Simple C++ Hierarchical State Machine

Features

• Ability to disallow entering or exiting states
• Arbitrary information can be passed to state classes
• Default states
• Event processing
• Thread safe

Usage

1. Create a main HSMachine class. Optionally provide a callback function to run whenever a state transition happens in order to, for example, update a UI.
2. Subclass HSMState to create your custom states, overriding the on_* and can_* functions if desired.
3. Optionally extend HSMInfo to create structs of information that can be passed to your HSMState-derived classes whenever a state transition or event occurs.
4. Allocate instances of your states, keeping pointers to them. Each pointer is an individual state, meaning you can have multiple states in the hierarchy that are represented by the same HSMState-derived class.
5. Create your hierarchy using the add_child_state function. This will take ownership of the pointers, and all state objects should be freed when the main HSMachine is destroyed, assuming there are no orphaned states. assert statements should ensure your machine is well-formed.
6. Optionally set default states using set_default_state.

See demo.cpp

Behavior

When a state transition is requested, can_enter and can_exit will be called for all intermediate state transitions that would need to happen. If any of these calls return false, the operation is aborted. If not, the corresponding on_enter and on_exit functions are called in the same order. Asking to transition to the current state does nothing.

If the final state has any default state set, the path of default states are recursively followed. For any given state that has a default state, the machine should never come to rest at that given state.

Events are processed starting from the current state and work upwards. If any on_event function returns true, the event is consumed and no further parent states will receive it.