Clarification of Rule 17.4

[sparker]

There is one significant difference between a pointer and an index: you can only compare pointers within the same aggregate object (or with NULL). So you can check if an integer index is within the range of an array, but given a pointer you can't check that the pointer is in a valid range (unless you invoke the undefined behaviour in ANSI C90 6.3.8).

For example, given:

Code:

int32 a[N];

you can do:

Code:

int32 lookup(int32 v)
{
   int32 r;

   if ((v < 0) || (v >= N))
   {
       /* handle error. */
       r = 0;
   }
   else
   {
        r = a[v];
   }
   return r;
}

but you can't (portably) do:

Code:

int32 lookup(int32 const *vp)
{
   int32 r;

   if ((vp < a) || (vp >= (a + N))
   {
       /* handle error. */
       r = 0;
   }
   else
   {
        r = *vp;
   }
   return r;
}

Is this important? I suspect not very much in practice.

stephen