is only needed if we intend to override this method
};

In this example list::get() is now a virtual method - as such any call to it will be communicated to the correct-child as you intended.
This is only possible however because the parent class has been informed that such a method exists, and can be over-ridden by the child classes.

[edit /]

As JonathanPotter stated in the comments, pay attention to the fact that the virtual keyword is only needed if you wish for a method called from a parent-pointer to be routed to the actual child object. And that this does incur some overhead.

Upvotes: 2

Jonathan Potter
Jonathan Potter

Reputation: 37192

lists[0] is a List* as far as the compiler's concerned.

UnOrderedList::get() won't override List::get(int pos) because the signatures are different. If you want the derived class to override a function in the base class the function signatures have to be the same.

As it is, you would need to cast lists[listNum] to UnOrderedList* to call UnOrderedList::get(), e.g. static_cast<UnOrderedList*>(lists[listNum])->get().

Upvotes: 1

Sam Varshavchik
Sam Varshavchik

Reputation: 118445

You need declare get() as a virtual method in the List class. Either that, or dynamic_cast it.

When you invoke get() using a pointer to a List, that particular pointer can point to any subclass of List. Just because one particular subclass of List implements a particular method doesn't mean that you can invoke it directly using a pointer to the base class.

Otherwise, there's no point to have base classes. And this is exactly what virtual methods are for: to allow you to invoke the methods in a subclass, using a pointer to the base class.

Upvotes: 2

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