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Groups > comp.lang.c++ > #71019

Re: Union type punning in C++

From boltar@nowhere.org
Newsgroups comp.lang.c++
Subject Re: Union type punning in C++
Date 2020-01-03 14:55 +0000
Organization Aioe.org NNTP Server
Message-ID <qunkkf$i7m$1@gioia.aioe.org> (permalink)
References <kl6PF.88068$CSS.31003@fx01.dcg> <48ade352-879d-4c02-849b-82dad161c3d6@googlegroups.com> <dd6814cc-7b64-463b-8a86-949c78273546@googlegroups.com>

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On Fri, 3 Jan 2020 02:21:28 -0800 (PST)
=?UTF-8?B?w5bDtiBUaWli?= <ootiib@hot.ee> wrote:
>On Friday, 3 January 2020 11:55:29 UTC+2, bol...@nowhere.org  wrote:
>> On Thu, 2 Jan 2020 10:26:35 -0800 (PST)
>> =?UTF-8?B?w5bDtiBUaWli?= <ootiib@hot.ee> wrote:
>> >On Thursday, 2 January 2020 19:22:52 UTC+2, bol...@nowhere.org  wrote:
>> >> On Thu, 2 Jan 2020 04:52:42 -0800 (PST)
>> >> =?UTF-8?B?w5bDtiBUaWli?= <ootiib@hot.ee> wrote:
>> >> >On Thursday, 2 January 2020 13:49:39 UTC+2, Bo Persson  wrote:
>> >> >> 
>> >> >> The C++ committee seems unwilling to change the union rules and so 
>> >> >> instead adds a bit_cast function to package the memcpy usage with a 
>> >> >> slightly better name:
>> >> >> 
>> >> >> template< class To, class From >
>> >> >> constexpr To bit_cast(const From& from) noexcept;
>> >> >> 
>> >> >> https://en.cppreference.com/w/cpp/numeric/bit_cast
>> >> >
>> >> >Thanks, I didn't somehow even notice that. So we can throw away
>> >> >our own type punning wrappers around memcpy in C++20.
>> >> 
>> >> You realise using a union (or a cast) in theory uses very few CPU cycles
>to
>> >do 
>> >> type punning, whereas memcpy has the overhead of a function call + loop.
>Its
>> >> an extremely inefficient way to do it if you don't need the new type to
>be in
>> >
>> >> a seperate variable.
>> >
>> >That is the typical behavior of gibberish team. In one post it is
>> >"de facto" in other "in theory". I wrote:
>> >
>> >Experiments of mine and others have shown that usage of memcpy
>> 
>> Lets see your test code then.
>
>You first. You demonstrated lack of knowledge with the "overhead of a

You made the claim so either back it up or fuck off back under your bridge.

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Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-01 19:24 +0000
  Re: Union type punning in C++ boltar@nowhere.org - 2020-01-02 09:36 +0000
  Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 03:34 -0800
    Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-02 12:49 +0100
      Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 04:52 -0800
        Re: Union type punning in C++ boltar@nowhere.org - 2020-01-02 17:22 +0000
          Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 10:26 -0800
            Re: Union type punning in C++ Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2020-01-02 21:54 +0000
              Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:49 +0000
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 17:11 +0100
                Re: Union type punning in C++ Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2020-01-03 17:55 +0000
            Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 09:55 +0000
              Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 11:20 +0100
              Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 02:21 -0800
                Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:55 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 16:04 +0100
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:45 -0800
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:26 +0100
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 16:55 -0800
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 01:02 +0000
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 17:06 -0800
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 07:34 -0800
                Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 16:12 +0000
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 08:49 -0800
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 19:28 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 18:54 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:25 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:28 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:41 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:52 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:08 +0100
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:34 +0100
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:43 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:20 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:25 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:25 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:25 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:27 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:36 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:41 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:49 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 09:01 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 10:02 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 09:19 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 10:38 +0100
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-04 12:45 +0100
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 12:00 +0000
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 13:03 +0100
                Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 14:22 +0100
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:46 -0800
                Re: Union type punning in C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-01-03 10:32 -0800
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 10:43 -0800
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 18:55 +0000
            Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 09:40 -0800
              Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-08 13:23 -0800
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 22:41 -0800
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 22:52 -0800
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-09 12:59 -0800
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-09 13:32 -0800
                Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-10 12:28 +0000
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-10 14:02 +0100
                Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-10 16:21 +0000
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-10 19:18 +0100
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-10 15:41 -0800
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 12:01 +0100
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-11 07:05 -0800
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 23:53 +0100
                Re: Union type punning in C++ Mr Flibble <flibble@i42.removethisbit.co.uk> - 2020-01-12 01:41 +0000
                Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-11 19:00 -0800
                Re: Union type punning in C++ Mr Flibble <flibble@i42.removethisbit.co.uk> - 2020-01-13 17:32 +0000
                Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 11:53 +0000
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 13:20 +0100
                Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 12:33 +0000
                Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 14:28 +0100
                Re: Union type punning in C++ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-11 10:48 +0000
                Re: Union type punning in C++ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-11 11:05 +0000
                Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 11:55 +0000
          Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-02 21:12 +0100
            Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:46 +0000
              Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-03 20:01 +0100
          Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-02 23:23 +0100
  Re: Union type punning in C++ peter koch <peter.koch.larsen@gmail.com> - 2020-01-02 12:39 -0800
  Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-02 15:39 -0800
    Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 16:58 -0800
      Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-02 17:55 -0800
        Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 00:40 -0800
          Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 14:04 +0000
            Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 15:24 +0100
              Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:34 -0800
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 22:59 +0000
                Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-04 00:11 +0000
                Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:21 +0000
                Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-04 00:26 +0000
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-04 16:51 -0800
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 18:47 -0800
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 19:07 -0800
            Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 06:47 -0800
              Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 14:57 +0000
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 07:38 -0800
                Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 17:06 +0000
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 11:05 -0800
          Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:38 -0800
            Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 22:37 -0800
      Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 07:10 -0800
        Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-11 10:23 -0800
          Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 09:01 -0800
            Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 11:03 -0800
              Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-13 12:48 -0800
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 14:15 -0800
                Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-13 21:47 -0500
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 23:44 -0800
                Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-14 08:26 -0500
              Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:30 -0800
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-14 09:11 -0800
                Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-14 21:52 -0500
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-14 23:15 -0800
                Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 02:03 -0800
    Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 07:28 -0800
      Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-11 20:40 -0800
        Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-12 20:16 -0500
          Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-12 17:28 -0800
            Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-12 21:53 -0500
              Re: Union type punning in C++ Manfred <noname@add.invalid> - 2020-01-13 18:50 +0100
              Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:21 -0800
            Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 08:57 -0800
              Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:27 -0800
                Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:05 -0800
                Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-14 20:58 -0800
                Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 03:47 -0800
            Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-15 01:22 -0800
              Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-15 08:04 -0500
                Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-15 09:38 -0800
                Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-15 11:01 -0800
        Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 08:37 -0800
          Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:29 -0800

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