Segmentation fault while accessing vector elements

Question

I am learning how to make a reference variable in C++ and I am having some trouble when using int &res = f[k] to make res a variable that refers to a given component of vector f.

In this code F(k) should return the fibonacci number of integer input k computed by memorizing the previous calls F(0), F(1),... F(k-2) and F(k-1) in vector f but I get a Segmentation Fault which makes me think I fail to refer res as a component of vector f.

Here's my code:

#include <iostream>
#include <vector>
using namespace std;
const int UNDEF = -1;
vector<int> f;

int F(int k) {
  int &res = f[k];  // I get "res: 0x0000000000000004 and &res: ??" when debugging
  if (res != UNDEF) return res; // EXC_BAD_ACCESS (code=1, address=0x4)
  if (k <= 1) return 1;
  return res = F(k-1) + F(k-2);
}

int main() {
  int k;
  cin >> k;
  vector<int> f(k+1, UNDEF);
  cout << F(k) << endl;
} 

I could really use some help over here! Thanks a lot:)

Àlex

Answer 1

The global vector variable f has no elements. And when you write:

int &res = f[k];

You're trying to access it's kth element which doesn't exist which is why you get segmentation fault at that point.

Note in your program you have two vector variable with the same name f. One of them is a local variable while the other one is a global variable.

When you write

int &res = f[k];

The vector f that is chosen is the global variable and since the global vector f has size 0(empty vector), when you try to access its kth element it gives you segmentation fault(undefined behavior) as can be seen here.