Lifetime has already started, if it is in shared memory or files? Yes and no. The process had no notion of objects being represented there.   -----Ursprüngliche Nachricht----- Von:Thiago Macieira via Std-Proposals <std-proposals_at_[hidden]> Gesendet:So 11.01.2026 17:31 Betreff:Re: [std-proposals] User-Defined Trivial Constructors Anlage:signature.asc An:std-proposals_at_[hidden]; CC:Thiago Macieira <thiago_at_[hidden]>; On Saturday, 10 January 2026 11:09:26 Brasilia Standard Time Jody Hagins via Std-Proposals wrote: > The implicit lifetime type requirement isn't academic. It's essential for: > > - **Shared memory** — inter-process communication via memory-mapped regions > - **Memory-mapped files** — persistent data structures > - **Custom allocators** — placement new into raw storage > - **`std::start_lifetime_as`** — explicit lifetime management Memory-mapped files and shared memory are the same thing: shared memory is implemented by memory-mapping a shared file. The only difference is that we call "memory mapped file" when your sharable data isn't being shared with any other process. So my 2¢, which may be an unpopular opinion: - for newly allocated memory (new mmapping of anonymous memory, brk(), custom allocators, etc,) you can and should just do a placement new or std::construct_at(). The stumbles on the problem of being able to call this memory storage region a "byte array" where no array had previously existed, but that can be solved as a much more restricted problem than for any implicit-lifetime type. - for shared memory (which includes memory-mapped files with content), the lifetime *had* *already* *started* before you did the mmaping. You've just made it visible to the application. Therefore, you should have no business using std::start_lifetime_as, std::launder or any such thing. The compilers and abstract machine need to be adapted to know the lifetime had started, not have programmers work around to declaring such. A corner case is when one read()s a file/socket/pipe/whatever. I'd mostly argue that this is a byte array and one can successfully reconstruct the integers and strings that comprise such without resorting to start_lifetime_as either. -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org  Principal Engineer - Intel Data Center - Platform & Sys. Eng. -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals