Re: Can I dynamically add new elements to vector while looping it?

From:

Richard Herring <junk@[127.0.0.1]>

Newsgroups:

comp.lang.c++

Date:

Tue, 13 Jun 2006 14:09:54 +0100

Message-ID:

<kManUPhikrjEFwGQ@baesystems.com>

In message <4f7pksF1hm779U1@individual.net>, Gernot Frisch
<Me@Privacy.net> writes

vector<int> ints;
vector<int>::iterator itr;
while (itr != ints.end()){
    int j = some_function(*i);
    if (j>0){
       ints.push_back(j);
    }
    ints.erase(itr);
}

instead of iterators you could use indices:

So where are they? I don't see anything looking like ints[i] anywhere in
the following, and it's still full of iterators. How is it supposed to
be an improvement?

int count = (int)ints.size();
while(itr != ints.begin() + count)

That's no different from saving the value of ints.end() and comparing
against the saved value. And itr hasn't been initialised yet.

{
   vector<int>::iterator ii(ints.begin() + i);
   int j=some_function(*ii);
   if(j>0) ints.push_back(j);

push_back() can invalidate all iterators. The following line is now UB.

ints.erase(itr); // Now you want a --i; as well!!!

erase(itr) invalidates itr. The idiom is

itr = ints.erase(itr)

which leaves itr pointing to the element after the one erased, or
ints.end() if there is no such element.

}

--
Richard Herring