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that use it are valid. Merged revisions 53047-53048 via svnmerge from https://svn.boost.org/svn/boost/trunk ........ r53047 | danieljames | 2009-05-16 15:17:20 +0100 (Sat, 16 May 2009) | 1 line Fix some validation errors. ........ r53048 | danieljames | 2009-05-16 15:23:59 +0100 (Sat, 16 May 2009) | 1 line Use a local copy of the valid HTML 4.01 icon. ........ [SVN r53258]
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<title>The Boost Statechart Library - Definitions</title>
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../index.htm"><img alt="C++ Boost" src=
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"../../../boost.png" border="0" width="277" height="86"></a></h3>
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<td valign="top">
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<h1 align="center">The Boost Statechart Library</h1>
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<h2 align="center">Definitions</h2>
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<hr>
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<h2>Introduction</h2>
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<p>The Boost.Statechart documentation uses a lot of terminology specific to
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state machines. Most of it is equal to the one used in the UML
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specifications. This document contains only definitions for terminology not
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used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML
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standard</a>. A short tour around UML terminology can be found <a href=
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"http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p>
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<h2>Definitions</h2>
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<dl class="page-index">
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<dt><a href="#Context">Context</a></dt>
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<dt><a href="#InnermostCommonContext">Innermost common context</a></dt>
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<dt><a href="#InnermostState">Innermost state</a></dt>
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<dt><a href="#InStateReaction">In-state reaction</a></dt>
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<dt><a href="#OutermostState">Outermost state</a></dt>
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<dt><a href="#PolymorphicEvents">Polymorphic events</a></dt>
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<dt><a href="#Reaction">Reaction</a></dt>
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<dt><a href="#UnstableState">Unstable state</a></dt>
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<dt><a href="#UnstableStateMachine">Unstable state machine</a></dt>
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</dl>
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<h3><a name="Context" id="Context">Context</a></h3>
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<p>The contexts of a state define its location in the state hierarchy. A
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state's <b>direct</b> context is defined by what is passed as the
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<code>Context</code> template parameter of the <code><a href=
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"reference.html#ClassTemplatesimple_state">simple_state</a></code> and
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<code><a href="reference.html#ClassTemplatestate">state</a></code> class
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templates. This can either be the state machine (which makes the state an
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<a href="#OutermostState">outermost state</a>) or its direct outer state. A
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state's <b>indirect</b> contexts follow from the direct context of its
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direct context and the direct context of the direct context of its direct
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context and so on. Examples:</p>
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<p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif"
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border="0" width="467" height="572"></p>
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<ul>
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<li>A's <b>direct</b> context is the state machine (not visible in this
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picture). A does not have any indirect contexts</li>
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<li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the
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state machine (not visible in this picture)</li>
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<li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A
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and the state machine (not visible in this picture)</li>
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<li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the
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state machine (not visible in this picture)</li>
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</ul>
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<h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost
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common context</a></h3>
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<p>The innermost common context of two states is the first direct or
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indirect context that both states have in common. Also known as Least
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Common Ancestor (UML).</p>
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<h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3>
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<p>An innermost state is a state that does not itself have inner states.
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Also known as leaf state or simple state (UML). Note that <code><a href=
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"reference.html#ClassTemplatesimple_state">boost::statechart::simple_state<></a></code>
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is <b>not</b> a model of the UML simple state.</p>
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<h3><a name="InStateReaction" id="InStateReaction">In-state
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reaction</a></h3>
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<p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither
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exits nor enters any states. Also known as inner transition or internal
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transition (UML).</p>
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<h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3>
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<p>An outermost state is a state that does not itself have outer states.
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Note that an outermost state is different from the UML top state. A state
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machine can have an arbitrary number of the former but only exactly one of
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the latter. Boost.Statechart only supports outermost states.</p>
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<h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic
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events</a></h3>
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<p>An FSM library supports polymorphic events if events can inherit from
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each other without restrictions <b>and</b> if it allows the definition of
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reactions for leafs and nodes of the resulting event inheritance
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tree.</p>
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<p>Example (using a hypothetical FSM library, as Boost.Statechart does not
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support polymorphic events):</p>
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<pre>
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struct EvButtonPressed : Event // node
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{
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/* common button pressed properties */
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};
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struct EvPlayButtonPressed : EvButtonPressed {}; // leaf
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struct EvStopButtonPressed : EvButtonPressed {}; // leaf
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struct EvForwardButtonPressed : EvButtonPressed {}; // leaf
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</pre>
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<p>If a state machine needs to react whenever <b>any</b> button (including
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the ones that may be added in the future) is pressed, a reaction for
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<code>EvButtonPressed</code> can be defined.</p>
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<h3><a name="Reaction" id="Reaction">Reaction</a></h3>
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<p>A reaction consists of all the side effects caused by the processing of
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one event. Reactions can be categorized as follows:</p>
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<ol>
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<li>In-state reaction</li>
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<li>Event deferral</li>
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<li>Transition</li>
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<li>Termination, also known as transition to the final state (UML)</li>
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</ol>
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<p>Note that it is possible to mix a reaction of type 1 with one of the
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other types (the in-state reaction is always executed first) but it is not
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possible to mix a reaction of type 2-4 with anything else but type 1.</p>
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<p>A reaction is always associated with exactly one state type and exactly
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one event type.</p>
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<h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3>
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<p>A state is unstable from the moment when it has been entered until after
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its last <b>direct</b> inner state has been entered. A state is also
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unstable from the moment just before its first <b>direct</b> inner state is
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exited until right before the state itself is exited.</p>
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<h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state
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machine</a></h3>
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<p>A state machine is unstable if at least one of its currently active
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states is unstable. This is the case during the following three
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operations:</p>
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<ul>
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<li>Initiation: From the moment after the first state has been entered
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until after the last state of the initial state configuration has been
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entered</li>
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<li>Transition: From the moment just before the first state of the
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current state configuration is exited until after the last state of the
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destination state configuration has been entered</li>
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<li>Termination: From the moment just before the first state is exited
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until right before the last terminated state is exited. A successfully
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executed termination (no exception was thrown) never leaves any states
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unstable. For example, consider the active state A with two orthogonal
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regions in which the inner states B and C are each active. Terminating
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either B or C does not make A unstable. Neither does terminating both, as
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that inevitably also terminates A</li>
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</ul>
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<p>Under normal circumstances a state machine has Run-To-Completion
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semantics, that is, it is always stable before the machine returns to the
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client or before the next event is dequeued. So, a state machine is usually
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only unstable when it is busy processing an event and becomes stable again
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right before it has finished processing the event. However, this can not be
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guaranteed when entry, exit or transition actions fail. Such a failure is
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reported by an event, which must be processed while the state machine is
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unstable. However, exception event processing rules ensure that a state
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machine is never unstable when it returns to the client (see <code><a href=
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"reference.html#process_event">state_machine<>::process_event()</a></code>
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for details).</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p>
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<p><i>Copyright © 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
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<a href="contact.html">Andreas Huber Dönni</a></i></p>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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