Debug assertion error - List iterators incompatible

Question

I'm working on a program that is supposed to put every Window in a list, resize it, and move it to a screen position according to a specified layout.

When I'm running this function however I get a debug assertion error saying "list iterators incompatible".

Here is the code:

void Control::checkForNewWindows()
{
    for (std::list<Window>::iterator i = mainDetector.getWindowList().begin(); i != mainDetector.getWindowList().end(); ++i)
    {
        bool forBreak = false;
        if ((i->getTitle().find("sample_title") != std::string::npos) && (i->getState() == false))
        {
            for (int y = 0; y < 3; y++)
            {
                for (int x = 0; x < 4; x++)
                {
                    if (activeLayout.windowLayout[y][x].getHandle() == 0)
                    {
                        moveWindow(*i, activeLayout.dimensionsLayout[y][x].x, activeLayout.dimensionsLayout[y][x].y, activeLayout.dimensionsLayout[y][x].width,
                            activeLayout.dimensionsLayout[y][x].height);
                        activeLayout.windowLayout[y][x] = *i;
                        activeLayout.windowLayout[y][x].setState(true);
                        forBreak = true;
                    }
                    if (forBreak)
                    {
                        break;
                    }
                }
                if (forBreak)
                {
                    break;
                }
            }
        }
    }
}

The error occurs during the first for loop, hope someone can help me fix this

Edit:

Here is the getWindowList function:

std::list <Window> Detector::getWindowList()
{
    return windowList;
}

and the windowList definition:

std::list <Window> windowList;

Answer 1

Your loop looks like this:

for (std::list<Window>::iterator i = mainDetector.getWindowList().begin(); 
     i != mainDetector.getWindowList().end(); 
     ++i)

Given the above, the issue is this:

std::list <Window> Detector::getWindowList()
{
    return windowList;
}

You're returning a copy of the list, not the original. Thus the iterator to the copy will be used in the loop and not the iterator of windowList . In fact, you are using two different iterators in the loop construct, and neither one of them refers to the original list, only copies.

The fix is to return a reference:

std::list <Window>& Detector::getWindowList()
{
    return windowList;
}

You're now returning a reference to the actual list, not a copy. Now the iterators you are using in the loop constraints refer to the same list, not different lists.

Answer 2

FYI for others researching this, you can also get the same error message when dealing with dangling pointers and deleted memory. I ran across this thread while tracking down that error message. In my case, there was an iterator to a list that had been deleted.