Dyanamic Array Class, Program runs well but with Error - c++

Question

This is my Code
#ifndef INTLIST_H_INCLUDED
#define INTLIST_H_INCLUDED
#include <iostream>
using namespace std;
class intList
{
int upper_bound;
int arr[0];
public:
intList(){ arr[0] = 0; upper_bound = 0; }
void append(int x);
void sort();
friend ostream & operator << (ostream &, intList&);
inline int len(){ return upper_bound; }
inline int &operator [](int x){ return arr[x]; }
private:
void increment(int *a, int &l);
void swap(int &a, int &b);
};
void intList::swap(int &a, int &b)
{
int temp = a;
a = b;
b = temp;
}
void intList::increment(int *a, int &b)
{
b++;
a[b] = 0;
}
void intList::append(int num)
{
arr[upper_bound] = num;
increment(arr, upper_bound);
}
void intList::sort()
{
for(int i = 0; i < upper_bound; i++)
{
int minLoc = i;
for(int j = i+1; j<upper_bound; j++)
{
if(arr[j] < arr[minLoc])
minLoc = j;
}
if(minLoc != i)
swap(arr[i], arr[minLoc]);
}
}
ostream& operator << (ostream & dout, intList &a)
{
dout << "[ ";
for(int i = 0; i<a.upper_bound-1; i++)
dout << a.arr[i] << ", ";
dout << a.arr[a.upper_bound-1] << " ]";
return dout;
}
#endif // INTLIST_H_INCLUDED
The Code does its work perfectly fine. But at the end the Program Crashes. Giving some error like
process returned -1073741819 (0xC0000005) execution time : some seconds.
Just didn't get where am I going wrong.

Answer 1

This looks bad:
int arr[0];
First, C++ doesn't allow zero-sized fixed size arrays. Second, your code certainly needs more than a zero sized array.
Whatever use you make of this code is undefined behaviour (UB). UB includes code seemingly "working perfectly fine".

Answer 2

Your code has several problems.
For example, you have a fixed array of 0 size. If you want a dynamically growable array, you can use std::vector: you can add new items at the end of the vector (dynamically resizing it) using push_back() method:
#include <vector>
// Start with an empty vector
std::vector<int> v;
// Add some items to it
v.push_back(10);
v.push_back(20);
....
Note also that in header files it's not good to insert a using namespace std;. In this way you pollute the global namespace with STL classes, which is bad. Just use std:: prefix in header files.
Moreover, if you want to print the class content to an output stream, you may want to take the class as a const reference, since instances of the class are input parameters (you observe them and print their content to the stream):
std::ostream& operator<<(std::ostream& os, const IntList& a)
{
....
}