Stack Overflow Accessing Large Vector

Question

I'm getting a stack overflow on the first iteration of this for loop

for (int q = 0; q < SIZEN; q++)
{
    cout<<nList[q]<<" ";
}

nList is a vector of type int with 376 items. The size of nList depends on a constant defined in the program. The program works for every value up to 376, then after 376 it stops working.

Any thoughts?

Answer 1

If by "stops working", you mean crashes, then you're probably reading past the end of the buffer. vector::operator[] is not range checked, so it will let you shoot yourself in the foot.

If you want to traverse a vector, use an iterator, or at the very least nList.size().

So with least modifications to your code:

for (int q = 0; q < nList.size(); q++)
{
    cout << nList[q] << " ";
}

or with iterators

for (std::vector<int>::const_iterator it = nList.begin();
     it != nList.end(); ++it) {
  cout << *it << " ";
}

Answer 2

My initial guess here would be that the vector is less than 376. The [] operator makes no guaratees as to running over the actual vector bounds. You'd be much, MUCH safer if you used the at function:

for(int i=0; i < nList.size(); ++i){
  cout << nList.at(q) << " ";
}

there, if q is outside of the vector it'll throw an exception. That'll help diagnose this type of runtime problem.

Answer 3

If the you have added 376 elements to the vector by using for example push_back it is normal that the access with values higher than 376 make the program fail, you are accessing surely uninitialized and not managed memory.