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

From	Andrey Tarasevich <andreytarasevich@hotmail.com>
Newsgroups	comp.lang.c++
Subject	Re: Function pointer size (was Re: nullptr is not 0)
Date	2022-05-18 02:14 -0700
Organization	A noiseless patient Spider
Message-ID	<t62ddu$41b$1@dont-email.me> (permalink)
References	(1 earlier) <t5a7h3$od0$1@gioia.aioe.org> <t5buj0$od3$1@dont-email.me> <66geK.6522$lQq1.6208@fx43.iad> <t610ls$8r7$1@dont-email.me> <t61tn1$1gd6$1@gioia.aioe.org>

Show all headers | View raw

On 5/17/2022 9:46 PM, Juha Nieminen wrote:
> Vir Campestris <vir.campestris@invalid.invalid> wrote:
>> I just experimented. Yes, it's 16 bytes, while an ordinary function
>> pointer is only 8 (Linux 64, g++).
>>
>> But the top 8 bytes always seem to be zero.
>>
>> When are they anything else? Because if they aren't there's no reason
>> for the different size.
> 
> If you have a reference or pointer to an object of type class A, and a
> member pointer to a member function of class A, which is virtual, if
> the actual object pointed to is of some derived type B which has its
> own specialization of that virtual function, calling the function using
> the pointer will call the derived implementation not, the one in class A.
> 
> For this to be possible the pointer to member function needs additional
> data.

No, that's incorrect.

This has nothing to do with virtual functions. On the contrary, 
pointers-to-virtual-functions are "easy": regular virtual call mechanism 
itself is already required to incorporate all necessary mechanics to 
properly invoke virtual functions across the entire hierarchy of 
classes. This mechanics is already present in "regular" virtual calls 
(without involving any pointers-to-members).

Pointers-to-members simply piggyback on that already existing 
functionality. For which reason pointers-to-virtual-functions in a 
traditional implementation do not need to store any extra info.

I have already provided exhaustive explanation for when this extra info 
is really needed: specifically for calls aimed at non-virtual functions, 
which are forced to implement "virtual-like" behavior.

-- 
Best regards,
Andrey

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

csiph-web