On Tue, Jul 8, 2025 at 5:02 PM Tom Honermann <tom_at_[hidden]> wrote:

> On 7/8/25 10:20 AM, Brian Bi via Std-Discussion wrote:
>
> On Tue, Jul 8, 2025 at 9:57 AM Yongwei Wu via Std-Discussion <
> std-discussion_at_[hidden]> wrote:
>
>> Assume I am working on a little-endian machine only. Is the following
>> code undefined behaviour?
>>
>> uint32_t data = 0x40400000;
>> auto f = *std::start_lifetime_as<float>(&data);
>>
>
> It's fine as long as `sizeof(float) <= sizeof(uint32_t)`. See
> https://eel.is/c++draft/obj.lifetime#2
>
> Is it not also the case that implicit object creation requires that the
> storage be provided by an object that meets the provides storage
> requirements? https://eel.is/c++draft/intro.object#3. I thought it was
> required, but I'm unable to find clear wording now.
>
No, the old object doesn't have to provide storage for the new object.

But because it doesn't (in this case), the new object is not "nested
within" the old object, triggering [basic.life]/2.5, ending the lifetime of
the old object.

In general, C++ allows you to overwrite an object of arbitrary type (with
some restrictions) with an object of some other arbitrary type. You've
always been able to do that with placement new, and now you can do it with
`std::start_lifetime_as` too. But you have to deal with the consequences of
it (e.g., the UB of the original implicit destructor call).

>
>
>>
>> If it is not, is the data variable still accessible now?
>>
>
> No. The `data` object's lifetime has ended because its storage has been
> reused. https://eel.is/c++draft/basic.life#2.5
>
> And the old object is not one that is transparently replaceable by the new
> object. https://eel.is/c++draft/basic.life#10
>
> Tom.
>
>
>
>>
>> Best regards,
>>
>> Yongwei
>>
>> --
>> Yongwei Wu
>> URL: http://wyw.dcweb.cn/
>> --
>> Std-Discussion mailing list
>> Std-Discussion_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>>
>
>
> --
> *Brian Bi*
>
>

-- 
*Brian Bi*