Pointers - Is it safe to point to invalid memory?

[Paul Backus]

On Saturday, 16 August 2025 at 22:28:15 UTC, Andy Valencia wrote:
> On Saturday, 16 August 2025 at 21:58:30 UTC, Paul Backus wrote:
>> Creating a pointer that points out-of-bounds does not, by 
>> itself, result in undefined behavior.
>>
>> However, such a pointer would not be considered a [safe 
>> value][1], because dereferencing it *would* result in 
>> undefined behavior.
>
> I'm just pondering whether the intention was to accomodate this 
> looping pattern:
>
> ```d
> int sum_values(int* p, uint nval) {
>   int result = 0;
>   foreach(_; 0 .. nval) {
>     result += *p++;
>   }
>   return result;
> }
> ```
>
> A C idiom (I've so very much embraced not using pointers in my 
> D world) which technically leaves "p" pointing beyond the 
> memory range.  Perhaps this is the special case being addressed?

I am not sure what "special case" you are referring to, so I will 
try to explain from first principles.

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