Why is an exception thrown whilst removing a node from a doubly linked list?

[Shishykish]

Hello,

I have written a function to relink and delete a node from a doubly linked list data structure. I keep getting exceptions thrown in relation to failing to access memory.

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

//const int MAXRECS = 3; // maximum number of records

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();

TeleType head, tail, current;

TeleType *phead = &head;
TeleType *pcurrent = &current;
TeleType *ptail = &tail; // 3 pointers to structures of
						 // type TeleType
						 // get a pointer to the first structure in the list

int main()
{
	int i;
	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, 2 for the middle and 3 for the tail" << endl;

	cin >> i;
	switch (i)
	{
		case 1:
			remove_head(phead);
			display(pcurrent);
		case 2: 
			remove_current(pcurrent);
			display(phead);
		case 3:
			remove_tail(pcurrent);
		break;
	}

	system("pause");

	return 0;
}

void make_list()
{
	//Input values for head structure
	populate(phead);
	//Assign nextaddr pointer to current
	head.nextaddr = pcurrent;
	//Input values for current structure
	populate(pcurrent);
	//Assign current structure's newaddr and preaddr
	current.prevaddr = phead;
	current.nextaddr = ptail;
	//Input values for tail
	populate(ptail);
	//Assign prevaddr pointer
	tail.prevaddr = pcurrent;
	//assign newaddr pointer as null to tail structure
	tail.nextaddr = NULL;
	// assign prevaddr pointer as current pointer  for tail structure
	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: ";
	getline(cin, record->name);

	cout << "Enter the phone number: ";
	getline(cin, record->phoneNo);

	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)
{
	current.prevaddr = NULL;
	current.nextaddr = ptail;

	tail.prevaddr = pcurrent;
	tail.nextaddr = NULL;

	delete record;

	return;
}

void remove_current(TeleType *record)
{
	head.nextaddr = ptail;

	tail.prevaddr = phead;
	tail.nextaddr = NULL;

	delete record;

	return;
}

void remove_tail(TeleType *record)
{

}

[Ganado]

You should only delete a variable created with new.
Your pointer is not pointing to dynamic memory, so it should not be deleted with new.

If you want to use delete, allocate phead/pcurrent/ptail with new. Otherwise, just remove the delete calls.

[ne555]

foo.cpp: In function ‘int main()’:
foo.cpp:51:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
foo.cpp:54:11: warning: this statement may fall through [-Wimplicit-fallthrough=]

	switch (i)
	{
		case 1:
			remove_head(phead);
			display(pcurrent);
			break; //<--- add this
		case 2: 
			remove_current(pcurrent);
			display(phead);
			break; //<--- add this
		case 3:
			remove_tail(pcurrent);
		break;
	}

without the breaks, the code keeps executing.

if you keep having issues, provide an example input.

[Shishykish]

Hello,

Since tidying up the switch statement in the main, I have been able to get it working.

#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 head, tail, current;

TeleType *phead = &head;
TeleType *pcurrent = &current;
TeleType *ptail = &tail; // 3 pointers to structures of
						 // type TeleType
						 // get a pointer to the first structure in the list

int main()
{
	int i;
	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, 2 for the middle and 3 for the tail, 4 to insert a new record" << endl;

	cin >> i;

	cout << endl;

	switch (i)
	{
	case 1:
		remove_head(phead);
		display(pcurrent);
		break;
	case 2:
		remove_current(pcurrent);
		display(phead);
		break;
	case 3:
		remove_tail(pcurrent);
		display(phead);
		break;
	case 4:
		insert(ptail);
		display(phead);
		break;
	}

	system("pause");

	return 0;
}

void make_list()
{
	//Input values for head structure
	populate(phead);
	//Assign nextaddr pointer to current
	head.nextaddr = pcurrent;
	//Input values for current structure
	populate(pcurrent);
	//Assign current structure's newaddr and preaddr
	current.prevaddr = phead;
	current.nextaddr = ptail;
	//Input values for tail
	populate(ptail);
	//Assign prevaddr pointer
	tail.prevaddr = pcurrent;
	//assign newaddr pointer as null to tail structure
	tail.nextaddr = NULL;
	// assign prevaddr pointer as current pointer  for tail structure
	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);

	cout << "Enter the phone number: ";
	getline(cin, record->phoneNo);

	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)
{
	current.prevaddr = NULL;
	current.nextaddr = ptail;

	tail.prevaddr = pcurrent;
	tail.nextaddr = NULL;

	return;
}

void remove_current(TeleType *record)
{
	head.nextaddr = ptail;

	tail.prevaddr = phead;
	tail.nextaddr = NULL;

	return;
}

void remove_tail(TeleType *record)
{
	current.nextaddr = NULL;
	current.prevaddr = phead;

	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 = ptail; //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;
}