> There is only one pointer value that is guaranteed to not point at
any object, and that's nullptr.
Sorry, not so.

It might not be an allocated object but 0 (zero) can be a valid address.
On some platforms it is the reset/interrupt vector.

On Fri, Jul 25, 2025 at 5:03 AM Andrey Semashev via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 25 Jul 2025 14:17, Frederick Virchanza Gotham via Std-Proposals wrote:
> > I'm writing some code at the moment, and the destructor looks like this:
> >
> > ~Governor(void) noexcept
> > {
> > try
> > {
> > delete this->p;
> > }
> > catch(...){}
> >
> > this->p = (void*)0xdeadbeef;
> > }
> >
> > I've set the pointer to 0xdeadbeef because I want to make sure that
> > there's nowhere else in the code that this particular object gets
> > deleted. If another part of the code goes to delete the object a
> > second time, it will segfault on 0xdeadbeef -- which is exactly what I
> > want to happen because I want the debugger to catch it.
>
> And what if 0xdeadbeef is a valid pointer?
>
> Even if it's not, you're not guaranteed to get a crash either. You may
> get a corrupted heap and continue running with garbage data in your
> memory, and undefined outcome.
>
> There is only one pointer value that is guaranteed to not point at any
> object, and that's nullptr. If you want to add another one, that would
> have to go beyond just defining a new constant. The whole runtime
> (including but not limited to the memory allocator) would have to be
> updated with support for this new constant.
>
> You can also create "invalid" pointer values yourself, which are "safe"
> to a certain degree. For example:
>
> void* make_invalid_ptr()
> {
> // Mark it const so that it is hopefully placed in a read-only
> // memory region, which would prevent modifications by the
> // memory allocator. Also make it large enough so that the
> // allocator doesn't corrupt neighbouring data.
> static const unsigned char dummy[PAGE_SIZE] = {};
> return dummy + PAGE_SIZE / 2;
> }
>
> --
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