C ++ Null Pointer

C ++ pointers

At the time of variable declarations, if not the exact address can be assigned, assign a NULL value for the pointer variable is a good programming practice. Fu NULL pointer value is called anull pointer.

A NULL pointer is defined in the standard library zero constants. Consider the following program:

#include <iostream>

using namespace std;

int main ()
{
   int  *ptr = NULL;

   cout << "ptr 的值是 " << ptr ;
 
   return 0;
}

When the above code is compiled and executed, it produces the following results:

ptr 的值是 0

On most operating systems, the program does not allow access to memory address 0, because the memory is reserved for the operating system. However, the memory address 0 has a special significance, it indicates that the pointer does not point to an accessible memory location. But according to the convention, if the pointer contains a null value (zero value), it is assumed that it does not point to anything.

To check for a null pointer, you can use the if statement, as follows:

if(ptr)     /* 如果 p 非空，则完成 */
if(!ptr)    /* 如果 p 为空，则完成 */

Therefore, if all unused pointers are assigned a null value, while avoiding the use of a null pointer, it is possible to prevent the misuse of an uninitialized pointer. In many cases, uninitialized variables there are some garbage value, causes the program difficult to debug.

C ++ pointers