Doubly Linked List of Structures - Overwriting - c++

Question

So I am attempting to make a program in C++ that performs basic operations on a doubly linked list. The main problem I am having is that every time I add a new element to the list, the id field I enter for the new element overwrites the id fields of all other elements. The id field is the only field that does this, which is extremely confusing to me. I believe my error could lie somewhere in my use of pointers, as this is my first program in C++ using them to manage a list(Pointers in Ada seem much simpler to me). I have ran my program through GDB step-by-step multiple times and still can't seem to determine what is causing the problem. This leads me to believe that it could be a logic error on my part.
Here is my main:
#include <iostream>
#include "list.h"
#include <string.h> // <string>
using namespace std;
int main (void)
{
int choice, printorder;
char idbuffer[100];
rec r;
do
{
cout << "Enter your choice: 1 - Add, 2 - Delete, 3 - Print, 0 - quit. " <<endl;
cin >> choice;
switch ( choice )
{
case 1: //AddItem
cout << "\nEnter ID ";
cin >> idbuffer;
r.id = idbuffer;
cout << "\nFirst Name ";
cin >> r.firstname;
cout << "\nLast Name ";
cin >> r.lastname;
if ( AddItem ( r ) )
{
cout << "\nSuccess!\n";
}
else
{
cout << "\nItem failed to be added.\n";
}
break;
case 2: //Delete
cout << "\nEnter id: ";
cin >> idbuffer;
if ( DeleteItem ( idbuffer ) )
{
cout << "\nDelete OK.\n";
}
else
{
cout << "\nDelete Failed for: " << idbuffer << endl;
}
break;
case 3: // Print
cout << "Enter order: 0 - Ascending, 1 - Descending. \n";
cin >> printorder;
PrintList (printorder);
break;
case 0: // quit
break;
default: // bad choice
break;
} // end switch
}
while ( choice != 0 );// end do while
} // end main
Here is my file list.h:
struct rec
{
char * id;
char firstname[15];
char lastname[15];
rec* prev;
rec* next;
};
int AddItem ( rec r );
int DeleteItem ( char* delid );
void PrintList ( int order );
And finally the AddItem function from my file list.cpp:
#include <iostream>
#include "list.h"
#include <string.h>
using namespace std;
rec * first = NULL;
rec * last = NULL;
int AddItem( rec r )
//Return: 1 if a success, 0 if failed.
//No duplicate id's, sort by lastname
{
rec * ptr = NULL;
rec * current;
rec * myStruct = new rec;
myStruct -> id = r.id;
strcpy(myStruct -> firstname, r.firstname);
strcpy(myStruct -> lastname, r.lastname);
ptr = first;
//Check for duplicate id's, currently commented out due to id's being overwritten
//while (ptr)
//{
// if (ptr -> id == myStruct -> id)
// {
// return 0;
// }
// ptr = ptr -> next;
//}
//ptr = first;
if (first == NULL) //Empty
{
first = myStruct; //Inserts node into empty list
last = myStruct;
myStruct -> prev = NULL;
myStruct -> next = NULL;
return 1;
}
else if (myStruct -> lastname > last -> lastname) //Add to end of list
{
last -> next = myStruct;
myStruct -> prev = last;
last = myStruct;
return 1;
}
else if (myStruct -> lastname && myStruct -> lastname < first -> lastname) //Add to beginning of list
{
ptr = first;
first = myStruct;
myStruct -> next = ptr;
ptr -> prev = myStruct;
return 1;
}
else
{
current = first;
while (current)
{
if (myStruct -> lastname > current -> lastname && myStruct -> lastname <= current -> next -> lastname)
{
ptr = current -> next;
current -> next = myStruct;
myStruct -> prev = current;
myStruct -> next = ptr;
ptr -> prev = myStruct;
return 1;
}
else
{
current = current -> next;
}
}
return 0;
}
I'm sure there are other errors throughout this code and some sloppy programming, but I plan on fixing those up and making the program neater/more efficient after I work out this overwriting problem, as it is also holding me back from testing other parts of my program. Thanks in advance and sorry for the lengthy post.
Update1: After realizing I needed to change the line
myStruct -> id = r.id;
to a string copy, such as:
strcpy(myStruct -> id, r.id);
I am now receiving a segmentation fault.. I would imagine something with my pointers is messed up. Back to work I go and thanks for all the help so far!
Update2: After much more editing and quite a few more errors, I have the program working! I am now in the process of cleaning things up and optimizing the program. Thanks to everyone for the great advice!

Answer 1

All id objects have the same value - the address of the first character of idbuffer.
In main
cin >> idbuffer;
r.id = idbuffer;
^^^^^^^^^
and then further in AddItem
int AddItem( rec r )
//Return: 1 if a success, 0 if failed.
//No duplicate id's, sort by lastname
{
rec * ptr = NULL;
rec * current;
rec * myStruct = new rec;
myStruct -> id = r.id;
^^^^^
//...
You should dynamically allocate memory that will be pointed to by id and there the actual string with id will be copied. Or you should declare data member id as a character array.
For example in main
cin >> idbuffer;
r.id = new char[std::strlen( idbuffer ) + 1];
std::strcpy( r.id, idbuffer );
and in AddItem
rec * myStruct = new rec( r );
A better approach is to use an object of type std::string.