C++ double iterator loop for collision detection - c++

Question

First things first, I'm very new to C++! So I've got a standard list of objects representing elements in 2d space, and I want to do some basic collision detection. My first idea (coming from a mostly Java background...) is to compare each object with each other object, calling an function to check for intersection on each pair or objects. This would have been simple in Java, taking the first item of the ArrayList, comparing it with the second, third, etc. then taking the second item, comparing it with the third, fourth, etc. This is the approach I took with the problem in C++, but I'm using an iterator (rather than direct element access like I'd do in Java), but iterators are meant to be used linearly, right? So direct access isn't suitable.
So my question is how do I perform this algorithm? I'm also reasonably sure this isn't the best way to do (very basic) collision detection, so any advice on that would be welcomed too. Here's my (non-working) code.
for (list<Box>::iterator p = mBoxes.begin(); p != mBoxes.end(); p++) {
for (list<Box>::iterator q = mBoxes.begin() + p); q != mBoxes.end(); q++) {
if (p->isIntersecting(q)) {
p->changeDirection();
q->changeDirection();
}
}
}
This should illustrate the method I'm attempting, but of course my try of mBoxes.begin() + p doesn't work!

Answer 1

EDIT: Responding to multiple comments.
for (list<Box>::iterator p = mBoxes.begin(); p != mBoxes.end(); ++p) {
for (list<Box>::iterator q = p); q != mBoxes.end(); ++q) {
if (p==q) continue;
if (p->isIntersecting(*q)) {
p->changeDirection();
q->changeDirection();
}
}
}

Answer 2

The list class is a linked list, you cannot directly index into it. Also, iterators are not indices, + just doesn't makes any sense.
However, in theory, you should be able to copy an iterator, if you change q = mboxes.begin() + p into q = p it should set q to be an iterator pointing to the same location of p, and that may just solve your problem.
for (list<Box>::iterator p = mBoxes.begin(); p != mBoxes.end(); p++) {
for (list<Box>::iterator q = p, q++; q != mBoxes.end(); q++) {
if (p->isIntersecting(q)) {
p->changeDirection();
q->changeDirection();
}
}
}
the q++ should simply skip the current element so you don't compare an item against itself.

Answer 3

Something like this should work:
for (int p = 0; p < mBoxes.size(); ++p)
{
for (int q = 0; q < mBoxes.size(); ++q)
{
if (p == q)
{
// don't compare for collision against itself
continue;
}
if (mBoxes[p]->isIntersecting(mBoxes[q]))
{
mBoxes[p]->changeDirection();
mBoxes[q]->changeDirection();
}
}
}
However, one problem with this is that as you iterate through you'll compare p intersect q and also q intersect p meaning that if they do intersect they'll change direction twice putting them back in the same direction each time. So some extra logic would be needed to avoid this.