draft VS mirage

I should have said the "latest standard", not "spec", if we're being technical. But EVERY bit of official material is very clear about asserting that C++23 is still a preview/in-progress, not a standard. Saying otherwise is, strictly speaking, incorrect.

https://isocpp.org/std/the-standard

https://www.iso.org/standard/79358.html

https://github.com/cplusplus/draft/blob/main/papers/n4951.md

[3] https://github.com/cplusplus/draft/releases/tag/n4917

*This is a joke, but only barely so.

pdf

I'm currently going through Annex A of C++20, but I can't find the definition of "designator brace-or-equal-initializer", and couldn't find much formal information on it in an obvious way. The newest source on [decl] (https://github.com/cplusplus/draft/blob/main/source/declarations.tex) also doesn't seem to have it. Am I missing anything, or is this a missing definition in the standard grammar?

> I don't have a copy of the standard at hand, can anyone quote the relevant section?

The C++ (draft) standard is on GitHub! [0] Compiling it needs Perl and some LaTeX packages, but is reasonably straightforwards otherwise. In addition, links to specific draft standards can be found on cppreference [1].

But anyways, in the first C++20 post-publication draft (N4868), the wording you're interested in is in multiple sections. Section 22.2.3 Sequence Containers [sequence.reqmts] has Table 78: Optional sequence container operations [tab:container.seq.opt] (starting on page 815), which states that a precondition of pop_back() is that empty() returns false. Section 16.3.2.4 Detailed Specifications [structure.specifications] (page 481) states:

> Preconditions: the conditions that the function assumes to hold whenever it is called; violation of any preconditions results in undefined behavior.

Therefore, calling pop_back() on an empty vector results in undefined behavior.

> Is this something that in practice is implemented in different (exception-throwing) ways?

Based on a quick glance at the major implementations (libc++ 15.0.7 at [2], MSVC at [3], libstdc++ at [4]), it looks like asserts are used. Whether those result in exceptions probably depends on whether the asserts are compiled in in the first place and how they are implemented, but it's definitely not a guaranteed exception.

[0]: https://github.com/cplusplus/draft

[1]: https://en.cppreference.com/w/cpp/links

[2]: https://github.com/llvm/llvm-project/blob/llvmorg-15.0.7/lib...

[3]: https://github.com/llvm/llvm-project/blob/8dfdcc7b7bf66834a7...

[4]: https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3...

Which standard specifically are you quoting there? I checked an old and a new C++ draft in https://github.com/cplusplus/draft/tree/main/papers, and in neither one did 6.3 have anything like that.

https://github.com/cplusplus/draft/releases/download/n4917/n4917.pdf (page 1, chapter 1 scope):

PRs for C++ are at https://github.com/cplusplus/draft But the discussion for a PR is via https://isocpp.org/std/submit-a-proposal

C/C++

By the way, https://github.com/cplusplus/draft/releases/tag/n4910 , says

Interesting, and thanks.

I didn't know about those. I kind of thought you may have used MirageOS, which I had read about earlier. It is done in OCaml.

https://mirage.io/

Unix system programming in OCaml (2014)

https://ocaml.github.io/ocamlunix/

"MirageOS is a library operating system that constructs unikernels for secure, high-performance network applications across a variety of cloud computing and mobile platforms."

https://mirage.io/

That was/is part of the promise of the whole unikernel thing, no?

https://mirage.io/ or similar could then let you boot your database. That said, it's not really taken off from what I can tell, so I'm guessing there's more to it than that.

MirageOS is not Rust, but in the ballpark!

https://mirage.io/

OCaml MirageOS? https://mirage.io/

MirageOS is a runtime for OCaml to create unikernels. They describe themselves as "library operating system". Probably not quite what you were asking for, but I think it's quite interesting for certain use cases (e.g. running services as standalone unikernels in VMs or embedded devices instead of "traditional" programs on top of a general purpose OS).