I made a rough attempt a little while ago to make something P-like with C++17 constructs, I think it turned out pretty nicely: https://gist.github.com/saltzm/f2acd7f656436dd63c9808ea11738...

There’s one extra validation hop to check valid state transitions that I don’t love, but if I take that feature out then I think it’s pretty low overhead for how readable it is