Re: why use static_cast ?

From	Bo Persson <bo@bo-persson.se>
Newsgroups	comp.lang.c++
Subject	Re: why use static_cast ?
Date	2022-10-22 18:31 +0200
Message-ID	<jrik6lF5araU2@mid.individual.net> (permalink)
References	<tiqa5e$8aju$1@dont-email.me> <Vqc4L.432362$wLZ8.363475@fx18.iad> <tis3if$ct2g$1@dont-email.me> <tit51u$1lna$1@gioia.aioe.org> <tiuq71$1380$1@gioia.aioe.org>

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On 2022-10-21 at 20:59, Lynn McGuire wrote:
> On 10/20/2022 10:52 PM, Manfred wrote:
>> On 10/20/2022 8:21 PM, Lynn McGuire wrote:
>>> On 10/20/2022 8:55 AM, Scott Lurndal wrote:
>>>> Lynn McGuire <lynnmcguire5@gmail.com> writes:
>>>>> I put the following cast into my code and one of my programmers 
>>>>> wants me
>>>>> to use static cast.  Why ?
>>>>>     https://en.cppreference.com/w/cpp/language/static_cast
>>>>>
>>>>>     longint nfac [8];
>>>>>     fem::str <8> * nfac_str = (fem::str <8> *) nfac;
>>>>>
>>>>> fem::str <8> is an 8 character object that acts like a Fortran
>>>>> character*8.  longint is a long long.
>>>>>
>>>>
>>>> Young C++ programmers don't understand C, so they think
>>>> that static_cast<fem::str<8>*> is more "readable". Heh.
>>>
>>> The programmer in question is 39 and has been writing C and C++ since 
>>> he was 13 or 14.
>>>
>>> His actual comment was that static_cast's are easier to grep which I 
>>> find hard to believe.
>>
>> If by grep you mean grep(1), then yes, static_cast is definitely 
>> easier to grep than a C-style cast.
>> However, the advantage of reinterpret_cast (static_cast wouldn't work) 
>> is not that it is more readable. It is that it hurts the eye.
>>
>> Depending on which compiler you use, I'd verify that what you intend 
>> to do actually works. As far as the standard is concerned, at least 
>> one necessary condition is that fem::str<8> is effectively an unsigned 
>> char[8].
>> Otherwise it's UB unless your compiler says otherwise (although it 
>> probably Just Works on any of them, in practice.)
>>
>>>
>>> Thanks,
>>> Lynn
> 
> I am using Visual Studio 2015 on Windows 10 Pro x64.
> 
> fem::str<8> is an array of unsigned char [8] that I pass around the 
> place to hold a Fortran string of given length that is blank filled 
> rather than null terminated.
>     https://cci.lbl.gov/fable/sources/fable/fem/str.hpp
> 
> And yes, I will be able to tell rather quickly if the cast is working 
> properly or not.  I am minimizing usage though as I prefer char * and 
> std::string.
> 

If it is always 8 bytes, I would use memcpy between two buffers and skip 
the pointer indirection.

The memcpy based implementation of C++20 bit_cast, as shown here

https://en.cppreference.com/w/cpp/numeric/bit_cast

is known to inline and optimize into

mov rax, From

for 64-bit values on common 64-bit compilers.

