C++26 Reflections adventures and compile-time UML

Post by einpoklum » Sun, Aug 03, 2025, 11:26 AM UTC

a_t48 wrote: post #55

Oh man, some of the code in the linked proposal: Old: template struct list {}; using types = list ; constexpr auto sizes = [] class L, class... T>(L ) { return std::array {{ sizeof(T)... }}; }(types{}); New: constexpr std::array types = {^^int, ^^float, ^^double}; constexpr std::array sizes = []{ std::array r; std::ranges::transform(types, r.begin(), std::meta::size_of); return r; }(); I'm so tired of parameter packs…

    constexpr std::array types = {^^int, ^^float, ^^double};
    auto sizes = std::whatever::transform(types, std::meta::size_of);

Post by dguest » Sun, Aug 03, 2025, 12:38 PM UTC

oyoman wrote: post #48

dguest wrote: post #45

Earlier quoted context omitted.

It would standardize something they've done in an ad-hoc way for decades. They have a library called "reflex" [1] which adds some reflection, and which was (re)written by cannibalizing a lot of llvm code. They actually use the reflection to serialize a lot of the LHC data. It's kind of neat that it works. It's also a bit fidgety: the cannibalized code can cause issues (which, e.g. prevented C++11 adoption for a while…

The library that you refer is not in use for a long time already. The document you pointed out is from 2006 (you can check the creation date). Since then, a lot has changed, and now it is all based on cling ( https://root.cern/cling/ ), that originates from clang and llvm. cling is responsible generates the serialization / reflection of the classes needed within the ROOT framework.

Post by germandiago » Sun, Aug 03, 2025, 1:14 PM UTC

tw061023 wrote: post #6

matt123456789 wrote: post #3

Whenever I start to feel like a real programmer making games and webapps and AI-enhanced ETL pipelines, I inevitably come across the blog post of a C++ expert and reminded that I am basically playing with legos and play-doh.

It's the other way around. You are the real programmer and the committee and the "modern C++" crowd are more interested playing with legos instead of shipping actual software. No way anything std::meta gets into serious production; too flexible in some ways, too inflexible in others, too much unpredictability, too high impact on compilation times - just like always with newer additions to the C++ standard. It takes o…

Post by Thom2000 » Sun, Aug 03, 2025, 1:52 PM UTC

zozbot234 wrote: post #47

tw061023 wrote: post #6

Earlier quoted context omitted.

It's the other way around. You are the real programmer and the committee and the "modern C++" crowd are more interested playing with legos instead of shipping actual software. No way anything std::meta gets into serious production; too flexible in some ways, too inflexible in others, too much unpredictability, too high impact on compilation times - just like always with newer additions to the C++ standard. It takes o…

> No way anything std::meta gets into serious production Rust proc macros get used in serious production, even though they're quite slow to compile. Sure, std::meta is probably a bit clunkier, but that's expected from new C++ features as you say.

Post by jcranmer » Sun, Aug 03, 2025, 2:17 PM UTC

Someone wrote: post #66

SleepyMyroslav wrote: post #51

Earlier quoted context omitted.

You are sounding like rose tinted glasses are on. I think your glass is half full if you recheck actual versions and features. And mine is half empty in gamedev. Anecdata: A year or so ago I have been in discussion if beta features of C++20 on platforms are good to be used on large scale. It makes it not a sum but an intersection of partial implementations. Anyway it looked positive until we needed a pilot project to…

> One of the projects came back with 'just flipping C++20 switch with no changes causes significant regression on build times Given that C++20 introduced modules, which are intended to make builds faster, I think just flipping C++20 switch with no changes and checking build times should not be the end of checking whether C++20 is worth it for your setup.