Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

The Boost Statechart Library

UML to Boost.Statechart mapping summary

State machine

A state machine is defined by deriving a class from either the state_machine or the asynchronous_state_machine class templates; example

States

Normal state

A state is defined by deriving a class from either the simple_state or state class templates; example
The position of a state in the state hierarchy is defined by what is passed as the Context template parameter. Outermost states pass the state_machine<> or asynchronous_state_machine<> subtype they belong to, inner states pass their direct outer state; example
A state can be made a member of a particular orthogonal region by passing an instantiation of the orthogonal class template nested in its outer state; example

Initial pseudostate

The state that is initially entered is identified by template parameters rather than by a default transition from the initial pseudostate (there is no such thing in Boost.Statechart)
For state_machine<> and asynchronous_state_machine<> the state that is initially entered needs to be specified with the InitialState template parameter; example
For simple_state<> and state<> the inner state(s) that is/are initially entered can be specified with the InnerInitial template parameter; example

Final pseudostate

There is no such thing as a final pseudostate in Boost.Statechart
This functionality is implemented with a special reaction (termination<>) and special reaction function (simple_state<>::terminate()); example

Shallow history pseudostate

A shallow history transition target or shallow history inner initial state can be specified with shallow_history<>

Deep history pseudostate

A deep history transition target or deep history inner initial state can be specified with deep_history<>; example

Junction point

Does not exist in Boost.Statechart; rationale

Dynamic choice point

Does not (yet?) exist in Boost.Statechart; rationale

Join and fork bars

Do not exist in Boost.Statechart; rationale

Events

A event is defined by deriving a class from the event class template; example
There is no distinction between signal and call events, see below

Call event

All events passed to state_machine<>::process_event() are implicitly call events

Signal event

All events passed to either simple_state<>::post_event() or fifo_scheduler<>::queue_event() are implicitly signal events

Time event

Does not exist in Boost.Statechart
A time event can be simulated with an external timer that is started in the entry action and stopped in the exit action of a particular state. When starting the timer, it is instructed to call fifo_scheduler<>::queue_event() when the due time elapses

Change event

Does not exist in Boost.Statechart
A change event can be simulated by packing to be monitored variables into an external wrapper with get and set methods. Whenever set is called the wrapper posts an appropriate event

Reactions

Guard

A guard can be implemented with a selection statement in a custom_reaction<>; example

Transition

A transition can be defined with either a transition<> or a call to simple_state<>::transit<> in a custom_reaction<>; example

Deferral

A deferral reaction can be defined with either a deferral<> or a call to simple_state<>::defer_event() in a custom_reaction<>; example

Internal transition (in-state reaction)

An in-state reaction can be defined with either a in_state_reaction<> or a call to simple_state<>::discard_event() in a custom_reaction<>

Actions

Entry action

A state entry action can be defined by adding a constructor to a state class; example

Exit action

A state exit action can be defined by adding a destructor to a state class; example

Transition action

A transition action can be defined by passing a function pointer referencing the action to either transition<> or simple_state<>::transit<>(); example

Do activity

Not supported in Boost.Statechart
A do activity can be simulated with a separate thread that is started in the entry action and cancelled (!) in the exit action of a particular state

Revised 03 December, 2006

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

The Boost Statechart Library

UML to Boost.Statechart mapping summary

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