1. #include <iostream>
2.  
3. template<typename T>
4. class Array
5. {
6. private:
7.         int m_capacity = 0;
8.         T* m_data = nullptr;
9. public:
10.         Array(int capacity) : m_capacity(capacity), m_data(new T[capacity]{})
11.         {
12.         }
13.  
14.         ~Array()
15.         {
16.                 delete[] m_data;
17.         }
18.  
19.         T& operator[](int index)
20.         {
21.                 return m_data[index];
22.         }
23.  
24.         friend std::ostream& operator<< (std::ostream& stream, const Array& array)
25.         {
26.                 for (int i = 0; i <= array.m_capacity - 1; i++)
27.                 {
28.                         // how can i check in here if value under this key contains something that user set manually?
29.                         stream << i << ": " << array.m_data[i] << std::endl;
30.                 }
31.                 return stream;
32.         }
33. };
34.  
35.  
36.  
37. int main()
38. {
39.         Array<int> test = Array<int>(5);
40.         test[0] = 1;
41.         test[1] = 0; // this 0 should be also displayed
42.         test[2] = 3;
43.  
44.         /*
45.         result right now:
46.         0: 1
47.         1: 0
48.         2: 3
49.         3: 0
50.         4: 0
51.  
52.         what i would like to achieve:
53.         0: 1
54.         1: 0
55.         2: 3
56.  
57.         so as you can see i would like to not display element 3,4 because i haven't set these values.
58.         */
59.         std::cout << test << std::endl;
60.         std::getchar();
61.         return 0;
62. }