Exception Thrown: Write Access Violation. **this** Was Nullptr. Occurred

Why is there an exception thrown after inserting a new record for a doubly linked list?

Hello,

I'm making a telephone directory as practice with a double linked list data structure. When trying to select my option to insert a new record after the tail node, I have an exception thrown

Exception thrown: write access violation.
record was nullptr.

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// Exercise13.5_4revised.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include <iostream> #include <iomanip> #include <string> using namespace std; struct TeleType { string name; string phoneNo; TeleType *nextaddr; TeleType *prevaddr; }; void populate(TeleType *); // function prototype needed by main() void display(TeleType *); // function prototype needed by main() void remove_head(TeleType *); //function prototype for main() void remove_current(TeleType *); //function prototype for main() void remove_tail(TeleType *); //function prototype for main() void make_list(); void insert(TeleType *); TeleType *findRec(string, TeleType *); TeleType *phead; TeleType *pcurrent; TeleType *ptail; int main() { int i; string search_term; cout << "The list will now be made" << endl; make_list(); cout << "\nThe list consists of the following records:\n"; display(phead); // display the structures cout << "Which record would you like to remove?" << endl; cout << "select 1 if you want to remove the head" << endl; cout << "2 for the middle " << endl; cout << "and 3 for the tail" << endl; cout << ", 4 to insert a new record "<< endl; cout << ", 5 to find a record" << endl; cout << "6 Display records" << endl; cin >> i; cin.ignore(); cout << endl; switch (i) { case 1: remove_head(phead); display(phead); break; case 2: remove_current(pcurrent); display(phead); break; case 3: remove_tail(pcurrent); display(phead); break; case 4: insert(ptail); display(phead); break; case 5: cout << "What are you looking for? " << endl; getline(cin, search_term); cin.ignore(); cout << "The address matching is " << findRec(search_term, phead) << endl; break; case 6: display(phead); break; } system("pause"); return 0; } void make_list() { //Setting the head of the linked list //assign head to a new structure //Call populate function to fill in data //set nextaddr and prevaddr pointers to null //assign tail with head int number_of_records =1; cout << "How many records would you like?" << endl; cin >> number_of_records; TeleType head, current, temp, tail; TeleType *phead = &head; TeleType *pcurrent = &current; TeleType *ptemp = &temp; TeleType *ptail = &tail; populate(phead); head.nextaddr = NULL; head.prevaddr = NULL; tail = head; for (int i = 0; i < number_of_records - 1; i++) { pcurrent = new TeleType; populate(pcurrent); pcurrent->nextaddr = NULL; pcurrent->prevaddr = ptail; //ptail->nextaddr = pcurrent; tail = current; } //set up a for loop to make records //make a new node assigned to current structure //call populate function to fill in data members //assign a pointer to to current's nextaddr as NULl //assign pointer prevaddr of current as the pointer of head. //assign tail to current return; } // input a name and phone number void populate(TeleType *record) // record is a pointer to a { // structure of type TeleType cout << "Enter a name: " << endl; getline(cin, record->name); cin.ignore(); cout << "Enter the phone number: "; getline(cin, record->phoneNo); cin.ignore(); return; } void display(TeleType *contents) // contents is a pointer to a { // structure of type TeleType while (contents != NULL) // display until end of linked list { cout << endl << setiosflags(ios::left) << setw(30) << contents->name << setw(20) << contents->phoneNo << setw(20) << contents->prevaddr << setw(20) << contents->nextaddr; contents = contents->nextaddr; } cout << endl; return; } void remove_head(TeleType *record) { return; } void remove_current(TeleType *record) { return; } void remove_tail(TeleType *record) { return; } void insert(TeleType *record) { TeleType *ptr_new_record = new TeleType; //create a new structure, return an address and assign it to a ptr //new record. record->nextaddr = ptr_new_record; //change the nextaddr pointer to point to the new record ptr_new_record->nextaddr = NULL; //assign new null pointer ptr_new_record->prevaddr = record; //assign new pointer of new record to point to the the old tail structure. populate(ptr_new_record); //call populate function to fill in data members of the structure. return; } TeleType *findRec(string term, TeleType *contents) { TeleType *pointer_to_address = phead; while (contents != NULL) { if (term == contents->name) { pointer_to_address = contents; break; } else { cout << "match not found" << endl; } contents = contents->nextaddr; } return pointer_to_address; }

In the function make_list, you create four TeleType objects. These are local to that function.

TeleType head, current, temp, tail;

They will all cease to exist when that function finishes. Any pointers to them will be pointing at garbage memory. ptail is a pointer pointing to garbage memory.

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Exception Thrown: Write Access Violation. **this** Was Nullptr. Occurred

Source: http://www.cplusplus.com/forum/beginner/228937/

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