Re: Function pointer size (was Re: nullptr is not 0)

From	Juha Nieminen <nospam@thanks.invalid>
Newsgroups	comp.lang.c++
Subject	Re: Function pointer size (was Re: nullptr is not 0)
Date	2022-05-20 07:15 +0000
Organization	Aioe.org NNTP Server
Message-ID	<t67f63$1lhn$1@gioia.aioe.org> (permalink)
References	(4 earlier) <t610ls$8r7$1@dont-email.me> <t61tn1$1gd6$1@gioia.aioe.org> <t62ddu$41b$1@dont-email.me> <t65lls$1nug$1@gioia.aioe.org> <t65rkf$5q3$1@dont-email.me>

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Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
> * In GCC on x86 (x64) the lower 8-byte word of a pointer that happens to 
> point a regular function is simply a pointer. At the same time a pointer 
> that happens to point to a virtual function represents an offset into 
> virtual table +1. These are two completely different representations.

I think I get now what you are saying.

A pointer-to-virtual-member doesn't have to point directly to the function
itself (as normal function pointers do). Instead, it points to a memory
location that has the actual memory address of the function (in other
words, the pointer points to a position in a vtable).

And the code generated by the compiler can make that distinction by
looking at the least-significant bit of the pointer (which can be used
to indicate the "type" of the pointer because functions always start
at even memory addresses, at least in x86, and thus this least-significant
bit is free space that can be used to store this flag.)

Thread

nullptr is not 0 Sam <sam@email-scan.com> - 2022-05-08 09:45 -0400
  Re: nullptr is not 0 "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-08 16:21 +0200
  Re: nullptr is not 0 Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-08 10:00 -0700
  Re: nullptr is not 0 Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-09 06:38 +0200
  Re: nullptr is not 0 Juha Nieminen <nospam@thanks.invalid> - 2022-05-09 05:06 +0000
    Re: nullptr is not 0 Vir Campestris <vir.campestris@invalid.invalid> - 2022-05-09 21:46 +0100
      Re: nullptr is not 0 Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-09 13:57 -0700
        Re: nullptr is not 0 Juha Nieminen <nospam@thanks.invalid> - 2022-05-10 05:04 +0000
          Re: nullptr is not 0 Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-09 22:33 -0700
      Re: nullptr is not 0 scott@slp53.sl.home (Scott Lurndal) - 2022-05-09 21:56 +0000
        Function pointer size (was Re: nullptr is not 0) Vir Campestris <vir.campestris@invalid.invalid> - 2022-05-17 21:30 +0100
          Re: Function pointer size (was Re: nullptr is not 0) scott@slp53.sl.home (Scott Lurndal) - 2022-05-17 21:00 +0000
            Re: Function pointer size (was Re: nullptr is not 0) Vir Campestris <vir.campestris@invalid.invalid> - 2022-05-19 21:52 +0100
            Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 14:08 -0700
          Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-17 17:05 -0700
          Re: Function pointer size (was Re: nullptr is not 0) Juha Nieminen <nospam@thanks.invalid> - 2022-05-18 04:46 +0000
            Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-18 02:14 -0700
              Re: Function pointer size (was Re: nullptr is not 0) Juha Nieminen <nospam@thanks.invalid> - 2022-05-19 14:53 +0000
                Re: Function pointer size (was Re: nullptr is not 0) "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-05-19 17:19 +0200
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 10:14 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Richard Damon <Richard@Damon-Family.org> - 2022-05-19 13:45 -0400
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 11:31 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Juha Nieminen <nospam@thanks.invalid> - 2022-05-20 06:58 +0000
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-20 00:11 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Richard Damon <Richard@Damon-Family.org> - 2022-05-19 12:27 -0400
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 09:47 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Richard Damon <Richard@Damon-Family.org> - 2022-05-19 13:36 -0400
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 11:24 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Richard Damon <Richard@Damon-Family.org> - 2022-05-19 21:32 -0400
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 20:28 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Richard Damon <Richard@Damon-Family.org> - 2022-05-20 10:57 -0400
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 09:35 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-19 10:10 -0700
                Re: Function pointer size (was Re: nullptr is not 0) Juha Nieminen <nospam@thanks.invalid> - 2022-05-20 07:15 +0000
          Re: Function pointer size (was Re: nullptr is not 0) "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-18 11:50 -0700
            Re: Function pointer size (was Re: nullptr is not 0) Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-05-18 15:46 -0700
              Re: Function pointer size (was Re: nullptr is not 0) "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-19 20:52 -0700

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