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Subject: Re: reading values as bytes without memcpu, from enum unsigned char?
From: Roman Babinicz (rb_at_[hidden])
Date: 2020-08-06 18:12:28


On 06/08/2020 03:47, Jason McKesson via Std-Discussion wrote:
> On Wed, Aug 5, 2020 at 1:58 AM Roman Babinicz via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
>>
>>
>> enum class mybyte : unsigned char{};
>> using T1 = mybyte;
>> auto ptr = new T1 [100];
>> *(ptr+10) = static_cast<T1>(50);
>> delete []ptr;
>>
>> Is this well formed way of accessing object via glvalue pointer
>> (as in basic.lval#11.3) ?
>>
>> Because for some other types T1, e.g. for int, one possible reading of
>> standard is that this is UB and you need to memcpy the buffer into a new
>> location first (and even more likelly needed for non-fundamental types
>> in T1).
>
> I don't understand what exactly would be ill-formed about this. So

Thanks, and now for following example (this is the issue I should has
posted first)

enum class mybyte : unsigned char{};
using T1 = mybyte;
auto buf = new T1 [100];
unsigned char * ptr = reinterpret_cast< unsigned char * > ( buf ) ;
*(ptr+10) = 50;
delete []buf;

so nowe we access enum objects via the unsigned char (pointer) that
happens to be the underlying type, but is that enough to avoid need for
memcpy?

Further question would be: for which T1 types you need to first memcpy it?

> long as `T1` is a type which is default constructible and `T1` can be
> constructed from the integer literal 50, this code is fine. At no
> point are you reading any values as bytes. The fact that `unsigned
> char` is involved is essentially irrelevant.
>
> ptr is a pointer to an array of 100 `T1`s. So adding `10` to `ptr` is
> well-defined; it points to the 10th element (zero-based) of the array.
> `ptr + 10` is a valid pointer, and it is a pointer of that points to
> the type `T1`. It is also a pointer to an object of type `T1`. `T1` is
> the dynamic type of the object being pointed to, so [basic.lval/11.1]
> is satisfied.
>
> There is no "reading values as bytes" happening here.
>


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