What are null pointers used for

Question

I have just started using c++ and saw that their is a null value for pointers. I am curious as to what this is used for. It seems like it would be pointless to add a pointer to point to nothing.

Answer 1

Well, the null pointer value has the remarkable property that, despite it being a well-defined and unique constant value, the exact value depending on machine-architecture and ABI (on most modern ones all-bits-zero, not that it matters), it never points to (or just behind) an object.

This allows it to be used as a reliable error-indicator when a valid pointer is expected (functions might throw an exception or terminate execution instead), as well as a sentinel value, or to mark the absence of something optional.

On many implementations accessing memory through a nullpointer will reliably cause a hardware exception (some even trap on arithmetic), though on many others, especially those without paging and / or segmentation it will not.

Answer 2

Generally it's a placeholder. If you just declare a pointer, int *a;, there's no guarantee what is in the pointer when you want to access it. So if your code may or may not set the pointer later, there's no way to tell if the pointer is valid or just pointing to garbage memory. But if you declare it as NULL, such as int *a = NULL; you can then check later to see if the pointer was set, like if(a == NULL).

Most of the time during initialization we assign null value to a pointer so that we can check whether it is still null or a address has been assign to it or not.

Answer 3

It seems like it would be pointless to add a pointer to point to nothing.

No, it is not. Suppose you have a function returning optional dynamically allocated value. When you want to return "nothing" you return null. The caller can check for null and distinguish between 2 different cases: when the return value is "nothing" and when the return value is some valid usable object.

Answer 4

Basically, pointers are just integers. The null pointer is a pointer with a value of 0. It doesn't strictly point to nothing, it points to absolute address 0, which generally isn't accessible to your program; dereferencing it causes a fault.

It's generally used as a flag value, so that you can, for example, use it to end a loop.

Update:

There seem to be a lot of people confused by this answer, which is, strictly, completely correct. See C11(ISO/IEC 9899:201x) §6.3.2.3 Pointers Section 3:

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

So, what's an address? It's a number n where 0 ≤ n ≤ max_address. And how do we represent such a number? Why, it's an integer, just like the standard says.

The C11 standard makes it clear that there's never anything to reference at address 0, because in some old pathologically non-portable code in BSD 4.2, you often saw code like this:

/* DON'T TRY THIS AT HOME */

int
main(){
    char target[100] ;
    char * tp = &target ;
    char * src = "This won't do what you think." ;
    void exit(int);

    while((*tp++ = *src++))
        ;

    exit(0);
}

This is still valid C:

$ gcc -o dumb dumb.c
dumb.c:6:12: warning: incompatible pointer types initializing 'char *' with an
      expression of type 'char (*)[100]' [-Wincompatible-pointer-types]
    char * tp = &target ;
           ^    ~~~~~~~
1 warning generated.
$ 

In 4.2BSD on a VAX, you could get away with that nonsense, because address 0 reliably contained the value 0, so the assignment evaluated to 0, which is of course FALSE.

Now, to demonstrate:

/* Very simple program dereferencing a NULL pointer. */

int
main() {
    int * a_pointer ;
    int a_value ;
    void exit(int);             /* To avoid any #includes */

    a_pointer = ((void*)0);
    a_value = *a_pointer ;

    exit(0);
}

Here's the results:

$ gcc -o null null.c
$ ./null
Segmentation fault: 11
$ 

Answer 5

null value in C and C++ is equal to 0. But nullptr in C++ is different from it, nullptr is always a pointer type in C++. We assign a null value to a pointer variable for various reason.

1. To check whether a memory has been allocated to the pointer or not
2. To neutralize a dangling pointer so that it should not create any side effect
3. To check whether a return address is a valid address or not etc.

Most of the time during initialization we assign null value to a pointer so that we can check whether it is still null or a address has been assign to it or not.