A Mental Model for C++ Coroutine
13 comments
·July 11, 2025valorzard
Note that with std::execution, c++26 will have a default async runtime (similar to how C# has a default async runtime).
This means that c++26 is getting a default coroutine task type [1] AND a default executor [2]. You can even spawn the tasks like in Tokio/async Rust. [3]
I’m not totally sure if this is a GOOD idea to add to the c++ standard but oh well.
[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p35...
[3] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p31...
uvdn7
> I’m not totally sure if this is a GOOD idea to add to the c++ standard
What are the downsides? Naively, it seems like a good idea to both provide a coroutine spec (for power users) and a default task type & default executor.
valorzard
well, Rust didn't do the same thing for a reason. Rust lets you pick and choose what async runtime to use (even though everyone has decided to use Tokio anyways). This is good because it allows for alternative async runtimes like Embassy (https://embassy.dev/) and it also doesn't freeze the API into something that can't change. It could totally be possible that people find a new style of async that works better than std::execution.
Rohansi
I don't know how it works for C++ but you're not locked down to a single implementation with how C# does it. You can have it use different executors/schedulers, different task types, etc.
nickelpro
Random switching between "Awaitor" and "awaiter" makes it seem like these are distinct concepts instead that the reader is supposed to understand.
In general this moves way too fast for the density of the grammar it's trying to introduce, lines like:
> We have seen Awaitors already - suspend_always is an empty awaiter type that has await_ready returns false always.
But we haven't "seen" suspend_always, it's mentioned in half a sentence in an earlier paragraph, with no further context or examples.
There's a reason Lewis Baker's writings about C++ coroutines are 5000 word monsters, the body of grammar which needs to be covered demands that level of careful and precise definition and exploration.
michaelg7x
Amen. Even with those 5k word monsters it's brutally hard. Andreas Fertig's cpp-insights is really helpful, when is able to complete the coroutine transform.
FWIW, I think a useful addition would be for compilers to output the intermediate source code, so you can reason more easily about behaviour and debug into readable code.
a_t48
I’m excited to actually getting around to trying coroutines - they should be a good replacement for simple state machines. Rather than an storing an object with a state enum, I can write simple declarative code.
spacechild1
In my latest personal project I have switched my asio networking code from callback functions to coroutines. It is such a big improvement! Repeated actions can be written as simple loops, error handling is done with exceptions and the code is generally much easier to follow. And here's the icing on the cake: most data can actually stay in local variables, which means I don't have to care about the lifetime!
mog_dev
Interesting article, but you should use a spell checker. Typos are distracting.
I recently decided that it was time to properly learn C++ coroutines. I looked at a few tutorials, but by far the best was Raymond Chen coroutine series[1]. It is a long series, but every article is just the right size. Strongly recommended.
[1] https://devblogs.microsoft.com/oldnewthing/20210504-01/?p=10...