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Re: Strange code-generation

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On 16.09.2019 15:53, Bonita Montero wrote:
> I just experimented a bit with downcasting to a derived class.
> So here's the code:
>
> struct A
> {
>      int a, b, c;
> };
>
> struct B
> {
>      int d, e, f;
> };
>
> struct D : public A, public B
> {
>      int i, j, k;
> };
>
> D *f( B &b )
> {
>      return &static_cast<D &>(b);
> }
>
> This is what you will expect ... with g++:
>
>      leaq    -12(%rdi), %rax
>      ret
>
> This is what makes MSVC from the above code
>
>      xor     edx, edx
>      lea     rax, QWORD PTR [rcx-12]
>      test    rcx, rcx
>      cmove   rax, rdx
>      ret     0
>
> So MSVC keeps a nullpointer when the input was also a nullpointer.
> Is there any requirement of the standard that mandates this behaviour?
> I think it would be even stupid to include that into the standard.

In standard C++ one cannot legally construct a "null reference", so the 
standard cannot mandate any behavior regarding them. The special 
handling of null only applies to the pointer form of static_cast<>.

However, an implementation may define behavior for things which are UB 
by the standard. MSVC supports things like this==nullptr (see e.g. 
CWnd::GetSafeHwnd() function), so maybe it supports "null references" as 
well, possibly for backward compatibility with their own old code.

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Strange code-generation Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-16 14:53 +0200
  Re: Strange code-generation Paavo Helde <myfirstname@osa.pri.ee> - 2019-09-16 16:59 +0300
    Re: Strange code-generation Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-16 21:08 +0200
    Re: Strange code-generation Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-18 08:47 -0700
      Re: Strange code-generation Paavo Helde <myfirstname@osa.pri.ee> - 2019-09-18 21:06 +0300
        Re: Strange code-generation Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-09 05:52 -0800
        Re: Strange code-generation "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-11-09 19:15 +0100
  Re: Strange code-generation Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-19 19:05 +0200
    Re: Strange code-generation David Brown <david.brown@hesbynett.no> - 2019-09-19 19:34 +0200
      Re: Strange code-generation Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-19 20:05 +0200
        Re: Strange code-generation Paavo Helde <myfirstname@osa.pri.ee> - 2019-09-19 22:00 +0300
        Re: Strange code-generation David Brown <david.brown@hesbynett.no> - 2019-09-20 08:21 +0200

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