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Re: Strange code-generation

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

> On 18.09.2019 18:47, Tim Rentsch wrote:
>
>> Paavo Helde <myfirstname@osa.pri.ee> writes:
>>
>>> 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.
>
> By that logic the C++ standard could also prescribe how the SQL
> language or the invisible unicorn in my garage must behave, in the
> case someone happens to incorporate them into their C++
> implementation.

These analogies are pretty much meaningless, because the terms
are not in the universe of discourse for the C++ language.  The
term "reference" is in the C++ universe of discourse, and every
C++ programmer understands what "null reference" means, in that
context, by analogy with "null pointer".  In fact the term "null
reference" appears in the C++ standard, as you point out.

> More to the point, the standard contains the following verbiage:
> "A reference shall be initialized to refer to a valid object or
> function.  [ Note: in particular, a null reference cannot exist in a
> well-defined program, because the only way to create such a reference
> would be to bind it to the ?object?  obtained by indirection through a
> null pointer, which causes undefined behavior. ]"

Saying "a null reference cannot exist in a well-defined program"
is a circular statement;  it has no information content, because
of what is meant by well-defined.  Also, just because a construct
has undefined behavior doesn't mean the result can't exist.
Obviously it can exist, because the implementation is free to
define the meaning.

> Without contradicting itself, the standard cannot say in one place "A
> reference shall be initialized to refer to a valid object" and
> "behavior is undefined" and then go on and discuss in another place
> what happens if a program violates this "shall", thus defining the
> behavior.

There is no contradiction.  What would (hypothetically) be being
defined is not declaring/initializing a null reference, but using
a null reference.  Do you not understand the distinction?


(Sorry to be so long in responding, life has been unusually
chaotic of late.)

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Thread

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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