Just loudly thinking: (not for this PR) we might do some scope-level RAII objects that would do end_object automagically. Or make begin_object to accept a lambda. So, end_object would be called automatically. I would probably prefer RAII as it might have nesting neater.

The point of RAII is to recover from exceptions cleanly. However, an exception when generating json would likely mean the entire json was invalid, so there's not much point in trying to fix up the superficial syntax, so there's not really anything useful for the RAII to recover/clean up.

We could add it (or a lambda) as an alternate syntax, however.

Well, why cannot state call end_object in its destructor? We'd end with something like this:

{
  auto objectScope = json.begin_object_scope();
  object.emit("checkpoints", checkpoints);
  ...
  object.emit("assertions", p4Assertions);
}

Here objectScope would hold reference to json, so it could close the scope properly on destruction.

or, alternatively, with function object:

{
  json.object([](auto &state) {
    state.emit("checkpoints", checkpoints);
    ...
    state.emit("assertions", p4Assertions);
 });
}

Note sure which variant I like better.

Ok, I made the 'state' value returned by begin_object/_vector automatically call end_object/_vector as appropriate when it goes out of scope, if it hasn't been called yet. So now you can have an explicit end call, or you can just leave it off.

Since all these functions are inline in JSONGenerator.h, the overhead should be minimal -- it should be able to optimize away the redundant state changes and checks, and the result should be just a series of ostream outputs.

maybe we can have emit_as_vector that would iterate over container?

That's basically what this is. Perhaps make a templated generate method that can take any iterable type (any type that has iterator, begin(), and end() -- C++20 concepts would be helpful here). But there would be some ambiguity whether a type should be a vector or an object.