Should r.length of type ulong be supported on 32-bit systems?

[Andrei Alexandrescu via Digitalmars-d]

On 10/01/2016 05:30 AM, Lodovico Giaretta wrote:
> On Saturday, 1 October 2016 at 02:31:23 UTC, Andrei Alexandrescu wrote:
>> https://github.com/dlang/phobos/pull/4827 still allows that but
>> specifies that phobos algorithms are not required to. -- Andrei
>
> I don't have much experience, but IMHO the length should be restricted
> to be any of the built-in unsigned integral types (ubyte, ushort, uint,
> size_t, ulong). This way, all comparisons for equality or inequality are
> defined and correct, and the common type exists and is the one you'd
> expect. Also, every other operation on them works as expected, because
> smaller types are widened and the absence of signed types guarantees no
> strange results.

I've looked through phobos and sizes smaller than size_t present less 
risk. The issue is that e.g. code is liable to do:

auto len = r.length;

and the proceeds to use len tacitly assuming it's a size_t. If r.length 
were allowed to return e.g. ubyte, doing something like ++len may 
overflow a lot earlier than the code would expect.

I made a quick pass through Phobos looking for potential issues. I found 
a couple in replaceLast (uses arithmetic on indexes obtained with auto) 
but not much else (in fairness I stopped at std/file.d in whatever order 
"git grep" lists files).

So in all likelihood sizes smaller than size_t may be easier to make 
work. However, this seems to complicate definitions without a 
corresponding payoff. For example a natural consequence would be to have 
operator[] take the same type as the length. That would definitely cause 
problems because people pass index computations to operator[] such as 1, 
idx + 1, or $ - 1.

> The only problem that arises with this definition of length is that
> functions cannot take a length parameter as size_t, because that could
> truncate it.

I think Jonathan is right: sizes longer than size_t have numerous issues 
- in fact enough that the limiting decision was made (by a group of 
contributors and approved by us) to force iota.length to return size_t.


Andrei