Unexpected vector behavior using set intersection - c++

Question

I am trying to compare a 2 dimensional vector against another 2 dimensional vector.
vector<vector<int> >::iterator rowit1;
for(rowit1 = aboveaveperms.begin(); rowit1 != aboveaveperms.end();rowit1++)
{
vector<vector<int> >::iterator row2it;
int s = 0;
for(row2it = afterave.begin(); row2it != afterave.end();row2it++)
{
vector<int> matches (9);
vector<int>::iterator itset;
itset = set_intersection(rowit1->begin(),rowit1->end(),row2it->begin() + 1,row2it->end(),matches.begin());
matches.resize(itset-matches.begin());
s = matches.size();
matches.erase(matches.begin(),matches.end()); // my attempt at trying to correct it
}
}
In the second loop on the first pass matches is created correctly holding 9 zeros. But once it begins the second loop, as soon as it hits the curly bracket, it comes out with 101 million instead of 9. Once it actually hits it's declaration it goes back to the 9 which is proper. I have noticed while debugging that it is holding all of the 2 dimensional vector I am checking against before the declaration corrects it to only holding 9 ints.
idzireit
APPENDUM:
As it has been pointed out I forgot to ask my question. The above code does work as I want the number of matches (between 5 and 9) but because of the erratic behaviour of the vector will it cause a memory leak or sigseg if the other vectors get too big?
Also, as whozcraig asked, I have row2it->begin() + 1 because the first element is the game number which is much bigger then the rest of the numbers. The rest of the numbers are sorted though.
Example of what I am trying to do is as follows:
perm vector 1
1 3 5 6 7
compare against 4 vectors and count for number of matches in each
5 8 9 10 11
3 7 11 14 18
1 5 6 7 8
so in running through the loop the first match should be 1 second should be 2 and third should be 4. Hope this helps to clarify what I am trying to do.
ADDENDUM 2:
Now that I have my windoze machine up and running and have reinstalled qt creator on it I have run the above code and works fine on windows. I do not get the 101 million elements like I did when running under ubuntu. Why does it work correctly on windoze and not ubuntu ?
idzireit

Answer 1

Firstly, according to https://en.cppreference.com/w/cpp/algorithm/set_intersection, both vectors be intersected must be sorted.
Secondly, you set an initial size of 9 to vector matches, but if the number of intersected elements is greater than 9, then std::set_intersection is writing out of bounds of matches, which may cause an undefined behaviour.
You can execute your code using a memory analyzer as Valgrind in order to detect a possible memory leak. You could use std::back_inserter to avoid this problem:
#include <iterator> // std::back_inserter
vector<int> matches;
std::back_insert_iterator<std::vector<int> > itset(matches);
itset = set_intersection(rowit1->begin(),rowit1->end(),row2it->begin() + 1,row2it->end(), std::back_inserter(matches));