Question

I know that variables allocated on that stack of a function become inaccessible when the function finishes execution. However, vector types allocate their elements on the heap no matter how they are allocated. So for instance,

vector<int> A;

will allocate space for its elements on the heap instead of the stack.

My question is, assume I have the following code:

int main(int argc, char *argv[]) {
    // initialize a vector
    vector<int> A = initVector(100000);

    // do something with the vector...

    return 0;
}


// initialize the vector
vector<int> initVector(int size) {
    vector<int> A (size);  // initialize the vector "on the stack"

    // fill the vector with a sequence of numbers...
    int counter = 0;
    for (vector<int>::iterator i = A.begin(); i != A.end(); i++) {
        (*i) = counter++;
    }

    return A;
}

Will I have memory access problems when using the vector A in the main function? I tried this several times and they all worked normally, but I’m scared that this might just be luck.

The way I see it is, the vector A allocates its elements on the heap, but it has some “overhead” parameters (maybe the size of the vector) allocated on the stack itself. Therefore, using the vector in the main function might result in a memory access problem if these parameters are overwritten by another allocation. Any ideas?

Answer 1

Yes, it will work normally because the memory for the elements are allocated and that is what will be used to build the vector<int> A = variable. However, performance wise, it is not the best idea.

I would suggest changing your function to be the following though

void initVector(vector<int>& a, int size) 

For additional references on usage, please see Returning a STL vector from a function… and [C++] Returning Vector from Function.

For an additional reference on performance (using C++11), please see Proper way (move semantics) to return a std::vector from function calling in C++0x