Mailinglist Archive: opensuse-programming (118 mails)

[<synthetoonz@xxxxxxxxxxxxx>]

> From: Colin Carter <colincarter@xxxxxxxxxxxxxx>
> On Thursday 21 April 2005 22:31, Randall R Schulz wrote:
> > On Thursday 21 April 2005 02:48, Michael Stevens wrote:

> > > In C++ it would seem that the 'const void * NULL = 0'  definition
> > > would be a good thing as opposed to the literal '0' which Bjarne is
> > > recommending.
> >
> > I used these routinely in my C++ work:
> >
> > const	void		*NIL		= 0;
> > const	char		NUL		= 0;
> >
> > I would use a naked 0 in source code only where the context was actually
> > integer, not pointer or character, in which case I'd use on of these.

> I like this idea - it makes it clear about what you mean, whereas (to me
> anyway) there is always confusion with  NULL  which is not even NUL.
> 
> How do you write that a pointer  ptr  is pointing nowhere?
> That is, the value of the pointer is zero.   I mean so that it is clear
> that it is not pointing to a zero value.
> Pardon my ignorance, but my preference is for FORTRAN and we are
> positively discouraged from using pointers because all of our variables
> are in fact addresses (pointers) and not 'values' as in C.

If you want to know if a pointer is NULL, that is, not a valid pointer, you just compare the pointer.  If you dereference the pointer  (that's the *  )  then you are examining what  the pointer is pointing at.

Explicitly stated,     if   (pointer   ==   NULL)    /* The pointer must be NULL */

The joy of   !     (that's NOT)   is that it helps do the same sort of check with less typing. 

So, you could say     if (  !pointer   )  and it means the same thing as above.

   !    can save you from that dreaded  =   vs   ==    typo   where you inadvertently write code that assigns a value instead of compares.


A random function that protects itself from being passed a 0 (that's NULL pointer)...

char * foo(int x, char * bar)
{
    if ( !bar ) /* pointer must be zero */
        return NULL;

    /* .... blah blah .... */

    return somethingnotzero;
}


elsewhere some code calls foo()...

...
char * mybar = NULL;

/*Maybe mybar is assigned something later, or maybe not */

if  (!foo(2, mybar) )
{
    /* foo returned NULL, so it is an error */
    /* do something about the error */
}
else
{
    /* I like success.  foo worked. */
}



Very common in code is something that checks if the pointer is (not) NULL and that what it points to is also (not) NULL (or zero) .   i.e.  you want to know that a string pointer is valid and that it points to a string with at least one real character in it....

char * p = NULL;

/*.... maybe we made p point to  a string or not.... */

if (   !p   ||    !*p   )
    printf ("There is no P!\n");