Re: Dereferencing and returning by reference!

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Thu, 16 May 2013 05:52:42 -0700 (PDT)
On 2013-05-15 23:21, wrote:

On Wednesday, May 15, 2013 10:00:02 AM UTC-4, Daniel Kr?gler wrote:

The last line of:

Has: We agreed that the approach in the standard seems okay: p =
0;; is not inherently an error. An lvalue-to-rvalue conversion
would give it undefined behavior.

I believe the intent is that, as return of '*obj' doesn't involve
an 'lvalue to rvalue' conversion there is no problem at this

I agree that the mentioned issue is related to this problem. The
most recent issue state can be found here:

Note that the current resolution tendency (the issue is still
drafting) seems to indicate that only references to a null object
or to a one past the last element of an array would be feasible,
not a reference to something arbitrary would be supported.

What is the point of endorsing binding a reference to a "null
object" or one past the end of an array, even if you forbid binding
them to the target of a garbage pointer value?

First, I need to correct my initial judgement a bit (thanks to Richard
Smith for pointing that out to me): CWG 232 still doesn't make binding
*references* to "null objects" valid, it only allows to form empty
glvalues in some contexts. The difference is that

    A* p1 = nullptr;
    A* p2 = &*p1;

would become valid, but still attempting to form

    A& r2 = *p1;

would be undefined behaviour. Supporting this idiom - especially for
arrays in the form of

int a[10];
int* pa = &a[10];

- would just be consistent with what C99 allows (but not C89).

It makes sense to add in this context that there exists indeed an
issue that follows more the direction I was describing when
incorrectly referring to CWG 232, which is:

Again, this is still drafting, it needs further adaption in the
context of constexpr, and any use of such a reference would still be
undefined behavior as it occurred in the revised example I mentioned
in my previous posting.

Up until I read this post, I assumed that the intent of (C++)
reference variables was that they would always be expected to be
bound to something you could actually reference -- that it was
undefined behavior and hence a no-no to do otherwise. This comment
suggests that the C++ is endorsing having un-referenceable

This is IMHO a Bad Thing(tm); Wil Evers' post gives one example of

Or am I missing something?

Neither you nor Wil are missing something, there is currently no
intended support for null references by the core language group. I
apologize for my misleading original reply.

HTH & Greetings from Bremen,

Daniel Kr?gler

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