Looking at Unity made me understand the point of C++ coroutines

You can roll stackful coroutines in C++ (or C) with 50-ish lines of Assembly. It's a matter of saving a few registers and switching the stack pointer, minicoro [1] is a pretty good C library that does it. I like this model a lot more than C++20 coroutines:

1. C++20 coros are stackless, in the general case every async "function call" heap allocates.

2. If you do your own stackful coroutines, every function can suspend/resume, you don't have to deal with colored functions.

3. (opinion) C++20 coros are very tasteless and "C++-design-commitee pilled". They're very hard to understand, implement, require the STL, they're very heavy in debug builds and you'll end up with template hell to do something as simple as Promise.all

[1] https://github.com/edubart/minicoro

Simon Tatham, author of Putty, has quite a detailed blog post [0] on using the C++20's coroutine system. And yep, it's a lot to do on your own, C++26 really ought to give us some pre-built templates/patterns/scaffolds.

[0] https://web.archive.org/web/20260105235513/https://www.chiar...

Not an expert in game development, but I'd say the issue with C++ coroutines (and 'colored' async functions in general) is that the whole call stack must be written to support that. From a practical perspective, that must in turn be backed by a multithreaded event loop to be useful, which is very difficult to write performantly and correctly. Hence, most people end up using coroutines with something like boost::asio, but you can do that only if your repo allows a 'kitchen sink' library like Boost in the first place.

More broadly the dimension of time is always a problem in gamedev, where you're partially inching everything forward each frame and having to keep it all coherent across them.

It can easily and often does lead to messy rube goldberg machines.

There was a game AI talk a while back, I forget the name unfortunately, but as I recall the guy was pointing out this friction and suggesting additions we could make at the programming language level to better support that kind of time spanning logic.

Looking at C++ made me understand the point of Rust.

Coroutines generally imply some sort of magic to me.

I would just go straight to tbb and concurrent_unordered_map!

The challenge of parallelism does not come from how to make things parallel, but how you share memory:

How you avoid cache misses, make sure threads don't trample each other and design the higher level abstraction so that all layers can benefit from the performance without suffering turnaround problems.

My challenge right now is how do I make the JVM fast on native memory:

1) Rewrite my own JVM. 2) Use the buffer and offset structure Oracle still has but has deprecated and is encouraging people to not use.

We need Java/C# (already has it but is terrible to write native/VM code for?) with bottlenecks at native performance and one way or the other somebody is going to have to write it?

Always jarring to see how Unity is stuck on an ancient version of C#. The use of IEnumerable as a "generator" mechanic is quite a good hack though.

As the author lays out, the thing that made coroutines click for me was the isomorphism with state machine-driven control flow.

That’s similar to most of what makes C++ tick: There’s no deep magic, it’s “just” type-checked syntactic sugar for code patterns you could already implement in C.

(Occurs to me that the exceptions to this … like exceptions, overloads, and context-dependent lookup … are where C++ has struggled to manage its own complexity.)

As I mentioned on the Reddit thread,

This is quite understandable when you know the history behind how C++ coroutines came to be.

They were initially proposed by Microsoft, based on a C++/CX extension, that was inspired by .NET async/await implementation, as the WinRT runtime was designed to only support asynchronous code.

Thus if one knows how the .NET compiler and runtime magic works, including custom awaitable types, there will be some common bridges to how C++ co-routines ended up looking like.

Coroutines is just a way to write continuations in an imperative style and with more overhead.

I never understood the value. Just use lambdas/callbacks.

In Haskell this technique has been called ‘reinversion of control’: http://blog.sigfpe.com/2011/10/quick-and-dirty-reinversion-o...

No serious devs even uses Unity coroutines. Terrible control flow and perf. Fine for small projects on PC.

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