[Qt-creator] Proposal: Adding std::(experimental::)optional

[charleyb123 .]

On Fri, Aug 26, 2016 at 9:08 AM, Christian Kandeler <
christian.kandeler at qt.io> wrote:

> On 08/26/2016 04:49 PM, charleyb123 . wrote:
> > An aside:  We are currently steeped in the tradition that "zero" is not
> > a legal address, but that is (quickly) becoming not-true for
> > process-containers, system-emulators and VMs, and new hardware
> > (including new CPUs).
>
> That has nothing to do with the null pointer in C(++). It never refers
> to a valid address, that's how the language is defined. If zero is a
> valid address in some environment, then it's the compiler's job to
> ensure that the null pointer maps to something else at runtime.
> That is also the reason that the following code is wrong:
>     struct s { void *p; } s;
>     memset(&s, 0, sizeof s); // s.p might not be a null pointer at runtime
>

Agree with what you're saying, but I'm highlighting a real-world ambiguity
as related to APIs, and as related to the compiler also.

Given a pointer on an architecture where *all* bit-patterns yield a valid
address, it is not possible for a compiler to do what you propose.

Possible work-arounds are to reserve a "well-known" address to imply a
"null-object-instance", but that's an artifact not required by the
architecture (but might be required by C(++) if we want to continue the
"null" concept.)

--charley
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