It is always a good practice to assign the pointer NULL to a pointer variable in
case you do not have exact address to be assigned. This is done at the time of
variable declaration. A pointer that is assigned NULL is called a null pointer.
The NULL pointer is a constant with a value of zero defined in several standard
libraries, including iostream. Consider the following program:
#include <iostream>
using namespace std;
int main ()
{
int *ptr = NULL;
cout << “The value of ptr is ” << ptr ;
return 0;
}
When the above code is compiled and executed, it produces the following result:

The value of ptr is 0

On most of the operating systems, programs are not permitted to access
memory at address 0 because that memory is reserved by the operating system.
However, the memory address 0 has special significance; it signals that the
pointer is not intended to point to an accessible memory location. But by
convention, if a pointer contains the null (zero) value, it is assumed to point to
nothing.
To check for a null pointer you can use an if statement as follows:

if(ptr) // succeeds if p is not null

if(!ptr) // succeeds if p is null

Thus, if all unused pointers are given the null value and you avoid the use of a
null pointer, you can avoid the accidental misuse of an uninitialized pointer.
Many times, uninitialized variables hold some junk values and it becomes
difficult to debug the program.

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