c++ - Linked List: Segmentation fault error when assigning a value to the next part of a node -

this first question here , english not good, please bear me.

i trying create linked list function insert element in position. need position predptr having trouble doing because whenever run program, segmentation fault. believe error in "predptr->next = first;" part inside insert() function, when set predptr (the line above) "predptr = new node()" works triggers second case in insert function.

here's code:

//class list class list { private:    //class node   class node   {   public:      string data;     node * next;      //node constructor     node()     {       data = "";       next = 0;     }      //node constructor value     node(string val)     {       data = val;       next = 0;     }   };    int mysize;   node * first;  public:    //list constructor   list()   {     mysize = 0;     first = 0;   } //insert function   void insert(string val, int pos)   {     node * newptr, * predptr;     newptr = new node(val);     predptr = new node();     predptr->next = first;      cout << "pred: " << newptr->data << endl;      //position predptr     for(int = 0; < pos; i++)       predptr = predptr->next;      //case 1: inserting @ middle or end of list     if(predptr != 0)     {       cout << "nf" << endl;       newptr->next = predptr->next;       predptr->next = newptr;     }     //case 2: inserting @ beginning of list     else     {       cout << "f"  << endl;       newptr->next = first;       first = newptr;     }      delete predptr;     mysize++;    } int main() {   list a;   cout << (a.empty() ? "yes" : "no") << endl;   cout << "inserting 5 elements..." << endl;   a.insert("asus", 1);   a.insert("acer", 2);   a.insert("sony", 3);   a.insert("toshiba", 4);   cout << "list a: ";   a.display();   cout << endl;    return 0; } 

the first clue might wrong here, in insert():

newptr = new node(val); predptr = new node(); 

rules of logic dictate if insert() expected add 1 value list, creating 2 new nodes, instead of one, wrong. expect create 1 new node, here. not two.

the logic in insert() wrong. can't salvage it. option here rewrite scratch:

node * newptr=new node(val);  node **predptr=&first;  while ( (*predptr) && pos) {     predptr= & (*predptr)->next;     --pos; }  newptr->next= *predptr; *predptr=newptr; 

that's it. if pos passed in 0, insert new node first position in list, if 1 second, etc... things of nature 0-based. if want position #1 instead of position #0 first element of list, decrement pos before loop.

if pos exceeds size of list, code won't crash, unlike attempt, add new node end of list.

i'll close quote:

"the more overthink plumbing, easier stop drain."

scotty, star trek iii.

insertion linked list simple operation. should not complicated process.