On Tue, Jul 8, 2025 at 9:57 AM Yongwei Wu via Std-Discussion <std-discussion@lists.isocpp.org> 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.

 

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

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Yongwei Wu
URL: http://wyw.dcweb.cn/
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Brian Bi