an odd STL "List" error

kris273 · 08-15-2004, 04:42 AM

I have the following code and I get a segmentation fault

Code:

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <list>		// list class library
using namespace std;

int main()
{
	list<int> int_list;
	int i;
	
	for ( i = 0; i < 20; i++)
	{
	  	int_list.push_front(i*3);
	}
	   
	list<int>::iterator int_iter;
	
	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		cout << *int_iter << " ";
	}

	cout << endl << "test1" << endl;

	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		if (*int_iter == 27)
		{
			int_list.erase(int_iter,(int_iter++));
		}
	}

	cout << endl << "test2" << endl;
	
	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		cout << *int_iter << " ";
	}

	cout << endl;

}

compiling and excution:

Quote:

[list tests]$ g++ -o listtest2 listtest2.cpp [list tests]$ ./listtest2
57 54 51 48 45 42 39 36 33 30 27 24 21 18 15 12 9 6 3 0
test1
Segmentation fault

The STL erase command:

Quote:

iterator erase( iterator pos );
iterator erase( iterator start, iterator end );

When I take out the erase the program works, but it does nothing, is there a bug in the stl list... or am I just doing something amazingly stupid that I fail to see, or did I find a bad site or what?

raxxor · 08-15-2004, 09:37 AM

Hi,

You should use:

Code:

int_list.erase (int_iter--);

Or else your loop will try to go through all 20 items of the list, the only problem is there are only 19 now, and the additional int_iter++ for arg 2 of the erase will make it go for 21.

r

kev82 · 08-15-2004, 09:54 AM

ok, the problem is with the erase as you assummed, you need to remember that end is one after the last element so erase(a,b) erases the range [a,b) that is from a to b excluding b

now you are calling erase(int_iter,(int_iter++)) which i reckon compiles to something like

temp=int_iter+1
erase(int_iter, int_iter)
int_iter=temp;

which isnt what you want as you can see. either of the following would work:

erase(int_iter++)
erase(int_iter,++int_iter)

kris273 · 08-15-2004, 02:59 PM

blah, I can't believe I left that in, if you take out the ++, it still doesn't work, that was me messing around with it and trying to figure out what's wrong

it was orginally "int_list.erase(int_iter);"

[Linux list tests]$ g++ -o listtest2 listtest2.cpp
[Linux list tests]$ ./listtest2
57 54 51 48 45 42 39 36 33 30 27 24 21 18 15 12 9 6 3 0
test1
Segmentation fault

kris273 · 08-16-2004, 10:12 PM

here's my code

Code:

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <list>		// list class library
using namespace std;

int main()
{
	list<int> int_list;
	int i;
	
	for ( i = 0; i < 20; i++)
	{
	  	int_list.push_front(i*3);
	}
	   
	list<int>::iterator int_iter;
	
	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		cout << *int_iter << " ";
	}

	cout << endl << "test1" << endl;

	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		if (*int_iter == 27)
		{
			cout << "it equals" << endl;
			int_list.erase(int_iter);
		}
	}

	cout << endl << "test2" << endl;
	
	for (int_iter = int_list.begin(); int_iter != int_list.end(); int_iter++ )
	{
		cout << *int_iter << " ";
	}

	cout << endl;

}

here's the result

[Linux list tests]$ g++ -o listtest2 listtest2.cpp
[Linux list tests]$ ./listtest2
57 54 51 48 45 42 39 36 33 30 27 24 21 18 15 12 9 6 3 0
test1
it equals
Segmentation fault
[Linux list tests]$

raxxor · 08-16-2004, 11:48 PM

Hi again,

Did you try using int_iter-- ? This mainly prevents another seg fault to be incase you delete the last item in the list... it also works for what you are doing:

Code:

~@thursday> ./test 
57 54 51 48 45 42 39 36 33 30 27 24 21 18 15 12 9 6 3 0 
test1
it equals

test2
57 54 51 48 45 42 39 36 33 30 24 21 18 15 12 9 6 3 0 
~@thursday>

Just change int_list.erase(int_iter); to int_list.erase(int_iter--);

Hope this helps,

r

kev82 · 08-17-2004, 06:00 AM

int_iter-- only works because list<> is doubally(sp?) linked, this wont work for a singly linked list. ie slist<>

the code i gave above "erase(int_iter++)" will work for both a singly and a doubally linked list. also there are no problems at the end because if you delete the last item int_iter++ will equal int_list.end() causing you to drop out of the loop before you try and dereference.