Show HN: CXXStateTree – A modern C++ library for hierarchical state machines
github.comHi HN!
I've built [CXXStateTree](https://github.com/ZigRazor/CXXStateTree), a modern C++ header-only library to create hierarchical state machines with clean, intuitive APIs.
It supports: - Deeply nested states - Entry/exit handlers - State transitions with guards and actions - Asynchronous transitions with `co_await` (C++20 coroutines) - Optional runtime type identification for flexibility
It's ideal for complex control logic, embedded systems, games, robotics, and anywhere you'd use a finite state machine.
I’d love feedback, use cases, or contributions from the community!
how is it better than https://github.com/boost-ext/sml ?
there are about 1 million c++ state machines, and sml happens to be the best, or one of them. how does yours differentiate?
Nice and compact. I only wound have two nitpicks:
The Readme sais "zero heap allocations" but the code uses list and unordered map and moves, did you mean "zero allocations after state tree building"?
Also for embedded it would be useful to separate all in/out, dot export etc. to a second library that you can omit on small targets.
yes, it means "zero allocations after state tree building". Thank you for the suggestions, I think we could separate target with compilation switch. If you want you can open an issue on the repo. Thank you so much
Related idea for those using python: https://github.com/baymotion/smax.
i am by no means a C++ expert, but isn't "pragma once" frowned upon?
All terrestrial compilers support `#pragma once`.
In the C++ community (as lots of other things are), rejecting `#pragma once` is a long-standing tradition of worshipping the decaying body of prehistoric compilers for
It's unclear what benefits this approach has achieved, but don't disturb it, or else.
No, it is the way. Edit: no one has time for inventing unique names for include guards.
Does anyone write those by hand anyway in any kind of project the size where it would matter?
#pragma once is broken by design
> Does anyone write those by hand anyway in any kind of project the size where it would matter?
I think you're suggesting that you don't need to make up the names for include guards because all tools / IDEs for C++ write them for you automatically anyway. But that isn't my experience. Many IDEs don't write include guards for you automatically ... because everybody uses #pragma once already.
> #pragma once is broken by design
I think you're referring to the historical problem with #pragma once, which is that it can be hard for the compiler to identify what is really the same file (and therefore shouldn't be included a second time). If you hard link to the same file, or soft link to it, is it the same? What if the same file is mapped to two different mount points? What if genuinely different files have the same contents (e.g., because the same library is included from two different installation paths)? In practice, soft/hard links to the same file are easily detectable, and anything more obscure indicates such a weird problem with your setup that you surely have bigger issues. #pragma once is fine.
(Historically, it also had the benefit that compilers would know not to even re-read the header, whereas with traditional include guards they would need to re-include the file (e.g. in case the whole file is not wrapped in the #ifdef, or in case something else has undefined it since) only to then discard the contents. I've even seen coding guidelines requiring external include guards wrapped around every use of headers with #include <...>. Yuck! But modern compilers can work out when include guards are meant to mean that so today that difference probably no longer exists.)
Even if you don't write header guards by hand you get issues. The amount of time I got bitten by someone naming a file "widget.h" or "utils.hpp" three levels of libraries down with the corresponding #ifndef WIDGET_H which broke the build in incredibly mysterious ways...
https://github.com/search?q=ifndef+WIDGET_H&type=code
I don't understand what you're saying here. #pragma once does the job that include guards used to do, but with less work, and in a less error prone way. How is it broken, and how is the size of a project relevant?
> I don't understand what you're saying here. #pragma once does the job that include guards used to do, (...)
They don't. They are not C++ and at most they are compiler-specific.
It's fine if you opt to not write C++ and instead target specific compilers instead. Just don't pretend it's not frowned upon or kosher.
TIL about the existence of a passionate #pragma once hating subculture.
Since you seem to be more knowledgeble about this, I'm curious to know which C++ compilers lack support? I know that at least the 3 big ones do (GCC, Clang, and MSVC) and they have for a very long time.
it's absolutely not frowned upon in 2025. All the compilers that matter (GCC, Clang and MSVC) support and have supported them for two decades. Major projects use #pragma once internally - I see files using it in Qt, LLVM, etc.
> No, it is the way.
No, this is completely wrong. Pragma once is non-standard compiler directive. It might be supported by some compilers such as msvc but technically it is not even C++.
There are only two options: include guards, and modules.
Yes, it is non-standard, but I don't know any compiler that does not support it.
What major compiler does not support it?
> What major compiler does not support it?
The whole point is that it's not supported and it's not standard, thus using #pragma once needlessly introduced the risk of having the code break.
You should ask yourself what are you doing and why are you using non-standard constructs that may or may not work, specially when it's rather obvious and trivial to just use include guards. Using #pragma once isn't even qualify as being clever to gain anything.
Maybe you should ask why you're using a non-standard compiler if it's not supported.
Not being part of the official standard doesn't necessarily mean it's not well supported.
The whole point of C/C++ is knowing your environment. It's not Java, it's not TypeScript. It's one level above assembly, and if you change the compiler and things break, then it's your fault.
If the standards still don't have a proper replacement for include guards, then too bad for the standards. The C++ standard wasn't aware of multithreading before C++11, this didn't stop people from writing multithreaded programs.
As to why - #pragma once is just cleaner to look at.
> the standards still don't have a proper replacement for include guard
It does with modules... and in ten year if modules support is widespread, I'll consider stoping using pragma once.
I've avoided #pragma once because of reports that it slows down gcc/g++: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58770
But given that I haven't seen any mention of that issue in other comments, I wonder if it really is an issue.
vcpkg it
You can open an Issue on that on the repo (https://github.com/ZigRazor/CXXStateTree/issues) so we can track these changes.
Another idea is to create a Python binding with a release of a package