Resize array values of "empty" indices

I have a trivial resize function that just doubles array size when original list is full. From what I see, C++ treats unset indices as "empty" with values of 0. After I declared new double-sized array and copied elems from old list, I tried to output it in main but once it goes past last index w/ an item, it starts printing weird values, rather than 0's to indicate unset indices. The old and new lists are respectively: hashtable_1, hashtable_2.

The code is very trivial. Please let me know.

Code:

---

#include <iostream>
using namespace std;
 
//Note: trivial resize...I think there's a better one yet
void resize_v1(int* old_list, int* new_list, int size)//Note: new list will be declared in main
{
 
 for ( int i = 0; i < size; ++i )
  new_list[i] = old_list[i];
}
 
int main()
{
 int hashtable_1[15] = {0,0,9,0,1000,44,19,201,666,444,57,88,88908,77,33};
 
 int size_1 = sizeof(hashtable_1)/sizeof(int);//this is capacity, NOT num elems in list so far
 
 int hashtable_2[30] = {};
 
 cout << "Empty slot: " << hashtable_2[29] << endl;//I'm talking about this line, why does it output 0, yet in the for-loop below, it
outputs: -1 etc
 
 int size_2 = sizeof(hashtable_2)/sizeof(int);
 
 resize_v1(&hashtable_1[0], &hashtable_2[0], size_2);
 
 cout << size_2 << endl;
 
 for ( int i = 0; i < size_2; ++i )//weird values outputted once its past 15th index (last index that's occupied from old list: hashtable_1
 {
  cout << "cur i: " << i << endl;
  cout << hashtable_2[i] << " , " << endl << endl;
 }
 cout << endl;
 
 return 0;
}

---

First of all, C++ does not initialize any elements by default at all. Unless you explicitly initialize them, their values are undefined. There is no "empty" element. If you partly initialize an array, it implicitly initializes the rest of the array to a default value (0 for integers).
Secondly, your function does not resize anything. It copies over elements from one array to another, essentially a std::copy.
Thirdly, you are invoking undefined behavior by copying more elements into hashtable_1 than its capacity. Fix that before wondering about the output.

You're asking it to copy 30 items out of an array of that only holds 15 items*, into an array that holds 30 items.
*That's a buffer overrun!