A pointer should point to a valid address but not necessarily to valid elements (like for arrays). These are called invalid pointers. Uninitialized pointers are also invalid pointers.
int *ptr2 = arr+20;
Here, ptr1 is uninitialized so it becomes an invalid pointer and ptr2 is out of bounds of arr so it also becomes an invalid pointer.
(Note: invalid pointers do not necessarily raise compile errors)
Additional information on NULL Pointers is available.
Null pointer is a pointer which point nowhere and not just an invalid address.
Following are 2 methods to assign a pointer as NULL;
int *ptr1 = 0;
int *ptr2 = NULL;
Please note that 0 in the above code is used in a pointer context, and it is different from 0 as an integer
"Pointers in C/C++ with Examples" by Abhirav Kariya, Geeks for Geeks is licensed under CC BY 4.0