What is wrong with this implementation of linked list?

Question

This is my code for creating a singly-linked list in C++.

The function del_alt() deletes every alternate element starting from the second element.

The compiler gives no error as such , but during run the program terminates abruptly after showing the original list.

I have tried my best to find the possible errors but can't find any.

Help appreciated.

Cheers.

    #include<iostream>

using namespace std;
class Node 
{
    public:
        Node()
        {
            next=0;
        }
        Node *next;
        int info;

};

class List
{
    private:
        Node *head,*tail;
    public:
        List()
        {
            head=0;
            tail=0;
        }
    void  del_alt();
    void add_to_head(int);
    void show_list();
};


void List :: del_alt()
{
  if(!head||(head==tail))
    return;
  else
   {
      Node *current,*after,*ptr;
      for(current=head,after=head->next;current!=0;current=current->next,after=current->next)
      {
        ptr=after;
        current->next==after->next;
        if(ptr==tail)
            tail=current;
        delete ptr; 
      }
    }
}

void List :: add_to_head(int el)
{
    Node *ptr;
    ptr->info=el;
    if(!head)
        {
            ptr->next=0;
            head=tail=ptr;
        }   
    else
        {
            ptr->next=head;
            head=ptr;
        }
}

void List::show_list()
{   Node *ptr;
    cout<<"\n\n";
    ptr=head;
    while(ptr!=0)
    {
        cout<<"\t"<<ptr->info;
        ptr=ptr->next;
    }
}   
int main()
{
    List l;
    int el;
    char ch;
    cout<<"\n\n\t\t enter elements\n\n\t";
    do
    {
        cin>>el;
        l.add_to_head(el);
        cout<<"\n\t want to enter more ? (y/n) \n";
        cin>>ch;
    }while(ch=='y'||ch=='Y');

    cout<<"\n\t Original list -> \n";
    l.show_list();
    l.del_alt();
    cout<<"\n\t After deletion -> \n";
    n.show_list();
    cout<<"\n\n \\===============================================";
}

Answer 1

The problem comes from the non-initialization of ptr in the method add_to_head.

Node *ptr;
ptr->info=el

At least ptr should be a new allocated cell

Node *ptr = new Node;
ptr->info=el