Организовать очередь, найти среднее арифметическое элементов очереди

#include<iostream>
using namespace std; 
 
struct item
{
int elem;
item *next;
};
class List 
{
private:
item *first,*end;
int data;
public:
List () {first=end=NULL;};
void Creation ();
void Print ();
void Add (int);
void Seredne();
void Kol_elem();
};
 
 
 
void List::Creation () 
{
item *r;
int el;
cout<<"enter element:\n";
cin>>el;
if  (el!=0)
{
r = new (item);
(*r).elem = el; 
(*r).next = NULL;
first = r;
 end = r; 
 cin>>el;
while  (el!=0)
{
r = new (item);
(*r).elem = el; 
(*r).next = NULL;
(*end).next = r; 
end = r;
 cin>>el;
}}
else
{r = NULL; 
first = r;
 end = r;}
}
 
void List::Print ()
{
item *r;
cout<<"Vyvod ochered: "; 
r = first;
while  (r!=NULL)
{
cout<<(*r).elem<<" "; 
r = (*r).next;
}
cout<<endl;
}
void List::Add (int el)
{
item *r;
r = new (item);
(*r).elem = el; (*r).next = NULL;
if  (first!=NULL)
{
(*end).next = r; end = r;
}
else
{ first= r; end= r;}
}
 
void List::Seredne()
{
    ////
}
    
    void List::Kol_elem()
{
/////
}
 
int main ()
{
List A;
int el;
A.Creation ();
A.Print ();
cout<<"enter element: ";
cin>>el;
A.Add(el);
cout<<"Print new : ";
A.Print ();
A.Seredne();
A.Kol_elem();
}

...........
double seredne() const;
.............
 
double List::seredne() const
{
    if(!first) return 0;
    
    int count = 0; 
    double sum = 0;
    
    item * current = first;
 
    while(current)
    {
        ++count;
        sum += current->elem;
        current = current->next;
    }
    cout <<sum / count;
    
    
}

void List::Kilk_elem()
{
    int size; 
      cout<<"Kilkist elementiv: ";
        ++size;
    cout << size-1;
}