Segfault when removing element from list

This is a discussion on Segfault when removing element from list within the C++ Programming forums, part of the General Programming Boards category; It's been a while, haven't used lists much, but I'm trying to remove a specific element from a list I ...

  1. #1
    C++ Enthusiast jmd15's Avatar
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    Segfault when removing element from list

    It's been a while, haven't used lists much, but I'm trying to remove a specific element from a list I have.
    I iterate through the list until my iterator is pointing to the correct element, then I call the erase member function, passing my iterator as an argument to the erase function. When that piece of code runs, it causes a segfault. Here's what it looks like:
    Code:
    for(iter=openlist.begin();iter!=openlist.end();iter++)
    		{
    			temp=iter->get_pos();
    			if(disp.x==temp.x&&disp.y==temp.y)
    			openlist.erase(iter);
    		}
    The temp and disp variables are structs that just contain two integers(x and y). The openlist is my list(obviously), and I want to remove the element in openlist that has the same coordinates as the set found in disp. The list contains a class I made. I'm wondering if there's a better, easier way to remove that specific element with those coordinates(and yes, the coordinates appear only once)? Or is there a reason that's giving me a segmentation fault? Any help or pointers are appreciated, thanks.
    Trinity: "Neo... nobody has ever done this before."
    Neo: "That's why it's going to work."
    c9915ec6c1f3b876ddf38514adbb94f0

  2. #2
    ZuK
    ZuK is offline
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    you are incrementing a deleted iterator.
    If there is only one element that you want to remove then a quick fix would be
    Code:
                    for(iter=openlist.begin();iter!=openlist.end();iter++)
    		{
    			temp=iter->get_pos();
    			if(disp.x==temp.x&&disp.y==temp.y) {
    			    openlist.erase(iter);
                                break;
                            }
    		}
    Kurt

  3. #3
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    Will you ever erase more than one element from the list? If not, then you should break the loop immediately after erasing.

    If it is possible to erase more than one element, then you need to modify your loop. Calling erase invalidates the iterator it erases. You then increment that iterator and compare it to the end() iterator. This is illegal. The common way to erase from an stl sequence container like this is:
    Code:
    iter=openlist.begin();
    while (iter!=openlist.end())
    {
    	temp=iter->get_pos();
    	if(disp.x==temp.x&&disp.y==temp.y)
    		iter = openlist.erase(iter);
    	else
    		++iter;
    }

  4. #4
    C++ Enthusiast jmd15's Avatar
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    Thank you both. That was exactly my problem. It didn't dawn on me that such was taking place. Thanks again, problem solved.
    Trinity: "Neo... nobody has ever done this before."
    Neo: "That's why it's going to work."
    c9915ec6c1f3b876ddf38514adbb94f0

  5. #5
    The larch
    Join Date
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    A good way to erase elements from a list based on some condition, is the following:
    Code:
    list.remove_if(RemovePredicate);
    where RemovePredicate is a function that takes a single parameter with the same type as the list is holding (the current value) and returns true if the element should be removed.

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