Can C++ Actor Framework be used in such a way that it guarantees message ordering between two actors? I couldn't find anything about this in the manual.
If you have only two actors communicating directly, CAF guarantees that messages arrive in the order they have been sent. Only multi-hop scenarios can cause non-determinism and message reordering.
auto a = spawn(A);
self->send(a, "foo");
self->send(a, 42); // arrives always after "foo"
At the receiving end, it is possible to change the message processing order by changing the actor behavior with become:
[=](int) {
self->become(
keep_behavior,
[=](const std::string&) {
self->unbecome();
}
);
}
In the above example, this will process the int before the string message, even though they have arrived in opposite order at the actor's mailbox.