Author here. I wrote this on a rainy weekend last year and mostly forgot about it, so it was a bit of a surprise to see it pop up on HN this morning!
Happy to answer any questions (including "why on earth would you do this?") :)
Happy to answer any questions (including "why on earth would you do this?") :)
Why on earth would you do this?
Good question! :)
It actually started out as a learning exercise -- I didn't (and still don't) know much about ray tracing, and so I thought it would be good to study a simple example by translating it from a language I didn't know (TypeScript) to one I was more familiar with.
This involved writing a simple vec3 class, and it seemed natural to make that constexpr. Then as I went through I realised that many more functions could be made constexpr too. And then if I hacked together some (admittedly very poor) replacement maths functions, even more of it could be constexpr...
At this point, it became a challenge to see whether I could make the whole thing run at compile-time. The only tricky part was avoiding virtual functions -- I'd originally naturally translated TypeScript "interface"s into C++ abstract classes. I ended up using two different approaches: firstly using std::variant and std::visit (the any_thing class in the source code), and secondly using a struct of function pointers (for the surfaces) -- because while virtual calls aren't allowed in constexpr code, calls via a function pointer are just fine.
In the end, I was pretty satisfied that I'd managed to push constexpr as far as I could take it. I had intended to blog about it, but never got round to it ... and now 18 months later someone has saved me the job by posting it to HN anyway :)
> Were there particular features of 17 that you were able to benefit from?
I used std::variant from C++17 for polymorphism, as a workaround for the fact that you can't have constexpr virtual functions (these have since been proposed for C++20). I also used `std::optional` when testing for intersections between a ray and a "thing", but it would have been possible to structure the code in such a way that this wasn't necessary (for example, by using a bool return value and an out parameter).
Lastly, I used constexpr lambdas (new in C++17) though again, it would have been possible to do things differently so these weren't required -- and indeed I did have to work around the fact that Clang (at the time) didn't support evaluating capturing lambdas at compile-time.
An analogy would be if we used only ball-and-socket joints everywhere in machines, even where a single-axis hinge would work just fine. And then just accepted that machines are inherently wobbly, rather than questioning whether extra degrees of freedom (whether needed or not) are always better than less.
I think sometimes what we really want when we use class inheritance is actually something like a std::variant where we don't have to know all of the constituent types in advance.
well, most of the software I use every day have some sort of plug-in interface which is done at some point by loading a .dll at runtime and getting derived instances of a base type defined in the host app.