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Re: `bool` in pointer arithmetic: when does the promotion occur, if ever?

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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Andrey Tarasevich <andreytarasevich@hotmail.com> writes:
>
>> Sound like a silly question (and it might just as well be), but
>> nevertheless:
>>
>>    int *a = <whatever>;
>>    bool b = true;
>>    a + b;
>>
>> How does the binary `+` work in this case?  Its operands are an `int *`
>> and a `bool`. `bool` is an integer type, so the requirements are met
>> and the expression is valid.  Naturally we expect that `bool` to be
>> promoted to an integer value of 1, and the rest is clear.
>>
>> But where does it say in the standard that `bool` should get promoted
>> in this context?  There is no global rule requiring an unconditional
>> promotion, i.e. each operator's specification mandates integral
>> promotions individually and explicitly.  And I don't see integral
>> promotions mentioned anywhere in [expr.add].
>>
>> It does mention usual arithmetic conversions (UAC), of course, and UAC
>> include integral promotions.  But UAC are only applicable when _both_
>> operands have enumeration or arithmetic type.  UAC are not applied when
>> one operand is a pointer.  (The definition of UAC does not accommodate
>> for such possibility.)
>>
>> So, how does pointer+bool addition work then?  When does `true` turn
>> into 1 here?
>
> Just a few comments..
>
> One, I think you are raising a good point.
>
> Two, my guess is that this issue reflects an oversight on the part
> of the C++ standards committee.  At the very least, assuming there
> is a provision in the C++ standard the gives this result, the
> reasoning needed is obscure and deserves a note.
>
> Three, an argument could be made (for C++20, n4860) that converting
> 'true' or 'false' to 1 or 0 is a consequence of the last sentence
> of section 7.3 paragraph 1:
>
>     A standard conversion sequence will be applied to an expression
>     if necessary to convert it to a required destination type.
>
> Granted, the reasoning is fraught with ambiguity, and that is never
> good.  But the 'if necessary' provision gives a lot of latitude.

I think this is an oversight in the standard, and one that would be
fairly straightforward to correct.

The C standard, under "Additive operators", says:

    If both operands have arithmetic type, the usual arithmetic
    conversions are performed on them.

The corresponding wording in the C++ standard says:

    The usual arithmetic conversions are performed for operands of
    arithmetic or enumeration type.

The description of the "usual arithmetic conversions" is similar in both
standards, and assumes that both operands are of arithmetic type.  I
can't think of any good reason why the "If both operands have arithmetic
type," wording *should* apply in C but not in C++.

It *should* say that if one operand is of pointer type, the other
operand should undergo the integral promotions (but *not* the usual
arithmetic conversions).  So if you add a pointer and a bool, the bool
should be converted to int (0 or 1).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

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`bool` in pointer arithmetic: when does the promotion occur, if ever? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-15 10:45 -0700
  Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? David Brown <david.brown@hesbynett.no> - 2022-08-15 20:09 +0200
    Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-15 11:29 -0700
      Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-15 11:35 -0700
      Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? David Brown <david.brown@hesbynett.no> - 2022-08-15 21:20 +0200
        Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? red floyd <no.spam.here@its.invalid> - 2022-08-15 13:49 -0700
        Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-15 13:59 -0700
          Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? David Brown <david.brown@hesbynett.no> - 2022-08-16 09:12 +0200
            Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-16 09:56 -0700
              Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-09-15 07:18 -0700
                Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Language Lawyer <language.lawyer@gmail.com> - 2022-09-19 03:28 -0700
                Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-10-02 12:37 -0700
  Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-15 14:36 -0700
    Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-15 14:57 -0700
  Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-08-16 21:13 +0200
  Re: `bool` in pointer arithmetic: when does the promotion occur, if ever? Language Lawyer <language.lawyer@gmail.com> - 2022-08-17 09:08 -0700

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