Access Violation Error

jthep

I'm trying to get this piece of code I converted from C to work in C++ but I'm getting an access violation error. Problem occurs at line 61. Someone can help me with this?

The function display(llist mylist) displays a list of choices for a record book:

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void display(llist mylist)

{

    char name[25], address[80], telno[15], input[100];

    int yearofbirth, choice, records;

    yearofbirth = 0;

    choice = -1;
 
    while(choice != 6)

    {

        cout << "*****************************************************\n";

        cout << "         Choice 1:  Add a new record\n";

        cout << "         Choice 2:  Modify a record\n";

        cout << "         Choice 3:  Delete an existing record\n";

        cout << "         Choice 4:  Display a record\n";

        cout << "         Choice 5:  Display all records\n";

        cout << "         Choice 6:  Exit Program\n";

        cout << "*****************************************************\n";          

        cout << "Please enter one of the above choices from 1 - 6:  ";

        cin >> input;

        choice = atoi(input);

        cin.ignore(100, '\n');
 
        switch (choice)

        {

            case 1:

                //get user input for name of record

                cout << "\nEnter a name:  ";

                while(!getfield(name, '\n', 25))

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (25 character)\n";

                    cout << "Pleas re-enter:  ";

                };
 
                //get user input for address of record

                cout << "\nEnter an address ending it with '@':  ";

                while(!getfield(address, '@', 80))

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (80 character)\n";

                    cout << "Pleas re-enter:  ";

                }
 
                //get user input for yearofbirth of record

                cout << "\nEnter the year of birth:  ";

                cin >> input;

                cin.ignore(100, '\n');

                while(!(yearofbirth = atoi(input)) || (numberofdigits(yearofbirth) != 4))

                {

                    cout << "Invalid year entered, please re-enter:  ";

                    cin >> input;

                    cin.ignore(100, '\n');

                }
 
                //get user input for telno of record

                cout << "\nPlease enter a telephone # with no special characters:  ";

                while(!getfield(telno, '\n', 15) || !checkTelno(telno))

                    cout << "Invalid phone number, please re-enter:  ";
 
                //adding the new record into the list and displaying the list

                records = mylist.addRecord(name, address, yearofbirth, telno);

                cout << "Record added sucessfully!\n";

                mylist.printAll();

                break;

            case 2: 

                //get user input for name of record

                cout << "\nEnter a name:  ";

                while(getfield(name, '\n', 25) == 0)

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (25 character)\n";

                    cout << "Pleas re-enter:  ";

                };
 
                //get user input for address of record

                cout << "\nEnter the new address ending it with '@':  ";

                while(getfield(address, '\n', 80) == 0)

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (80 character)\n";

                    cout << "Pleas re-enter:  ";

                }  
 
                //get user input for telno of record

                cout << "\nPlease enter the new telephone # with no special characters:  ";

                while(getfield(telno, '\n', 15) == 0 || !checkTelno(telno))

                    cout << "Invalid phone number, please re-enter:  ";
 
                //modify records (if exists) and display feed back to user

                records = mylist.modifyRecord(name, address, telno);

                if(records == -1)

                    cout << "The list is empty, there were no records to modify\n";

                else

                {

                    mylist.printAll();

                    cout << "Records found and modified:  " << records << "\n";

                }

                break;

            case 3:

                //get user input for name of record

                cout << "\nEnter the name of record you would like to delete:  ";

                while(getfield(name, '\n', 25) == 0)

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (25 character)\n";

                    cout << "Pleas re-enter:  ";

                };
 
                //delete record requested and display feedback to user

                records = mylist.deleteRecord(name);

                if(records == -1)

                    cout << "The list is empty, there were no records to delete\n";

                else

                {

                    mylist.printAll();

                    cout << "Records found and deleted:  " << records << "\n";

                }

                break;

            case 4:

                //get user input for name of record

                cout << "\nEnter the name of record you would like to display:  ";

                while(getfield(name, '\n', 25) == 0)

                {

                    cout << "ERROR! Length of name entered exceeds character limit! (25 character)\n";

                    cout << "Pleas re-enter:  ";

                };
 
                records = mylist.printRecord(name);

                if(records == -1)

                    cout << "ERROR!!! The list is empty\n";

                else

                    cout << "Records found:  " << records << "\n";
 
                break;

            case 5:   

                mylist.printAll();

                break;

            case 6:

                mylist.~llist();
 
                cout << "********************Exiting Program********************\n";

                break;

            default:

                cout << "\n   ERROR! You have entered an incorrect choice     \n";

        }

    }

}

The part where the error comes up is during case 1 within the switch statement (At line 61). When I call printAll() an access violation error is thrown. The function prints all the records just fine but when it reaches the end, the error is thrown.

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void llist::printAll()

{

    record *begin = start;

    int count = 0;
 
    while(begin != NULL)

    {
 
        cout << "Name:  " << begin -> name << "\n";

        cout << "Address:  " << begin -> address << "\n";

        cout << "Year of Birth:  " << begin -> yearofbirth << "\n";

        cout << "Telephone Number:  " << begin -> telno << "\n\n";

        begin = begin -> next;

    }

}

weaknessforcats

WHy is there all this code in a display function?? It looks like your main() hase wandered in here.

I see the display funciton argument is a llist and not an llist& or llist*. That menas a copy was made when the display was called. And that menas you may have copy constructor problems. (Actually, it means you shoud have used a pointer or referecne argument).

Next, the printall() uses a magic variable named start that drops in from nowhere. I suspect a global variable. Probably the value in the global has gotten screwed up. You see, there has to be an llist that was used to make the copy for the display function argunment. That means there are at least two llist variables using the same start. Bad news. That is a race condition.

Read the C/C++ Article http://www.thescripts.com/forum/thread736028.html.

Try to get your functions to do only one thing. I would start by renaming your display function to main() and go from there.

jthep

Thanks. The variable start is a private member of class llist that is defined in a header file. Unfortunately I cannot change the definition of the class. As for the copy constructor, you are right, I was actually making a copy. However, even as I consolidate the display() function with int main(), the problem still persist. I was able to figure out the problem was actually coming from my addRecord function.

Regards,
Jthep

Access Violation Error

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