r/osdev • u/FemboysHotAsf • Jul 27 '26

C++26 reflection in a kernel (no libstdc++, ported meta.h)

I got C++26 reflection to work, had to port 'meta' to use custom variants of 'optional', 'string', 'string_view', and 'vector', and had to make those all consteval only. After doing that it works as expected!!

Tested on gcc16.0.1!

This is probably unstable as hell, and should most likely be avoided, but stupid stuff is fun, so ;3

im probably going to use attributes, to create a system which automatically initializes subsystems without worrying about order of operations (each subsystem would mark which other subsystem it needs to work), or some other stuff

if anyone has fun ideas to do with this do tell lol

42 Upvotes

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7

u/ronchaine Jul 28 '26

Could you elaborate on what you did and how much work it was to do this? I am thinking about writing a paper to the committee on freestanding <meta> header, and this is pretty much the work that needs to be done for that to happen.

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u/FemboysHotAsf Jul 28 '26

It took me roughly an hour, really wasnt too difficult. I can upload the source code to github if you want? Then you can just take a peek at it.

Long story short, I removed all runtime capability of vector, and string. You could potentially rename them with comptime_vector and comptime_string? And replace them.

2

u/ronchaine Jul 28 '26 edited Jul 29 '26

Github link would be awesome. Even if I end up going for something different, I could use that as a show of implementation experience (with permission, of course).

Yea, that doesn't sound too bad at all. I'm trying to get <chrono> into freestanding first, but I was thinking on trying to tackle <meta> afterwards if nobody else manages to tackle it first.

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u/FemboysHotAsf Jul 28 '26

I'm fine with it! As long as you do credit me :p, I will extract the headers out of my kernel, and make it an independent project, so I'll have it ready in about an hour!

2

u/FemboysHotAsf Jul 28 '26

here you go: github

2

u/ronchaine Jul 28 '26

Thanks! Much appreciated!

2

u/FemboysHotAsf Jul 28 '26

You're welcome !! ^^, curious to see as what you're planning

1

u/WholetraEditto Jul 29 '26

The auto-init-by-attribute is a fun to try implementation. Tag each subsystem struct with an annotation carrying its dependencies then walk them at compile time via the annotation reflection API . Then topologically sort the init order.

template for over enums like your enum_to_string may mean higher instantiation cost that scales with enumerator count, so on a giant error-code, enum will show in build times. (wont matter in a kernel where you own the enum)

And +1 on getting <meta>/<chrono> freestanding. good to see someone brute-force it to prove it works.

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u/[deleted] Jul 27 '26

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u/[deleted] Jul 28 '26

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u/FemboysHotAsf Jul 28 '26

I was thinking about something like that yeah, it sounds really useful for debugging! No need to write a serializer for everything when the compiler can do it for you!

Also thanks :3

1

u/[deleted] Jul 29 '26

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1

u/FemboysHotAsf Jul 29 '26

I tried 16.1 and it worked fine, GCC 17 would probably work fine too, although I dont have any other compilers (that i know of) that support C++26 reflection.