Using a field of an abstract class as a default argument of a method (c )-CodePudding

I wrote an abstract class called "List", which I basically use as an interface for other implementation methods of lists (such as dynamic array and linked list).

Now, I have a method called "add", with 2 arguments, the first one is the data to add and the second is the position where to add. I want to set the default value of this position to be the end of the list (i.e, its size).

Here's what I wrote:

#ifndef LIST_H
#define LIST_H

template <class T>
class List
{
    public:
        List(): m_size(0) {}
        virtual bool isEmpty() const=0;
        virtual void set(int index, T value)=0;
        virtual int getSize() const=0;
        virtual void add(T data, int index = m_size )=0;        //The problem is here    
        virtual T remove(int index)=0;
        virtual ~List(){}
        virtual T operator[](int index) const =0;
        virtual bool operator==(const List<T>& other) const =0;

    protected:

        int m_size;

};
#endif // LIST_H

But I get the following error:

error: invalid use of non-static data member 'List<T>::m_size'|

Is there a way to fix this problem?

Also, is it correct to implement the constructor as I did? do I need to call this constructor in the init line of the derived classes constructors or does the compiler does it implicitly?

Thanks in advance.

CodePudding user response：

You cannot use members as default arguments of methods. Instead write an overload (it doesnt need to be virtual when it calls the virtual add(T,int):

void add(T data) {
     add(data,m_size );
}

CodePudding user response：

As you may know, all class instances share the same VTABLE - table of virtual methods. Given that, you can't introduce instance-specific knowledge to that shared source. Just imagine you have two lists of the same type, first one with m_size=10 and another one with m_size=42. How to resolve that conflict? What to do if m_size is changed over time? etc.

So, it's may be worth to introduce two methods, like insert(data, pos) and append(data).

CodePudding user response：

The problem is that you are using m_size as a default parameter for your implementation. As pointed out here, it is not possible to do that straight away. What you could do is overload this function as follows:

virtual void add(T data, int index)=0     // Original - remove the default parameter
void add(T data)                          // Overload - call with m_size
{
    add(data, m_size);
}

This way, all you have to do is override the original function.