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

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On 5/19/2022 8:19 AM, Alf P. Steinbach wrote:
> On 19 May 2022 16:53, Juha Nieminen wrote:
>> Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
>>>> 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).
>>
>> I'm not sure that's correct.
>>
>> When you have a pointer-to-member, in the location of the call the 
>> compiler
>> doesn't know *which* member function it's pointing to. It only knows its
>> signature, not its name. The class may have several differently-named
>> member fuctions with the same signature. I don't think the compiler can
>> even know (from the source code alone) if the pointed-to function is
>> virtual or not.
>>
>> Thus, the compiler needs to create code that somehow figures out if
>> that member function is virtual, and which function it is, and then
>> use the vtable as normal to jump to the actual most-derived 
>> implementation.
>> I don't think it can do this without the extra data in the pointer.
>> (Well, I suppose it theoretically could, but that might require some
>> searching).
>>
>> Please correct me if I'm wrong. (Honestly. This isn't sarcasm.)
> 
> Just an example of what you're saying:
> 
> 
> #include <stdio.h>
> 
> struct A
> {
>      void foo() const { printf( "A::foo\n" ); }
>      virtual void bar() const { printf( "A::bar\n" ); }
> };
> 
> struct B: A
> {
>      void bar() const override { printf( "B::bar\n" ); }
> };
> 
> auto main() -> int
> {
>      auto        f = &A::foo;
>      const A&    o = B();
> 
>      f = &A::bar;
>      (o.*f)();    // "B::bar"
> }
> 

Not sure what this is supposed to illustrate. That 
pointers-to-member-functions implement "late binding" at the point of 
the call? Yes, that's true and that's banal.

However, this still does not in any way mean that 
pointers-to-member-function has to distinguish between pointing to a 
"regular" function and pointing to a virtual function.

-- 
Best regards,
Andrey

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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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