How to properly point back to std::list after erasing en element (double for loop)?

Question

I want to erase an element from the std::list and then point back to this list but when I do it this way

for(std::list<CvRect>::iterator it = listOfCvRects.begin(); it != listOfCvRects.end(); it++)
{
    for(std::list<CvRect>::iterator jt = listOfCvRects.begin();  jt != listOfCvRects.end(); jt++)
    {
        if( it == jt )
        { continue;}

        if( (jt->x) > (it->x) //.. more conditions...)
        {
            jt = listOfCvRects.erase(jt);
            //OR 
            //listOfCvRects.erase(jt++);
        }
    }

}

I got and unhandled exception : iterator is unincrementable

[possible duplicate](http://stackoverflow.com/q/596162/624900) — jterrace, Jan 04 '12 at 23:01
If the condition is symmetric, you could start the inner loop at `jt = it;`. — Kerrek SB, Jan 04 '12 at 23:08

score 4 · Accepted Answer · answered Jan 04 '12 at 23:04

I believe the problem is that in some cases (those where you delete an element), you are double incrementing the iterator. Your for loop looks like this:

for(std::list<T>::iterator jt = l.begin();  jt != l.end(); jt++) {
    ....
}

But inside of it you are doing something like this:

jt = l.erase(jt);

So, if the case happens that you do the erase, you erase it and at the same time, set the iterator to the next element... But, you also increment it with the jt++!

The simple approach to fixing this is re-write the for loop slightly to fit this form factor:

for(std::list<T>::iterator it = l.begin(); it != l.end(); ) { // notice no increment!
    // ...
    if(cond) {
        it = l.erase(it);
    } else {
        ++it;
    }
}

so you are doing one or the other increments, but never both.

@Patryk: see update, I was trying to get the basics in there quick while I was typing the full answer :-). — Evan Teran, Jan 04 '12 at 23:07

score 0 · Answer 2 · answered Jan 04 '12 at 23:05

Erasing an element from a list invalidates iterators that point at that element, but not other iterators. So you need to do the increment before the erase:

for(std::list<CvRect>::iterator it = listOfCvRects.begin(); it != listOfCvRects.end(); it++) {
    std::list<CvRect>::iterator jt = listOfCvRects.begin();
    while (jt != listOfCvRects.end()) {
        if( it == jt) continue;
        if( (jt->x) > (it->x) //.. more conditions...) {
            listOfCvRects.erase(jt++);
        } else {
            jt++;
        }
    }
}

`erase` returns an iterator to the next element (or `end()`). — Marcelo Cantos, Jan 04 '12 at 23:06

How to properly point back to std::list after erasing en element (double for loop)?

2 Answers2