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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Strange code-generation
Date: Wed, 18 Sep 2019 08:47:52 -0700
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Xref: csiph.com comp.lang.c++:68506

Paavo Helde <myfirstname@osa.pri.ee> writes:

> 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.  [...]

Sure it could.  The Standard doesn't say that they can't exist,
and even if they couldn't exist the Standard could still specify
a behavior for how they must be treated.  And it might even be
useful to give such a specification, for implementations that
choose to provide null references by means of an extension.