trouble with linked list values C++ - c++

Question

I'm trying to teach myself c++. to do so I made a challenge for myself to write a prime finder app. I've succeeded once in python (to learn python) with a less efficient algorithm. I'm using a doubly linked list to store the primes. currently I'm just trying to run this in a single thread but I made it doubly linked so I could multithread it later on.
anyway, TL;DR the debugger is showing the program getting stuck trying to assign a value to the start link's prm int in the Prime constructor I've done a bunch of searching but I can't figure out what I'm doing wrong. (also note the bings are debug messages)
#include <iostream>
#include <math.h>
#include <cmath>
using namespace std;
using std::cout;
struct PLink{
int prm;
PLink *next;
PLink *prev;
};
class Prime{
public:
PLink *start, *end;
Prime(){
start -> prm = 2;
end -> prm = 3;
start->next = end;
end->next = NULL;
start->prev = NULL;
end->prev = start;
addToEnd(5);
cout <<"cbing" << endl;
}
void insert(int val){
}
void addToEnd(int val){//adds a new prime to the end of the list
PLink *tmp = new PLink;
tmp->prm = val;
tmp->prev = end;
end->next = tmp;
tmp->next = NULL;
tmp = end;
cout << tmp->prm << endl;
cout << "addbing" << endl;
}
bool comp(int pot){ //compares the potential prime against known primes via modulo
int lim = sqrt(pot);
PLink * current = start;
bool check = false;
cout<<"bing " << pot << endl;
while (current->prm < lim && check == false){
if (pot%current->prm == 0) {
check = true;}
current = current->next;
}
return check; //false means its prime true means its not
}
};
int main()
{
Prime primeList;
int cap = 10000;
int beg = 5;
int count = 3;
bool toggle = false;
bool check = false;
cout << "2 \n3 \n5" << endl;
while(count < cap){
beg += 2;
cout << "bing" << endl;
if (toggle){
beg += 2;}
toggle = !toggle;
check = primeList.comp(beg);
if (check == false){
primeList.addToEnd(beg);
count++;
cout << "bing2" << endl;
}
}
};

Answer 1

using namespace std;
using std::cout;
the second using std::cout; is redundant, you can read some documents about C++ name visibility, like this:
http://www.cplusplus.com/doc/tutorial/namespaces/
http://www.tutorialspoint.com/cplusplus/cpp_namespaces.htm
Prime(){
start -> prm = 2;
end -> prm = 3;
start->next = end;
end->next = NULL;
start->prev = NULL;
end->prev = start;
addToEnd(5);
cout <<"cbing" << endl;
}
Note: when you declare a pointer like PLink *start, *end; C++ complier(say 'gcc' or clang) only allocate memory to store that pointer, but not allocate memory to store what your pointer is pointed(here it means your PLink object).
So, you should allocate memory for your PLink object pointed by these two pointers: PLink *start, *end;, that is to say, you have to change the above code to:
Prime(){
start = new PLink(); // use the default constructor generated by C++ complier since you haven't declared one in struct PLink
end = new PLink()
start -> prm = 2;
end -> prm = 3;
start->next = end;
end->next = NULL;
start->prev = NULL;
end->prev = start;
addToEnd(5);
cout <<"cbing" << endl;
}
Well, in order not to cause memory leak and double free the same pointer, you should carefully manipulate the object you created.