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Re: TIL: C++17 sequences assignment the wrong way.

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On 2019-07-19 16:45:11 +00, Kaz Kylheku wrote:
> On 2019-07-19, Barry Fishman <barry@ecubist.org> wrote:
>>
>> On 2019-07-19 03:36:55 +00, Kaz Kylheku wrote:
>>> C++17 adds a sequence point to assignment. E1 = E2 requires E2 to be
>>> completely evaluated first, then E1.
>>>
>>> Wow, what a jaw-droppingly moronic moment in computing history.
>>>
>>> They had the right motivation to fix a flaw, at least in one small area,
>>> and establish an order where there is chaos.
>>>
>>> Then they drop the ball by making it backwards.
>>
>> int *p = an_array_of_ints;
>> for (int i = 0; i < 3; i++) {
>>   *p++ = *p + 1;
>> }
>
> In C++17, this will reliably calculate the prior value of *p + 1,
> store it in *p, and increment p.
>
> But under left-to-right, that can be done just fine:
>
>   *p = *p++ + 1;

Yes.  But to my Infix language sense, this is less clear, especially
where the right hand side is more complex.

Also, I think infix assignment (ignoring side effects) usually makes more sense
in a right to left evaluation order.

In gcc and g++, although I am not sure if legal generally in C++17:

 b = a = 3;

as opposed to Lisp, where one would do:

(setq a 3 b a)

Or explicit ordered:

(setq b (setq a 3))

Personally I just try to avoid side effects (and C/C++).

--
Barry Fishman

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Thread

TIL: C++17 sequences assignment the wrong way. Kaz Kylheku <847-115-0292@kylheku.com> - 2019-07-19 03:36 +0000
  Re: TIL: C++17 sequences assignment the wrong way. Barry Fishman <barry@ecubist.org> - 2019-07-19 10:14 -0400
    Re: TIL: C++17 sequences assignment the wrong way. Kaz Kylheku <847-115-0292@kylheku.com> - 2019-07-19 16:45 +0000
      Re: TIL: C++17 sequences assignment the wrong way. Barry Fishman <barry@ecubist.org> - 2019-07-19 15:55 -0400
  Re: TIL: C++17 sequences assignment the wrong way. Stefan Monnier <monnier@iro.umontreal.ca> - 2019-07-19 13:30 -0400
    Re: TIL: C++17 sequences assignment the wrong way. Kaz Kylheku <847-115-0292@kylheku.com> - 2019-07-19 18:44 +0000

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