C++: Optimizing speed with vector/array?

Question

I have a nested for-loop structure and right now I am re-declaring the vector at the start of each iteration:

void function (n1,n2,bound,etc){

    for (int i=0; i<bound; i++){
             vector< vector<long long> > vec(n1, vector<long long>(n2));
             //about three more for-loops here
    }
}

This allows me to "start fresh" each iteration, which works great because my internal operations are largely in the form of vec[a][b] += some value. But I worry that it's slow for large n1 or large n2. I don't know the underlying architecture of vectors/arrays/etc so I am not sure what the fastest way is to handle this situation. Should I use an array instead? Should I clear it differently? Should I handle the logic differently altogether?

EDIT: The vector's size technically does not change each iteration (but it may change based on function parameters). I'm simply trying to clear it/etc so the program is as fast as humanly possible given all other circumstances.

EDIT:

My results of different methods:

Timings (for a sample set of data):
reclaring vector method: 111623 ms
clearing/resizing method: 126451 ms
looping/setting to 0 method: 88686 ms

Answer 1

I have a clear preference for small scopes (i.e. declaring the variable in the innermost loop if it’s only used there) but for large sizes this could cause a lot of allocations.

So if this loop is a performance problem, try declaring the variable outside the loop and merely clearing it inside the loop – however, this is only advantageous if the (reserved) size of the vector stays identical. If you are resizing the vector, then you get reallocations anyway.

Don’t use a raw array – it doesn’t give you any advantage, and only trouble.