StateMachine

This library implements lightweight header only finite state machine using the c++17 features std::optional and std::variant and NOT inheritance / virtual function calls.

Goals

The main goals were to evaluate:

• if a full featured state machine can be implemented by only using std::optional, std::variant and templates and NOT by using inheritance / virtual function calls
• if the implementation is scalable to real world production size state machines.

The goals were met partially:

• It can be done
• The code is small (~400sloc, requires 4 classes wich are loosely coupled but have rather complex templates, see devel/src/*) but not all features are implemeneted yet (namely external transitions, historizing)
• Scalability remains open to be evaluated.

Status

The current implementation is a proof of concept.

Examples

see

• devel/examples/simple/genericStates/lightSwitch_generic.cpp
• devel/examples/simple/customStates/lightSwitch_custom.cpp

Features

Currently the following features are implemented:

• Unrestricted type of events, eg could be int or std::string
• Unrestricted type of states, custom states need only provide the functions they want to support
• Nesting of state machines
• Run to completion
• Arbitrary guards and actions on transitions
• Arbitrary actions on entry, exit and self transtion
• Lightweight (~400sloc) and therefore easy to reason about
• No direct memory allocations by the library (indirect allocations happend due to using eg std::vector)
• Header Only, with no depencies except for the stl

Drawbacks

The drawbacks of the approach are:

• defining state machines with mixed custom- and generic states is rather complicated due to templates
• defining hierachical state machines is complex due to templates
• compiler-messages are hard to read due to extensive template usage (could be improved by static_asserts)
• Raw Pointer to states are beeing used, which cannot be avoided easily

Todo

The following issues are currently open:

• needs more testing
• usability on production scale state machines must be evaluated
• examples missing
• features missing, eg historizing, external/internal transitions
• speed and assembly size must be evaluated
• documentation
• compliance of names with https://en.wikipedia.org/wiki/UML_state_machine should be improved
• consistency of pointers in template arguments should be checked
• nullptr checks missing
• add public/private qualifier