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         <h1 align="center">The Boost Statechart Library</h1>

         <h2 align="center">Definitions</h2>
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   <hr>

   <h2>Introduction</h2>

   <p>The Boost.Statechart documentation uses a lot of terminology specific to
   state machines. Most of it is equal to the one used in the UML
   specifications. This document contains only definitions for terminology not
   used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML
   standard</a>. A short tour around UML terminology can be found <a href=
   "http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p>

   <h2>Definitions</h2>

   <dl class="page-index">
     <dt><a href="#Context">Context</a></dt>

     <dt><a href="#InnermostCommonContext">Innermost common context</a></dt>

     <dt><a href="#InnermostState">Innermost state</a></dt>

     <dt><a href="#InStateReaction">In-state reaction</a></dt>

     <dt><a href="#OutermostState">Outermost state</a></dt>

     <dt><a href="#PolymorphicEvents">Polymorphic events</a></dt>

     <dt><a href="#Reaction">Reaction</a></dt>

     <dt><a href="#UnstableState">Unstable state</a></dt>

     <dt><a href="#UnstableStateMachine">Unstable state machine</a></dt>
   </dl>

   <h3><a name="Context" id="Context">Context</a></h3>

   <p>The contexts of a state define its location in the state hierarchy. A
   state's <b>direct</b> context is defined by what is passed as the
   <code>Context</code> template parameter of the <code><a href=
   "reference.html#ClassTemplatesimple_state">simple_state</a></code> and
   <code><a href="reference.html#ClassTemplatestate">state</a></code> class
   templates. This can either be the state machine (which makes the state an
   <a href="#OutermostState">outermost state</a>) or its direct outer state. A
   state's <b>indirect</b> contexts follow from the direct context of its
   direct context and the direct context of the direct context of its direct
   context and so on. Examples:</p>

   <p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif"
   border="0" width="467" height="572"></p>

   <ul>
     <li>A's <b>direct</b> context is the state machine (not visible in this
     picture). A does not have any indirect contexts</li>

     <li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the
     state machine (not visible in this picture)</li>

     <li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A
     and the state machine (not visible in this picture)</li>

     <li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the
     state machine (not visible in this picture)</li>
   </ul>

   <h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost
   common context</a></h3>

   <p>The innermost common context of two states is the first direct or
   indirect context that both states have in common. Also known as Least
   Common Ancestor (UML).</p>

   <h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3>

   <p>An innermost state is a state that does not itself have inner states.
   Also known as leaf state or simple state (UML). Note that <code><a href=
   "reference.html#ClassTemplatesimple_state">boost::statechart::simple_state&lt;&gt;</a></code>
   is <b>not</b> a model of the UML simple state.</p>

   <h3><a name="InStateReaction" id="InStateReaction">In-state
   reaction</a></h3>

   <p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither
   exits nor enters any states. Also known as inner transition or internal
   transition (UML).</p>

   <h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3>

   <p>An outermost state is a state that does not itself have outer states.
   Note that an outermost state is different from the UML top state. A state
   machine can have an arbitrary number of the former but only exactly one of
   the latter. Boost.Statechart only supports outermost states.</p>

   <h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic
   events</a></h3>

   <p>An FSM library supports polymorphic events if events can inherit from
   each other without restrictions <b>and</b> if it allows the definition of
   reactions for leafs and nodes of the resulting event inheritance
   tree.</p>

   <p>Example (using a hypothetical FSM library, as Boost.Statechart does not
   support polymorphic events):</p>
   <pre>
 struct EvButtonPressed : Event // node
 {
   /* common button pressed properties */
 };

 struct EvPlayButtonPressed : EvButtonPressed {}; // leaf
 struct EvStopButtonPressed : EvButtonPressed {}; // leaf
 struct EvForwardButtonPressed : EvButtonPressed {}; // leaf
 </pre>

   <p>If a state machine needs to react whenever <b>any</b> button (including
   the ones that may be added in the future) is pressed, a reaction for
   <code>EvButtonPressed</code> can be defined.</p>

   <h3><a name="Reaction" id="Reaction">Reaction</a></h3>

   <p>A reaction consists of all the side effects caused by the processing of
   one event. Reactions can be categorized as follows:</p>

   <ol>
     <li>In-state reaction</li>

     <li>Event deferral</li>

     <li>Transition</li>

     <li>Termination, also known as transition to the final state (UML)</li>
   </ol>

   <p>Note that it is possible to mix a reaction of type 1 with one of the
   other types (the in-state reaction is always executed first) but it is not
   possible to mix a reaction of type 2-4 with anything else but type 1.</p>

   <p>A reaction is always associated with exactly one state type and exactly
   one event type.</p>

   <h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3>

   <p>A state is unstable from the moment when it has been entered until after
   its last <b>direct</b> inner state has been entered. A state is also
   unstable from the moment just before its first <b>direct</b> inner state is
   exited until right before the state itself is exited.</p>

   <h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state
   machine</a></h3>

   <p>A state machine is unstable if at least one of its currently active
   states is unstable. This is the case during the following three
   operations:</p>

   <ul>
     <li>Initiation: From the moment after the first state has been entered
     until after the last state of the initial state configuration has been
     entered</li>

     <li>Transition: From the moment just before the first state of the
     current state configuration is exited until after the last state of the
     destination state configuration has been entered</li>

     <li>Termination: From the moment just before the first state is exited
     until right before the last terminated state is exited. A successfully
     executed termination (no exception was thrown) never leaves any states
     unstable. For example, consider the active state A with two orthogonal
     regions in which the inner states B and C are each active. Terminating
     either B or C does not make A unstable. Neither does terminating both, as
     that inevitably also terminates A</li>
   </ul>

   <p>Under normal circumstances a state machine has Run-To-Completion
   semantics, that is, it is always stable before the machine returns to the
   client or before the next event is dequeued. So, a state machine is usually
   only unstable when it is busy processing an event and becomes stable again
   right before it has finished processing the event. However, this can not be
   guaranteed when entry, exit or transition actions fail. Such a failure is
   reported by an event, which must be processed while the state machine is
   unstable. However, exception event processing rules ensure that a state
   machine is never unstable when it returns to the client (see <code><a href=
   "reference.html#process_event">state_machine&lt;&gt;::process_event()</a></code>
   for details).</p>
   <hr>

   <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
   "../../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
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   <p>Revised
   <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p>

   <p><i>Copyright &copy; 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
   <a href="contact.html">Andreas Huber D&ouml;nni</a></i></p>

   <p><i>Distributed under the Boost Software License, Version 1.0. (See
   accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
   copy at <a href=
   "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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	<h3><a href="../../../index.htm"><img alt="C++ Boost" src=
	"../../../boost.png" border="0" width="277" height="86"></a></h3>
	</td>

	<td valign="top">
	<h1 align="center">The Boost Statechart Library</h1>

	<h2 align="center">Definitions</h2>
	</td>
	</tr>
	</table>
	<hr>

	<h2>Introduction</h2>

	<p>The Boost.Statechart documentation uses a lot of terminology specific to
	state machines. Most of it is equal to the one used in the UML
	specifications. This document contains only definitions for terminology not
	used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML
	standard</a>. A short tour around UML terminology can be found <a href=
	"http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p>

	<h2>Definitions</h2>

	<dl class="page-index">
	<dt><a href="#Context">Context</a></dt>

	<dt><a href="#InnermostCommonContext">Innermost common context</a></dt>

	<dt><a href="#InnermostState">Innermost state</a></dt>

	<dt><a href="#InStateReaction">In-state reaction</a></dt>

	<dt><a href="#OutermostState">Outermost state</a></dt>

	<dt><a href="#PolymorphicEvents">Polymorphic events</a></dt>

	<dt><a href="#Reaction">Reaction</a></dt>

	<dt><a href="#UnstableState">Unstable state</a></dt>

	<dt><a href="#UnstableStateMachine">Unstable state machine</a></dt>
	</dl>

	<h3><a name="Context" id="Context">Context</a></h3>

	<p>The contexts of a state define its location in the state hierarchy. A
	state's <b>direct</b> context is defined by what is passed as the
	<code>Context</code> template parameter of the <code><a href=
	"reference.html#ClassTemplatesimple_state">simple_state</a></code> and
	<code><a href="reference.html#ClassTemplatestate">state</a></code> class
	templates. This can either be the state machine (which makes the state an
	<a href="#OutermostState">outermost state</a>) or its direct outer state. A
	state's <b>indirect</b> contexts follow from the direct context of its
	direct context and the direct context of the direct context of its direct
	context and so on. Examples:</p>

	<p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif"
	border="0" width="467" height="572"></p>

	<ul>
	<li>A's <b>direct</b> context is the state machine (not visible in this
	picture). A does not have any indirect contexts</li>

	<li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the
	state machine (not visible in this picture)</li>

	<li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A
	and the state machine (not visible in this picture)</li>

