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Comment by LatencyKills | Hacker News Reader

std::future doesn't give you a state machine. You get the building blocks you have to assemble into one manually. Coroutines give you the same building blocks but let the compiler do the assembly, making the suspension points visible in the source while hiding the mechanical boilerplate.

This is why coroutine-based frameworks (e.g., C++20 coroutines with cppcoro) have largely superseded future-chaining for async state machine work — the generated code is often equivalent, but the source code is dramatically cleaner and closer to the synchronous equivalent.

(me: ex-Visual Studio dev who worked extensively on our C++ coroutine implementation)

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It doesn't seem like a clear win to me. The only "assembly" required with std::future is creating the associated promise and using it to signal when that async step is done, and the upside is a nice readable linear flow, as well as ease of integration (just create a thread to run the state machine function if want multiple in parallel).

With the coroutine approach using yield, doesn't that mean the caller needs to decide when to call it again? With the std::future approach where it's event driven by the promise being set when that state/step has completed.

You are describing a single async step, not a state machine. "Create a promise, set it when done", that's one state. A real async state machine has N states with transitions, branching, error handling, and cleanup between them.

> "The only 'assembly' required is creating the associated promise"

Again, that is only true for one step. For a state machine with N states you need explicit state enums or a long chain of .then() continuations. You also need to the manage the shared state across continuations (normally on the heap). You need to manage manual error propagation across each boundary and handle the cancellation tokens.

You only get a "A nice readable linear flow" using std:future when 1) using a blocking .get() on a thread, or 2) .then() chaining, which isn't "nice" by any means.

Lastly, you seem to be conflating a co_yield (generator, pull-based) with co_await (event-driven, push-based). With co_await, the coroutine is resumed by whoever completes the awaitable.

But what do I know... I only worked on implementing coroutines in cl.exe for 4 years. ;-)

I only mentioned co_yield() since that's what the article was (ab)using, although perhaps justifiably so. It seems the coroutine support was added to C++ in a very flexible way, but so low level as to be daunting/inconvenient to use. It needs to have more high level facilities (like Generators) built on top.

What I was thinking of as a state machine with using std::future was a single function state machine, using switch (state) to the state specific dispatch of asynch ops using std::future, wait for completion then select next state.

> as to be daunting/inconvenient to use

I don't even know how to respond to that. How in the world are you using C++ professionally if you think coroutines are "daunting"? No one uses C++ for it's "convenience" factor. We use it for the power and control it affords.

> What I was thinking of as a state machine with using std::future was a single function state machine, using switch (state) to the state specific dispatch of asynch ops using std::future, wait for completion then select next state.

Uh huh. What about error propagation and all the other very real issues I mentioned that you are just ignoring? Why not just let the compiler do all the work the way it was spec'ed and implemented?

So it's meant to be inconvenient, and that's the only right and proper way?!

Sounds more like punishment than software design, but each to their own.

I get what you’re saying, but you kicked off this thread like an expert — even though you knew you were talking to someone who helped build the very thing you’re critiquing.

It’s pretty clear you’ve never built a production-grade async state machine.

C++ is designed to provide the plumbing, not the kitchen sink. It’s a language for building abstractions, not handing them to you — though in practice, there’s a rich ecosystem if you’d rather reuse than reinvent.

That flexibility comes at the cost of convenience, which is why most new engineers don’t start with C++.

What you call “intimidating,” I call powerful. If coroutines throw you off, you’re probably using the wrong language.

Last thought — when you run into someone who’s built the tools you rely on, ask them questions instead of trying to lecture them. I would have been more than happy to work through a pedagogical solution with you.

/ignored

> It’s pretty clear you’ve never built a production-grade async state machine.

Haha .. you have no idea.

FWIW I've built frameworks exactly for that, and it's highly likely that you've unwittingly used one of them.

Uh huh. The person who gets confused by how co_wait() actually works and thinks that coroutines are "intimidating" wrote frameworks that I would have used to build our C++ compiler. Do you not understand that cl.exe doesn't use external frameworks? lmfao

I said used, as in used in your everyday life, by interacting with computer systems whose backend implementations you are blissfully ignorant of.

But yeah, if you want to win arguments then arguing against yourself and your own hallucinations, is in your case the best way to go.

Um... you might want to look at my profile. In addition to working at MS and Apple for two decades (where I touched everything from firmware, ring-0, and ring-3), I was on the team that created SoftICE [0]: the first commercial ring-0 debugger for Windows. I also created the automated deadlock detector for BoundsChecker [1], which requires an in-depth understanding of operating system internals.

> computer systems whose backend implementations you are blissfully ignorant of

I am extremely confident in my "backend" knowledge (of course, an actual systems engineer would never refer to their work as "backend").

You wrote a "C++ framework" that runs in the "backend" of a "computer system"? Do I have that right? Please let me know what it is so that I can decompile it and see how it was implemented!

[0]: https://en.wikipedia.org/wiki/SoftICE

[1]: https://en.wikipedia.org/wiki/BoundsChecker

> You wrote a "C++ framework" that runs in the "backend" of a "computer system"? Do I have that right?

Probably not.

You seem to be struggling with technical language.

See? I have no problem sharing/discussing what I've built. Throughout this entire thread they only thing you've done is bullshit. And again, remember that this started because you thought coroutines were "intimidating" while trying to mansplain how they worked to someone who literally built that functionality.

But sure... let's keep going while you do nothing but whine without displaying any technical acumen. This is fun!

Why don't you hop into the discussion of the early Windows ring-3-to-ring-0 transition issues [0] we had in the 90s? You know... since I am "blissfully ignorant" about internals. I can't wait to learn from your "backend" expertise!!

[0]: https://news.ycombinator.com/item?id=47611921#47612953

You do love your scare quotes, eh?

How old are you? 14?

Bye!

I'm the one trying to have a meaningful technical discussion with someone who finds a basic part of C++ intimidating.

One of us worked on two of the most popular development tools of all time, and the other makes shit up and refuses to engage with technical details.

Also, since you know what I've worked on, it should be pretty obvious how old I am.

I feel like thats really oversellign coro -- theres still a TON of boilerplate

My response specifically addressed the question of why you might choose one option over the other.

Do you believe that std::future is the better option?

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