On Fri, 29 Aug 2025 at 11:20, Jonathan Wakely <cxx_at_[hidden]> wrote:

>
>
> On Thu, 28 Aug 2025 at 23:44, Levo D via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>>
>> Why Not Span
>>
>> Span doesn't have special rules for bounds checking. The following would
>> assert, but we're looking for a compile-time error, which does not happen.
>>
>
> It could do though. std::span seems like the obvious solution for array
> slices in C++.
>
> Why not work on improving static analysis in existing compilers so that
> they detect out of bounds span accesses at compile time? Do we really need
> changes to the type system to do that?
>
>
>
>
>> The test1 and test2 only differ in the function signature.
>>
>> #include <span>
>> int test1(std::span<int> file, int n) { // span<int>
>> if (file[0] != 0x12 && file[2] != 0x34)
>> return -1;
>> return file[n] + file[100];
>> }
>> int test2(std::span<int, 32> file, int n) { // span<int, 32>
>> if (file[0] != 0x12 && file[2] != 0x34)
>> return -1;
>> return file[n] + file[100];
>> }
>> int main() {
>> int tooSmall[]={1};
>> int fileBadHeader[64]={1,2,3,4,5,6,7,8,9};
>> int fileOkHeader[64]={0x12,0x34,3,4,5,6,7,8,9};
>>
>> //test1({tooSmall}, 100); // will assert
>> test1({fileBadHeader}, 100); // will not assert since header
>> check fails
>> //test1({fileOkHeader}, 100); // will assert
>> test2(std::span{fileBadHeader}.subspan<10, 32>(), 100); // will
>> not assert
>> //test2(std::span{fileOkHeader}.subspan<0, 32>(), 100); // will
>> assert
>> }
>>
>> By using arrays, we can catch `file[100]` along with the
>> problematic index `n` if the user opts into the non-literal bounds check.
>>
>
> Nothing prevents compilers from checking the uses of std::span and
> diagnosing provably out of bounds accesses.
>

And if we reach for a new hack in the type system every time there's a
problem, and so *don't* work on improving the ergonomics and safety of
std::span, then span will never get more safety benefits. We'll just need
to keep adding more special case hacks to the type system to try to make
arrays slightly less useless, one kludge at a time.

We have std::span already. Let's make improvements there, not by trying to
make arrays be both plain arrays and also slices of arrays.