Is there a guarantee that a cast of a pointer does not change its representation and size?
 

-----Ursprüngliche Nachricht-----
Von: Gašper Ažman via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: Mo 27.01.2025 02:34
Betreff: Re: [std-proposals] Make all data pointers intercompatible
An: std-proposals@lists.isocpp.org;
CC: Gašper Ažman <gasper.azman@gmail.com>;

You know all pointers are the same size because they're all static_cast-able to void* and back.

On Mon, Jan 27, 2025 at 9:12 AM Jeremy Rifkin via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

> It's possible if the compiler hides it from the programmer. For instance if CHAR_BIT==8, and if the CPU instructions deal with 16-Bit operands, then you can have:
>
>    CHAR_BIT == 8
>    sizeof(int*) == 2
>    sizeof(char*) == 3

 

All the language cares about is that you can in fact address every byte of memory. From a language perspective, if CHAR_BIT == 8 then the smallest addressable unit of memory is 8 bits irrespective of hardware details.

 

Between https://eel.is/c++draft/basic.memobj#intro.memory-1 and https://eel.is/c++draft/basic.compound#7 and the fact struct Frog; Frog* ptr1 = nullptr; Frog* foo(); Frog* ptr2 = foo(); has to work I think it's sufficiently covered by the standard that all pointers to non-fundamental objects are the same size (or at least compatible in some way to make everything here work). Even if you had a system where sizeof(char*) == 3 and sizeof(int*) == 2 you would know sizeof(void*) >= 3.

 

Cheers,

Jeremy

 

On Sun, Jan 26, 2025 at 5:42 PM Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

On Sun, Jan 26, 2025 at 11:17 PM Jeremy Rifkin wrote:
>
> > For example on a system where CHAR_BIT==8 but the smallest addressable unit of memory is 16 bits
>
> That's not possible https://eel.is/c++draft/basic.memobj#intro.memory-1


It's possible if the compiler hides it from the programmer. For
instance if CHAR_BIT==8, and if the CPU instructions deal with 16-Bit
operands, then you can have:

    CHAR_BIT == 8
    sizeof(int*) == 2
    sizeof(char*) == 3

An int* is a 16-Bit number representing a memory address.
A char* is a 16-Bit number representing a memory address, with an
extra byte to indicate the offset.
--
Std-Proposals mailing list
Std-Proposals@lists.isocpp.org
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

--
Std-Proposals mailing list
Std-Proposals@lists.isocpp.org
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

-- 
 Std-Proposals mailing list
 Std-Proposals@lists.isocpp.org
 https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
 

--
Std-Proposals mailing list
Std-Proposals@lists.isocpp.org
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals