On 8/17/25 8:05 AM, Paul Backus wrote:
> In C, it is UB to *create* an out-of-bounds pointer, *except* for a
> pointer that is one element past the end of an array, which is allowed.
> (Source: [C11 § 6.5.6 ¶ 8][1]) The intent of this exception is to allow
> idioms like the one above.
>
> In D, merely *creating* an out-of-bounds pointer is never UB. In
> general, D tries to avoid making things UB unless it is absolutely
> necessary to do so, and that is probably why D is less strict than C
here.
>
> In both C and D, it is always UB to *dereference* an out-of-bounds
pointer.
>
> [1]: https://port70.net/~nsz/c/c11/n1570.html#6.5.6p8
That's exactly my understanding. If a C library has a function like the
following,
void foo(T* beginning, T* one_past_the_end);
and since D is allowed to call C functions, I would call that C function
like this:
// Expected in D:
T[] arr;
foo(arr.ptr, arr.ptr + length); // Second argument is pointing outside
I claim the call is valid and that D's spec is missing an explicit
mention to allow this.
Otherwise, I would have to make the following in D, which would be
ridiculous:
// Ridiculous in D:
T[] arr;
auto larger_arr = new T[arr.length + 1];
larger_arr[0..$ - 1] = arr[];
foo(larger_arr.ptr, larger_arr.ptr + length);
I still like the book. ;) The spec needs improvement.
Ali
