Permalink
Reply
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, because dereferencing it would result in undefined behavior.
I'm just pondering whether the intention was to accomodate this looping pattern:
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) 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.