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

[charleyb123 .]

On Fri, Aug 26, 2016 at 9:54 AM, Tobias Hunger <tobias.hunger at gmail.com>
wrote:

> On Fri, Aug 26, 2016 at 5:08 PM, 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.
>
> 0 is a valid address. The OS usually makes sure the first page is not
> mapped, thus triggering a segfault if you try to dereference 0 (or in
> fact the first 4k or so, depending on page size).
>
> Linux had a security issue once where an attacker could map code into
> the 0-page, triggering that code by accessing a
> null-(function-)pointer.
>
> > 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.
>
> I highly doubt that: 0 is a valid address, so the compiler can not
> just go ahead and map it elsewhere as it pleases.


Good points by Tobias, but I want to also elaborate to my original thought:

On these new architectures, all bit-patterns for the native "address-type"
are *user-process-addressable* as valid addresses (although an operating
system may reserve some ranges, or impose OS-specific address partitioning
restrictions, as Tobias explains).

I know at present we can "cheat" with stuff like 64-bit pointers that
merely rely upon 48-bit addressing for today's architectures, but that's an
anachronism.

--charley
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