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11  <title>The Boost Statechart Library - Definitions</title>
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19        <h3><a href="../../../index.htm"><img alt="C++ Boost" src=
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23      <td valign="top">
24        <h1 align="center">The Boost Statechart Library</h1>
25
26        <h2 align="center">Definitions</h2>
27      </td>
28    </tr>
29  </table>
30  <hr>
31
32  <h2>Introduction</h2>
33
34  <p>The Boost.Statechart documentation uses a lot of terminology specific to
35  state machines. Most of it is equal to the one used in the UML
36  specifications. This document contains only definitions for terminology not
37  used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML
38  standard</a>. A short tour around UML terminology can be found <a href=
39  "http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p>
40
41  <h2>Definitions</h2>
42
43  <dl class="page-index">
44    <dt><a href="#Context">Context</a></dt>
45
46    <dt><a href="#InnermostCommonContext">Innermost common context</a></dt>
47
48    <dt><a href="#InnermostState">Innermost state</a></dt>
49
50    <dt><a href="#InStateReaction">In-state reaction</a></dt>
51
52    <dt><a href="#OutermostState">Outermost state</a></dt>
53
54    <dt><a href="#PolymorphicEvents">Polymorphic events</a></dt>
55
56    <dt><a href="#Reaction">Reaction</a></dt>
57
58    <dt><a href="#UnstableState">Unstable state</a></dt>
59
60    <dt><a href="#UnstableStateMachine">Unstable state machine</a></dt>
61  </dl>
62
63  <h3><a name="Context" id="Context">Context</a></h3>
64
65  <p>The contexts of a state define its location in the state hierarchy. A
66  state's <b>direct</b> context is defined by what is passed as the
67  <code>Context</code> template parameter of the <code><a href=
68  "reference.html#ClassTemplatesimple_state">simple_state</a></code> and
69  <code><a href="reference.html#ClassTemplatestate">state</a></code> class
70  templates. This can either be the state machine (which makes the state an
71  <a href="#OutermostState">outermost state</a>) or its direct outer state. A
72  state's <b>indirect</b> contexts follow from the direct context of its
73  direct context and the direct context of the direct context of its direct
74  context and so on. Examples:</p>
75
76  <p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif"
77  border="0" width="467" height="572"></p>
78
79  <ul>
80    <li>A's <b>direct</b> context is the state machine (not visible in this
81    picture). A does not have any indirect contexts</li>
82
83    <li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the
84    state machine (not visible in this picture)</li>
85
86    <li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A
87    and the state machine (not visible in this picture)</li>
88
89    <li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the
90    state machine (not visible in this picture)</li>
91  </ul>
92
93  <h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost
94  common context</a></h3>
95
96  <p>The innermost common context of two states is the first direct or
97  indirect context that both states have in common. Also known as Least
98  Common Ancestor (UML).</p>
99
100  <h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3>
101
102  <p>An innermost state is a state that does not itself have inner states.
103  Also known as leaf state or simple state (UML). Note that <code><a href=
104  "reference.html#ClassTemplatesimple_state">boost::statechart::simple_state&lt;&gt;</a></code>
105  is <b>not</b> a model of the UML simple state.</p>
106
107  <h3><a name="InStateReaction" id="InStateReaction">In-state
108  reaction</a></h3>
109
110  <p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither
111  exits nor enters any states. Also known as inner transition or internal
112  transition (UML).</p>
113
114  <h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3>
115
116  <p>An outermost state is a state that does not itself have outer states.
117  Note that an outermost state is different from the UML top state. A state
118  machine can have an arbitrary number of the former but only exactly one of
119  the latter. Boost.Statechart only supports outermost states.</p>
120
121  <h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic
122  events</a></h3>
123
124  <p>An FSM library supports polymorphic events if events can inherit from
125  each other without restrictions <b>and</b> if it allows the definition of
126  reactions for leafs and nodes of the resulting event inheritance
127  tree.</p>
128
129  <p>Example (using a hypothetical FSM library, as Boost.Statechart does not
130  support polymorphic events):</p>
131  <pre>
132struct EvButtonPressed : Event // node
133{
134  /* common button pressed properties */
135};
136
137struct EvPlayButtonPressed : EvButtonPressed {}; // leaf
138struct EvStopButtonPressed : EvButtonPressed {}; // leaf
139struct EvForwardButtonPressed : EvButtonPressed {}; // leaf
140</pre>
141
142  <p>If a state machine needs to react whenever <b>any</b> button (including
143  the ones that may be added in the future) is pressed, a reaction for
144  <code>EvButtonPressed</code> can be defined.</p>
145
146  <h3><a name="Reaction" id="Reaction">Reaction</a></h3>
147
148  <p>A reaction consists of all the side effects caused by the processing of
149  one event. Reactions can be categorized as follows:</p>
150
151  <ol>
152    <li>In-state reaction</li>
153
154    <li>Event deferral</li>
155
156    <li>Transition</li>
157
158    <li>Termination, also known as transition to the final state (UML)</li>
159  </ol>
160
161  <p>Note that it is possible to mix a reaction of type 1 with one of the
162  other types (the in-state reaction is always executed first) but it is not
163  possible to mix a reaction of type 2-4 with anything else but type 1.</p>
164
165  <p>A reaction is always associated with exactly one state type and exactly
166  one event type.</p>
167
168  <h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3>
169
170  <p>A state is unstable from the moment when it has been entered until after
171  its last <b>direct</b> inner state has been entered. A state is also
172  unstable from the moment just before its first <b>direct</b> inner state is
173  exited until right before the state itself is exited.</p>
174
175  <h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state
176  machine</a></h3>
177
178  <p>A state machine is unstable if at least one of its currently active
179  states is unstable. This is the case during the following three
180  operations:</p>
181
182  <ul>
183    <li>Initiation: From the moment after the first state has been entered
184    until after the last state of the initial state configuration has been
185    entered</li>
186
187    <li>Transition: From the moment just before the first state of the
188    current state configuration is exited until after the last state of the
189    destination state configuration has been entered</li>
190
191    <li>Termination: From the moment just before the first state is exited
192    until right before the last terminated state is exited. A successfully
193    executed termination (no exception was thrown) never leaves any states
194    unstable. For example, consider the active state A with two orthogonal
195    regions in which the inner states B and C are each active. Terminating
196    either B or C does not make A unstable. Neither does terminating both, as
197    that inevitably also terminates A</li>
198  </ul>
199
200  <p>Under normal circumstances a state machine has Run-To-Completion
201  semantics, that is, it is always stable before the machine returns to the
202  client or before the next event is dequeued. So, a state machine is usually
203  only unstable when it is busy processing an event and becomes stable again
204  right before it has finished processing the event. However, this can not be
205  guaranteed when entry, exit or transition actions fail. Such a failure is
206  reported by an event, which must be processed while the state machine is
207  unstable. However, exception event processing rules ensure that a state
208  machine is never unstable when it returns to the client (see <code><a href=
209  "reference.html#process_event">state_machine&lt;&gt;::process_event()</a></code>
210  for details).</p>
211  <hr>
212
213  <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
214  "http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional"
215  height="31" width="88"></a></p>
216
217  <p>Revised
218  <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p>
219
220  <p><i>Copyright &copy; 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
221  <a href="contact.html">Andreas Huber D&ouml;nni</a></i></p>
222
223  <p><i>Distributed under the Boost Software License, Version 1.0. (See
224  accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
225  copy at <a href=
226  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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