Question

I have an array of pointers to vectors vector<int> *F[12]; and I wish to initialize some vectors and store their addresses in F.

for(uint i = A; i <= B; ++i){
    vector<uint> newvec(pow2(i), 0);
    F[i] = &newvec;
}

But when I try to access them later, I find that they are not all zero:

for(uint i = A; i <= B; ++i){
    for(uint j = 0; j <= pow2(i); ++j){
        if((*F[i]).at(j) != 0){
            cout << i << "/" << j << "/" << (*F[i]).at(j) << "|";
        }
    }
}

I assume this has something to do with the scope of the declared vector. Why has the memory been freed? Do I have to use malloc and create a vector from that?

Answer 1

newvec is being created on the call stack each iteration. Each time through the loop you create it, make a pointer to it, then it vanishes. You want to use the ‘new’ operator to create a longer-lasting instance of the vector in the heap.