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Subject: Re: reading values as bytes without memcpu, from enum unsigned char?
From: Jason McKesson (jmckesson_at_[hidden])
Date: 2020-08-07 12:03:42


On Fri, Aug 7, 2020 at 11:43 AM <language.lawyer_at_[hidden]> wrote:
>
> On 07/08/2020 18:16, Jason McKesson via Std-Discussion wrote:
> > On Fri, Aug 7, 2020 at 10:06 AM <language.lawyer_at_[hidden]> wrote:
> >>
> >> On 07/08/2020 16:51, Jason McKesson via Std-Discussion wrote:
> >>> The cast is legal. Accessing the `unsigned char` at that address is
> >>> legal.
> >>
> >> You can't access `unsigned char` objects, `reinterpret_cast<unsigned char*>(buf)` points to the object of `mybyte` type.
> >
> > According to [basic.lval]/8.8, I can:
> >
> >> If a program attempts to access the stored value of an object through a glvalue of other than one of the
> > following types the behavior is undefined:
> >> ...
> >> a char, unsigned char, or std::byte type.
> >
> > So it doesn't matter what `T1` is; you can access that byte as an
> > `unsigned char`. And indeed, if you could get a pointer to *any* byte
> > within `T1`, you can access it as an `unsigned char`.
> >
> > It's getting a pointer to those bytes that is the problem.
>
> That is what was meant by "you can't access `unsigned char` objects". You can't get a pointer to them.

You can't get a pointer to any bytes *other than* the first. That's
why I said "those bytes".

> >> But accessing an object of `mybyte` type through a glvalue of `unsigned char` type is indeed legal, because an enumeration with a fixed underlying type has the same values as the underlying type.
> >
> > Actually, that's not true. I mean, it is true that the value
> > representations are the same, but that doesn't mean you can access
> > them through different types. [basic.lval] has no special provisions
> > for an enum and its underlying type.
>
> I was speaking exactly about the case of `enum mybyte : unsigned char`.
> If there were no rule explicitly saying
> > For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type.
>
> accessing an object of `mybyte` type through an `unsigned char` glvalue would be UB because of [expr.pre]/4, even though [basic.lval] "allows" such access.

[expr.pre] doesn't apply. Remember: I was talking *specifically* about
having a pointer of type `unsigned char` that's pointing to a
`mybyte`. If you have such a pointer that's valid (ie: you *didn't* do
the pointer arithmetic, and thus didn't invoke UB), then the act of
*accessing* it is valid. But it's only valid because you used
`unsigned char`; it would not be valid if the underlying type were not
a bytewise type and you casted it to that type.

Just to spell it out, this is fine:

```
auto ptr = new AnyType(...);
auto byte_ptr = reinterpret_cast<unsigned char*>(ptr);
*byte_ptr;
```

What is not fine in C++ as it currently stands is trying to do pointer
arithmetic on `byte_ptr`.

> Your
> > The cast is legal. Accessing the `unsigned char` at that address is legal. But without P1839, actually doing the pointer arithmetic needed to access more than one such byte is not.
>
> sounds like that now, without P1839, there is some "access to one byte", which is not the case.

[basic.lval]/8.8 says otherwise.


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