Catch Action

Definition

A catch action is an action that is invoked whenever an exception is caught by the state machine.

Why You Need Catch Actions

Maki doesn't throw exceptions itself. There are however some cases where an exception may be thrown during a call to a member function of maki::machine:

• an action, a guard, or a hook throws an exception;
• an event is about to be copied into the run-to-completion queue, but the copy constructor throws an exception;
• an event is about to be copied into the run-to-completion queue, but memory allocation fails and throws an exception.

Maki provides basic exception guarantee; if the code it calls throws an exception, the state machine stays in a valid state.

However, by default, Maki doesn't catch these exceptions. If you want it to, you must set up a catch action, which inserts a try/catch(...) block at the topmost level of every non-const member function of maki::machine.

How to Use Catch Actions

To set up a catch action, you must call maki::machine_conf::catch_mx(), like so:

constexpr auto machine_conf = maki::machine_conf{}
    .context_a<context>()
    .transition_tables(transition_table)
    .catch_mx([](auto& mach, const std::exception_ptr& eptr)
    {
        /*
        Note: The possibility of the call below to throw in its turn should also
        be taken care of.
        */
 
        mach.process_event_no_catch(error{eptr});
    })
;

The given callable is invoked from the inserted catch(...) blocks.

Processing some kind of error event, like in the above example, is a common way to recover from an exception. This can be coupled with an error state to which the topmost region transitions whenever this error event occurs, as described in The Undefined State.