Segmentation Fault 11 whenever I run this. Would like assistance/feedback - c++

Question

So I'm making a really rudimentary implementation of a circular list. I haven't made the remove function yet. Whenever I run the cpp, I get a seg fault 11. Any feedback would be much appreciated. Thank you.
#include <iostream>
using namespace std;
struct node{
node* next=NULL;
bool tail= false;
int contents;
};
node* start;//start is a pointer that exists at the start of the list before the first element
class CircList{
node *seek;
public:
CircList (){ //creates a list of one node that points to itself
node *b= new node;
b->contents=0;
b->next = b;
start->next=b;
b->tail=true;
}
bool empty(){
if(start->next==NULL){
return true;
}
return false;
}
int size(CircList a){
if(start->next==NULL){
cout<<"size is 0 \n";
return true;
}
seek=start->next;
for(int i=0; i++;){
if(seek->tail==true){
cout<<"size is "<<i;
}
seek=seek->next;
}
return 0;
}
void insert(int pos, int val){
if(start->next ==NULL){//if inseting when the list is empty
node *b= new node;
b->next = b;
b->tail=true;
return;
}
node *b= new node;
b->contents= val;
seek=start->next;
for(int i=0;i<=pos; i++){
if(seek->tail==true){//if inserting at the end
seek->tail=false;
b->tail=true;
seek->next=b;
b->next=start->next;
}
if(pos==i){//if inserting between two nodes
b->next = seek->next;
seek->next = b;
}
seek=seek->next;
}
}
void remove(int a){
seek=start->next;
for(int i=0;i<=a-1; i++){
if(i<a){
seek=seek->next;
}
if(i==a-1){
}
}
}
void display(){
cout<<start->next->contents; //will also be completed in the near future
seek=start->next;
for(int i=0; ;i++){
if(seek->tail==false){
cout<<seek->contents<<"\n";
}
if(seek->tail==true){
cout<<seek->contents<<"\n";
return;
}
}
}
};
That was the .h file. The following is the cpp. I just plugged in numbers to test. I want to get the program running so that I can test how it behaves.
#include <iostream>
#include "CircList.h"
using namespace std;
int main(){
CircList a;
a.insert (5,5);
a.insert (5,5);
a.insert (1,4);
a.insert (20,65);
a.insert (3,7);
a.size(a);
a.display();
}

Answer 1

I kept treating start as a node instead of a pointer. By making start = Null and replacing all the "start->next"'s with "start", I got it to compile and run. But now it's only infinitely inserting nodes with a value of 0 in the contents.
Edit: I fixed it. By changing that weird for loop in the display function to a while loop, it doesn't do infinite inserts of the node in the constructor, anymore. It seems to work decently enough now.

Answer 2

This here causes a seg fault
start->next=b;
because start is NULL at the start of the program so you are de-referencing a null pointer.
instead set start to the first node in your constructor
start = b;

Answer 3

Your global variable start is an uninitialized pointer, yet you dereference it all over the place.