Thread

why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-19 21:01 -0500
  Re: why use static_cast ? "daniel...@gmail.com" <danielaparker@gmail.com> - 2022-10-19 19:40 -0700
    Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-20 00:21 -0500
      Re: why use static_cast ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-10-19 23:21 -0700
        Re: why use static_cast ? Juha Nieminen <nospam@thanks.invalid> - 2022-10-20 07:19 +0000
          Re: why use static_cast ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-10-20 00:21 -0700
      Re: why use static_cast ? Paul N <gw7rib@aol.com> - 2022-10-20 04:23 -0700
        Re: why use static_cast ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-10-20 14:14 +0100
        Re: why use static_cast ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-10-20 09:17 -0700
          Re: why use static_cast ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-10-20 09:52 -0700
            Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-15 17:10 -0800
              Re: why use static_cast ? Juha Nieminen <nospam@thanks.invalid> - 2022-11-16 09:26 +0000
                Re: why use static_cast ? David Brown <david.brown@hesbynett.no> - 2022-11-16 14:36 +0100
                Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-20 05:54 -0800
      Re: why use static_cast ? "daniel...@gmail.com" <danielaparker@gmail.com> - 2022-10-20 08:11 -0700
  Re: why use static_cast ? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-10-19 20:09 -0700
  Re: why use static_cast ? Juha Nieminen <nospam@thanks.invalid> - 2022-10-20 05:44 +0000
    Re: why use static_cast ? Gawr Gura <gawrgura@mail.hololive.com> - 2022-10-20 14:50 +0000
      Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 17:53 +0300
  Re: why use static_cast ? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-20 08:52 +0200
    Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 17:52 +0300
  Re: why use static_cast ? scott@slp53.sl.home (Scott Lurndal) - 2022-10-20 13:55 +0000
    Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 17:55 +0300
      Re: why use static_cast ? scott@slp53.sl.home (Scott Lurndal) - 2022-10-20 15:01 +0000
        Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 18:32 +0300
          Re: why use static_cast ? scott@slp53.sl.home (Scott Lurndal) - 2022-10-20 16:07 +0000
            Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 20:14 +0300
              Re: why use static_cast ? scott@slp53.sl.home (Scott Lurndal) - 2022-10-20 17:30 +0000
                Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-20 20:41 +0300
              Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-21 14:05 -0500
                Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-22 00:09 +0300
                Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-22 08:47 +0300
                Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-22 15:03 -0500
                Re: why use static_cast ? Muttley@dastardlyhq.com - 2022-10-23 07:35 +0000
                Re: why use static_cast ? JiiPee <kerrttuPoistaTama11@gmail.com> - 2022-10-23 15:28 +0300
                Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-23 14:13 -0500
                Re: why use static_cast ? Manfred <noname@add.invalid> - 2022-10-23 20:27 +0200
                Re: why use static_cast ? Michael S <already5chosen@yahoo.com> - 2022-10-23 15:04 -0700
                Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-23 23:14 -0500
          Re: why use static_cast ? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-21 17:21 +0200
      Re: why use static_cast ? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-21 17:24 +0200
    Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-20 13:21 -0500
      Re: why use static_cast ? Manfred <noname@add.invalid> - 2022-10-21 05:52 +0200
        Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-21 13:59 -0500
          Re: why use static_cast ? Bo Persson <bo@bo-persson.se> - 2022-10-22 18:31 +0200
            Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-22 15:05 -0500
            Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-22 17:34 -0500
        Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-02 17:54 -0700
          Re: why use static_cast ? Öö Tiib <ootiib@hot.ee> - 2022-11-03 08:51 -0700
            Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-15 16:19 -0800
              Re: why use static_cast ? Öö Tiib <ootiib@hot.ee> - 2022-11-15 23:02 -0800
                Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-20 05:59 -0800
                Re: why use static_cast ? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-20 15:05 +0100
                Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-20 07:22 -0800
                Re: why use static_cast ? Öö Tiib <ootiib@hot.ee> - 2022-11-20 08:40 -0800
                Re: why use static_cast ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-28 20:43 -0800
    Re: why use static_cast ? Juha Nieminen <nospam@thanks.invalid> - 2022-11-04 07:27 +0000
      Re: why use static_cast ? scott@slp53.sl.home (Scott Lurndal) - 2022-11-04 13:47 +0000
        Re: why use static_cast ? Vir Campestris <vir.campestris@invalid.invalid> - 2022-11-05 21:34 +0000
        Re: why use static_cast ? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-11-06 03:25 -0500
        Re: why use static_cast ? Paavo Helde <eesnimi@osa.pri.ee> - 2022-11-06 17:34 +0200
          Re: why use static_cast ? Juha Nieminen <nospam@thanks.invalid> - 2022-11-07 07:13 +0000
      Re: why use static_cast ? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-04 09:09 -0700
  Re: why use static_cast ? Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-20 13:21 -0500

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