There was a proposal to change this sent to WG14 already, with motivated
reasoning due to in-the-wild code using them wrong:

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2541.pdf

The problem is that the wording was extremely intentional for C. People use
the int-based return type to add additional stored information into the
return value, such as classification bits for the given input character
(packed as bit flags in the return value). It was meant to make mixed
table-based, macro-based implementations be fast, back in the day when
compilers were terrible. (And for C, given the wide berth of quality of
implementations, still are too terrible to optimize properly.) Even if you
could overcome ABI arguments ("bool is returned in different registers from
int"), you would need to tell all the implementations using the
implementation-definedness non-zero return value to stop doing that.

While everyone in the room evaluating the proposal understood, ultimately
for many of the stability reasons the proposal was rejected. You'd have
better luck designing new, multi-code-unit encoding (e.g. UTF-8) compatible
character classification functions for C and C++ to get the ball rolling,
killing 2 birds with one stone (can handle variable-encoding data, and have
better return values).

Sincerely,
JeanHeyd

On Mon, Oct 17, 2022 at 12:59 AM Thiago Macieira via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Sunday, 16 October 2022 03:42:33 PDT blacktea hamburger via
> Std-Proposals
> wrote:
> > Character classification functions in C return int, but C++ has built-in
> > bool type, so they should return bool.
> >
> > And they are usually used like:
> >
> > if (std::isdigit(c))
> > { ... }
> >
> > So it basically won't break existing code.
>
> Those functions are inherited from C and come from a C header, even if it
> has
> a C++ name (cctype). You need to propose this to WG14 first.
>
> What possible gain would we have for this change? What is that legacy int
> hurting?
>
> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Software Architect - Intel DCAI Cloud Engineering
>
>
>
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