Link lists in C++

Hey guys, sorry for, but I'm new to doubly-linked lists and was wondering if anyone could tell me why my program crashes when i use add_end()?

 #include <iostream>
 using namespace std;

 node *start_ptr = NULL;
 node *current;
 int option = 0;

 void add_end()
 {
     node *temp, *temp2;
     temp = new node;
     cout << "Enter name: ";
     cin >> temp->name;
     cout << "Enter profession: ";
     cin >> temp->profession;
     cout << "Enter age: ";
     cin >> temp->age;
     temp->nxt = NULL;
     if (start_ptr = NULL)
     {
     start_ptr = temp;
     current = start_ptr;
     }
     e开发者_JS百科lse
     {
         temp2 = start_ptr;
         while (temp2->nxt != NULL)
         {
            temp2 = temp2->nxt;
         }
         temp2->nxt = temp;
         temp->prv = temp2;
     }
 }

I bet that this if ( start_ptr = NULL ) is not what you intended... Did you forget an =? The if condition will never be met, as that statement is equivalent to start_ptr = 0; if (0), and then your code will assume that start_ptr can be dereferenced. You assing it to temp2 and then dereference NULL to access static_cast<node*>(0)->next...

This line is obviously wrong:

if (start_ptr = NULL)