Issue when I copy a char array to another, using a template

Question

Here is my code:

#include <iostream>
#include <string>
#include <stdio.h>
#include <stdlib.h>
using namespace std;

template <class T1, class T2>
void copy2(const T1 source[], T2 destination[] , int size){
    for (int i=0 ; i < size ; ++i){
        destination[i]=static_cast<T1>(source[i]);
    }
}



int main() {

    const char one[] = "hello";
    char two[5];

    cout << "one: " << one << endl;
    cout << "two: " << two << endl;

    copy2(one, two, 6);

    cout << "one: " << one << endl;
    cout << "two: " << two << endl;


    return 0;
}

but it outputs:

one: hello

two:

one:

two: hello

Moreover, the array "one" is const, and therefore shouldn't be changed.

PS: When I initiate the array "two" in the following way, it works (but WHY??):

 char two[8];

However when I initiate it in both of the following ways, I get weird errors:

 char two[6];

or

 char two[7];

Answer 1

My best guess is that two and one are on the stack next to each other like this:

  t   w   o   -   -   o   n   e   -   -    -
 --------------------------------------------
|   |   |   |   |   | h | e | l | l | o | \0 |
 --------------------------------------------

Since you are overflowing two's buffer by passing size 6 to copy2 when two has size 5, the memory will end up like this:

  t   w   o   -   -   o    n   e   -   -   -
 --------------------------------------------
| h | e | l | l | o | \0 | e | l | l | o | \0 |
 --------------------------------------------

Which is why two appears to hold "hello" and one shows nothing (since two overran its buffer and now the null terminator is the first character in one).