Vector.erase(Iterator) causes bad memory access

Question

I am trying to do a Z-Index reordering of videoObjects stored in a vector. The plan is to identify the videoObject which is going to be put on the first position of the vector, erase it and then insert it at the first position. Unfortunately the erase() function always causes bad memory access.

Here is my code:

testApp.h:

vector<videoObject> videoObjects;
vector<videoObject>::iterator itVid;

testApp.cpp:

// Get the videoObject which relates to the user event
for(itVid = videoObjects.begin(); itVid != videoObjects.end(); ++itVid) {
  if(videoObjects.at(itVid - videoObjects.begin()).isInside(ofPoint(tcur.getX(), tcur.getY()))) {
   videoObjects.erase(itVid);
  }
}

This should be so simple but I just don't see where I'm taking the wrong turn.

Bacteroid

Why videoObjects.at(itVid - videoObjects.begin()) instead of (*itVid)? – Puiia

tried, but it causes the following error: no matching function for call to 'std::vector<videoObject, std::allocator<videoObject> >::at(videoObject&)' – Bacteroid

You don't need to use at() at all; dereferencing the iterator returns a reference to the pointed-to element. – Puiia

Duplicate problem to this question... https://mcmap.net/q/911297/-c-iterators-problem/… - do we close off same-solution questions? This is the same old erase-invalidates-iterator problem. – Whitlow

Answer 1

You should do

itVid = videoObjects.erase(itVid);

Quote from cplusplus.com:

[vector::erase] invalidates all iterator and references to elements after position or first.

Return value: A random access iterator pointing to the new location of the element that followed the last element erased by the function call, which is the vector end if the operation erased the last element in the sequence.

Update: the way you access the current element inside your condition looks rather strange. Also one must avoid incrementing the iterator after erase, as this would skip an element and may cause out-of-bounds errors. Try this:

for(itVid = videoObjects.begin(); itVid != videoObjects.end(); ){
  if(itVid->isInside(ofPoint(tcur.getX(), tcur.getY()))){
    itVid = videoObjects.erase(itVid);
  } else {
    ++itVid;
  }
}

Besprent

To anyone using Visual C++ 2010: You need to disable iterator debugging to get this to work (basically, iterator debugging is hopelessly broken in this scenario): connect.microsoft.com/VisualStudio/feedback/details/557029 – Puiia

I've tried: `for(itVid = videoObjects.begin(); itVid != videoObjects.end(); ++itVid){ if(videoObjects.at(itVid - videoObjects.begin()).isInside(ofPoint(tcur.getX(), tcur.getY()))){ itVid = videoObjects.erase(itVid); break; } } but still get a bad memory access... – Bacteroid

@Péter: I'm greatly appreciating your help. Unfortunately I do still get the bad memmory access :( – Bacteroid

@xon1c, have you tried debugging? How many times the loop is executed before the failure? Are multiple elements erased, or does the very first erase fail? – Buote

@Péter: I have 3 videoObjects in the vector. All 3 are being displayed on screen. By clicking on one, it gets deleted. The bad memory access happens with either one. So the function will always erase only one videoObject but I've tried, it doesn't matter on which index position that object is stored. – Bacteroid

@xon1c, if the error happens right in the call to erase, it may also be a destructor issue. Could you publish the constructor(s) and destructor of videoObject? – Buote

@xon1c, how about the embedded videoClip? Try commenting out that member (and any other non-primitive member fields) and see whether the bug still occurs... – Buote

@Péter: I am using the ofVideoPlayer class provided by openframeworks. I've discovered that its constructor has a method tex.clear(); that causes the exception. After putting that line in a comment, the erase() function now works correctly. Hopefully not calling 'tex.clear();` won't bring up any other potential problems. However, thank you very much for your help & patience! – Bacteroid

@Péter: sorry, I was wrong... still getting bad memory access when I uncomment the videoClip part. :( – Bacteroid

@xon1c, that at least proves that the culprit is that class. – Buote

@Péter: ok, now finally the videoObject gets removed properly. The ofVideoPlayer uses the class ofTexture which also causes problems when the destructor is called. I've removed all lines from both destructors and all works fine now but I'm just waiting for the next error coming up because of this workaround... sigh – Bacteroid

Answer 2

Beware, erasing elements one by one from a vector has quadratic complexity. STL to the rescue!

#include <algorithm>
#include <functional>

videoObjects.erase(
    std::remove_if(
        std::bind2nd(
            std::mem_fun_ref(&videoObject::isInside),
            ofPoint(tcur.getX(), tcur.getY())
        ),
    ),
    videoObjects.end()
);

Williamwilliams

Answer 3

You cannot delete while iterating over the list because the iterator gets invalid. You should use the return iterator of Erase to set it to your current iterator.

Alaska

Answer 4

erase function returns the next valid iterator.

You would have to make a while loop and do something like

iterator = erase(...)

with corresponding checks.

Teishateixeira