Question

Note: This is not a question whether I should “use list or deque”. It’s a question about the validity of iterators in the face of insert().

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This may be a simple question and I’m just too dense to see the right way to do this. I’m implementing (for better or worse) a network traffic buffer as a std::list<char> buf, and I’m maintaining my current read position as an iterator readpos.

When I add data, I do something like

buf.insert(buf.end(), newdata.begin(), newdata.end());

My question is now, how do I keep the readpos iterator valid? If it points to the middle of the old buf, then it should be fine (by the iterator guarantees for std::list), but typically I may have read and processed all data and I have readpos == buf.end(). After the insertion, I want readpos always to point to the next unread character, which in case of the insertion should be the first inserted one.

Any suggestions? (Short of changing the buffer to a std::deque<char>, which appears to be much better suited to the task, as suggested below.)

Update: From a quick test with GCC4.4 I observe that deque and list behave differently with respect to readpos = buf.end(): After inserting at the end, readpos is broken in a list, but points to the next element in a deque. Is this a standard guarantee?

(According to cplusplus, any deque::insert() invalidated all iterators. That’s no good. Maybe using a counter is better than an iterator to track a position in a deque?)

Answer 1

if (readpos == buf.begin())
{
    buf.insert(buf.end(), newdata.begin(), newdata.end());
    readpos = buf.begin();
}
else
{
    --readpos;
    buf.insert(buf.end(), newdata.begin(), newdata.end());
    ++readpos;
}

Not elegant, but it should work.