Help to understand the friendship of functions related to classes

[ferenczi]

Hi. I'm developing a stack using a linked list.

I have to develop two friend functions to do something with the list.

But I've tried to understand the friendship, without any consistent result.

Extract of code:

class Element {
    
        public:
            int Data;
            Element *nextElement;
            Element(int Element) {Data = Element; nextElement = NULL;}
            ~Element() { }
            
};

class Stack {
    
    private:    
        Element *firstElement;
    
    public:
        Stack() { firstElement = NULL; }
        ~Stack() { }
        bool isEmpty() { return firstElement == NULL; }
        void push(int El)
        {
            Element *pElement = new Element(El);
            pElement->nextElement = firstElement;
            firstElement = pElement;
        }
        int pop()
        {
            if (isEmpty())
            {
                cout << "Empty stack";
                exit(EXIT_FAILURE);
            }
            else
            {
                Element *pElement = firstElement;
                firstElement = firstElement->nextElement;
                int temp = pElement->Data;
                delete pElement;
                return temp;
            }
        }
        
        void show()
        {
            Element *pElement = firstElement;
            while(pElement != NULL)
            {
                cout << pElement->Data << "\n";
                pElement = pElement->nextElement;
            }
        }
        
        friend int ::count(Stack& stack);
        friend void ::extract(Stack& stack);

};

When I try to define the friend function outside the class in order to do something (e.g. count the element of the stack) I get errors.

I try to define them this way:

int Stack::count(Stack& stack)
{
    //Do something
}

I have: Method count does not have declaration.

If I declare like:

int count(Stack& stack)
{
    //Do something
}

I have no errors, but I cannot access the private properties of the class.

The thing I would like to do is: sum the elements of the stack with a friend function.

Please help me understand how it works.

Thank you.

[laserlight]

You probably forgot to declare count to be a friend.

Personally, I think it should be a member function instead, in which case it should be:

int Stack::count() const
{
    //Do something
}

Though I note that size is a more canonical name for such a member function, and indeed count risks a conflict with std::count if you go around using namespace std indiscriminately.

The thing I would like to do is: sum the elements of the stack with a friend function.

I do not think that summing the elements of a stack should be part of the interface of a stack. If you want to sum the elements of a stack, pop the elements one by one to sum it (e.g., in a non-member non-friend function). If that is inappropriate, then perhaps a stack is inappropriate to begin with.

[ferenczi]

I have to do that way because it's an exercise.

[antred]

class Stack {
    public:
        int pop()
        {
            if (isEmpty())
            {
                cout << "Empty stack";
                exit(EXIT_FAILURE);
            }
            else
            {
                Element *pElement = firstElement;
                firstElement = firstElement->nextElement;
                int temp = pElement->Data;
                delete pElement;
                return temp;
            }
        }
};

This is usually not a good idea. A library class / function should never just summarily terminate the process in case of an error. The proper way to inform client code about failure of an operation is either through exceptions or return values.

[ferenczi]

Thank you for the tip, but it doesn't concern the thread.

[grumpy]

Not withstanding laserlight's legitimate concern that the function should be a member the second version of the function count() is the correct one.

It will need to access members of the argument (for example using stack.firstElement. It cannot access a private member (like firstElement) of a class without knowing what object that member belongs to.

The colons in the friend declaration are possible superfluous in your case. If you don't know what they actually mean, leave them out.

[phantomotap]

I have no errors, but I cannot access the private properties of the class.

O_o

The `friend' statements says that you can access `private' components.

I imagine this is an issue born of not copying and pasting what you actually have tried.

What have you actually done?

Soma

[ferenczi]

Now they work.
I've removed the colons and used the second version for the function.
Thank you!