求助 重载+运算符

请问下面的程序错在哪里？为什么会出现异常？
#include"iostream"
using namespace std;

struct set
{
char data;
set *next;
};

class Set
{
set *first,*last;
public:
Set(){first=last=new set;first->next=0;}
Set(Set &);
~Set(){make_empty();delete first;}
void make_empty();
void print();
void create();
bool add(const char);
Set operator+(const Set &);
};

int main()
{
Set s1,s2,s3;
s1.create();
s2.create();
s3=s1+s2;
s3.print();
return 0;
}

Set::Set(Set &r)
{
set *p1=r.first->next;
set *p2=first=new set;
while(p1)
{
p2->next=new set;
p2=p2->next;
p2->data=p1->data;
p1=p1->next;
}
p2->next=0;
last=p2;
}

void Set::make_empty()
{
set *p1=first->next,*p2;
while(p1)
{
p2=p1;