	<li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the
	state machine (not visible in this picture)</li>
	</ul>

	<h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost
	common context</a></h3>

	<p>The innermost common context of two states is the first direct or
	indirect context that both states have in common. Also known as Least
	Common Ancestor (UML).</p>

	<h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3>

	<p>An innermost state is a state that does not itself have inner states.
	Also known as leaf state or simple state (UML). Note that <code><a href=
	"reference.html#ClassTemplatesimple_state">boost::statechart::simple_state<></a></code>
	is <b>not</b> a model of the UML simple state.</p>

	<h3><a name="InStateReaction" id="InStateReaction">In-state
	reaction</a></h3>

	<p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither
	exits nor enters any states. Also known as inner transition or internal
	transition (UML).</p>

	<h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3>

	<p>An outermost state is a state that does not itself have outer states.
	Note that an outermost state is different from the UML top state. A state
	machine can have an arbitrary number of the former but only exactly one of
	the latter. Boost.Statechart only supports outermost states.</p>

	<h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic
	events</a></h3>

	<p>An FSM library supports polymorphic events if events can inherit from
	each other without restrictions <b>and</b> if it allows the definition of
	reactions for leafs and nodes of the resulting event inheritance
	tree.</p>

	<p>Example (using a hypothetical FSM library, as Boost.Statechart does not
	support polymorphic events):</p>
	<pre>
	struct EvButtonPressed : Event // node
	{
	/* common button pressed properties */
	};

	struct EvPlayButtonPressed : EvButtonPressed {}; // leaf
	struct EvStopButtonPressed : EvButtonPressed {}; // leaf
	struct EvForwardButtonPressed : EvButtonPressed {}; // leaf
	</pre>

	<p>If a state machine needs to react whenever <b>any</b> button (including
	the ones that may be added in the future) is pressed, a reaction for
	<code>EvButtonPressed</code> can be defined.</p>

	<h3><a name="Reaction" id="Reaction">Reaction</a></h3>

	<p>A reaction consists of all the side effects caused by the processing of
	one event. Reactions can be categorized as follows:</p>

	<ol>
	<li>In-state reaction</li>

	<li>Event deferral</li>

	<li>Transition</li>

	<li>Termination, also known as transition to the final state (UML)</li>
	</ol>

	<p>Note that it is possible to mix a reaction of type 1 with one of the
	other types (the in-state reaction is always executed first) but it is not
	possible to mix a reaction of type 2-4 with anything else but type 1.</p>

	<p>A reaction is always associated with exactly one state type and exactly
	one event type.</p>

	<h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3>

	<p>A state is unstable from the moment when it has been entered until after
	its last <b>direct</b> inner state has been entered. A state is also
	unstable from the moment just before its first <b>direct</b> inner state is
	exited until right before the state itself is exited.</p>

	<h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state
	machine</a></h3>

	<p>A state machine is unstable if at least one of its currently active
	states is unstable. This is the case during the following three
	operations:</p>

	<ul>
	<li>Initiation: From the moment after the first state has been entered
	until after the last state of the initial state configuration has been
	entered</li>

	<li>Transition: From the moment just before the first state of the
	current state configuration is exited until after the last state of the
	destination state configuration has been entered</li>

	<li>Termination: From the moment just before the first state is exited
	until right before the last terminated state is exited. A successfully
	executed termination (no exception was thrown) never leaves any states
	unstable. For example, consider the active state A with two orthogonal
	regions in which the inner states B and C are each active. Terminating
	either B or C does not make A unstable. Neither does terminating both, as
	that inevitably also terminates A</li>
	</ul>

	<p>Under normal circumstances a state machine has Run-To-Completion
	semantics, that is, it is always stable before the machine returns to the
	client or before the next event is dequeued. So, a state machine is usually
	only unstable when it is busy processing an event and becomes stable again
	right before it has finished processing the event. However, this can not be
	guaranteed when entry, exit or transition actions fail. Such a failure is
	reported by an event, which must be processed while the state machine is
	unstable. However, exception event processing rules ensure that a state
	machine is never unstable when it returns to the client (see <code><a href=
	"reference.html#process_event">state_machine<>::process_event()</a></code>
	for details).</p>
	<hr>

	<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
	"../../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
	height="31" width="88"></a></p>

	<p>Revised
	<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p>

	<p><i>Copyright © 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
	<a href="contact.html">Andreas Huber Dönni</a></i></p>

	<p><i>Distributed under the Boost Software License, Version 1.0. (See
	accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
	copy at <a href=
	"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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