c++ LinkedList when calling display function - c++

Question

I"m trying to understand link list but every example I've tried to copy gives me a segmentation fault. This is my example program that I am working on.
in contacts.h
struct people{
string last;
string first;
int age;
people *next;
};
class Contacts {
private:
people *head;
public:
Contacts() {
head=NULL;
};
void add(string, string, int);
void display();
};
in main.cpp
//menu items
//cin >> option
//if option == 1, addContact
//if option == 8, displayContact
//basic in and out here no need to show code
in main.h
void addContact() {
Contacts *ptr;
int i, loop=0, a=0;
string l, f;
cout << "number of contacts " << endl;
cin >> loop;
for(i=0; i<loop; i++) {
cout << "enter last name ";
cin >> l;
cout << "enter first name ";
cin >> f;
cout << "enter age ";
cin >> a;
}
ptr->add(l,f,a);
}
void display() {
Contacts *ptr;
ptr->display();
}
in contacts.cpp
void Contacts::add(string l, string f, int a) {
people *node = new people;
node->last=l;
node->first=f;
node->age=a;
node->next=NULL;
}
void Contacts::display() {
people *tmp = head;
while(tmp!=NULL) {
cout << tmp->last << endl;
cout << tmp->first << endl;
cout << tmp->age << endl;
tmp->next;
}
the add function works
then display() gives me a segfault

Answer 1

Member function add should be defined the following way
void Contacts::add( const string &l, const string &f, int a )
{
head = new people { l, f, a, head };
}
Or if your compiler does not support initializer lists with the new operator then
void Contacts::add( const string &l, const string &f, int a )
{
people *p = new people;
p->last = l;
p->first = f;
p->age = a;
p->next = head;
head = p;
}
Member fuunction display should be declared as
void Contacts::display() const;
and defined as
void Contacts::display() const
{
for ( people *tmp = head; tmp; tmp = tmp->next )
{
cout << tmp->last << endl;
cout << tmp->first << endl;
cout << tmp->age << endl;
cout << endl;
}
}
The other two functions should be defined the following way
void addContact( Contacts &contacts )
{
int n = 0;
cout << "number of contacts " << endl;
cin >> n;
for ( int i = 0; i < n; i++ )
{
string l, f;
int a = 0;
cout << "enter last name ";
cin >> l;
cout << "enter first name ";
cin >> f;
cout << "enter age ";
cin >> a;
contacts.add( l, f, a );
}
}
void display( const Contacts &contacts )
{
contacts.display();
}
And in main you should define an object of type
Contacts contacts;
and use it as the argument of the functions above.