C++ LANGUAGE NULL POINTERS

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C++ LANGUAGE NULL POINTERS

 
Null Pointers 


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.