Vector.erase(Iterator) causes bad memory access

问题:

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.

回答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;
  }
}

回答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()
);

回答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.

回答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.