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<h3><a href="../../../index.htm"><img alt="C++ Boost" src=
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<h1 align="center">The Boost Statechart Library</h1>
<h2 align="center">Frequently Asked Questions (FAQs)</h2>
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<hr>
<dl class="page-index">
<dt><a href="#StateLocalStorage">What's so cool about state-local
storage?</a></dt>
<dt><a href="#HideInnerWorkings">How can I hide the inner workings of a
state machine from its clients?</a></dt>
<dt><a href="#MachineInheritance">Is it possible to inherit from a given
state machine and modify its layout in the subclass?</a></dt>
<dt><a href="#Uml2">What about UML2.0 features?</a></dt>
<dt><a href="#AssertInStateDestructor">Why do I get an assert when I
access the state machine from a state destructor?</a></dt>
<dt><a href="#EmbeddedApplications">Is Boost.Statechart suitable for
embedded applications?</a></dt>
<dt><a href="#HardRealtime">Is your library suitable for applications
with hard real-time requirements?</a></dt>
<dt><a href="#TemplatedStates">With templated states I get an error that
'inner_context_type' is not defined. What's wrong?</a></dt>
<dt><a href="#CompilerError">My compiler reports an error in the library
code. Is this a bug in Boost.Statechart?</a></dt>
<dt><a href="#DisableHistory">Is it possible to disable history for a
state at runtime?</a></dt>
<dt><a href="#Dll">How can I compile a state machine into a dynamic link
library (DLL)?</a></dt>
<dt><a href="#PolymorphicEvents">Does Boost.Statechart support
polymorphic events?</a></dt>
<dt><a href="#WrongExitActionOrder">Why are exit-actions called in the
wrong order when I use multiple inheritance?</a></dt>
</dl>
<h2><a name="StateLocalStorage" id="StateLocalStorage">What's so cool about
state-local storage?</a></h2>
<p>This is best explained with an example:</p>
<pre>
struct Active;
struct Stopped;
struct Running;
struct StopWatch : sc::state_machine&lt; StopWatch, Active &gt;
{
// startTime_ remains uninitialized, because there is no reasonable default
StopWatch() : elapsedTime_( 0.0 ) {}
~StopWatch()
{
terminate();
}
double ElapsedTime() const
{
// Ugly switch over the current state.
if ( state_cast&lt; const Stopped * &gt;() != 0 )
{
return elapsedTime_;
}
else if ( state_cast&lt; const Running * &gt;() != 0 )
{
return elapsedTime_ + std::difftime( std::time( 0 ), startTime_ );
}
else // we're terminated
{
throw std::bad_cast();
}
}
// elapsedTime_ is only meaningful when the machine is not terminated
double elapsedTime_;
// startTime_ is only meaningful when the machine is in Running
std::time_t startTime_;
};
struct Active : sc::state&lt; Active, StopWatch, Stopped &gt;
{
typedef sc::transition&lt; EvReset, Active &gt; reactions;
Active( my_context ctx ) : my_base( ctx )
{
outermost_context().elapsedTime_ = 0.0;
}
};
struct Running : sc::state&lt; Running, Active &gt;
{
typedef sc::transition&lt; EvStartStop, Stopped &gt; reactions;
Running( my_context ctx ) : my_base( ctx )
{
outermost_context().startTime_ = std::time( 0 );
}
~Running()
{
outermost_context().elapsedTime_ +=
std::difftime( std::time( 0 ), outermost_context().startTime_ );
}
};
struct Stopped : sc::simple_state&lt; Stopped, Active &gt;
{
typedef sc::transition&lt; EvStartStop, Running &gt; reactions;
};
</pre>
<p>This StopWatch does not make any use of state-local storage while
implementing the same behavior as the <a href=
"tutorial.html#BasicTopicsAStopWatch">tutorial StopWatch</a>. Even though
this code is probably easier to read for the untrained eye, it does have a
few problems that are absent in the original:</p>
<ul>
<li>This StopWatch class has data members that have a meaningful value
only if the state machine happens to be in a certain state. That is, the
lifetimes of these variables are not identical with the one of the
StopWatch object containing them. Since the lifetimes are managed by the
entry and exit actions of states, we need to use an ugly switch over the
current state (see <code>StopWatch::ElapsedTime()</code>) if we want to
access them from a context where the current state is unclear. This
essentially duplicates some of the state logic of the FSM. Therefore,
whenever we need to change the layout of the state machine we will likely
also need to change the ugly switch. Even worse, if we forget to change
the switch, the code will probably still compile and maybe even silently
do the wrong thing. Note that this is impossible with the version in the
tutorial, which will at least throw an exception and often just refuse to
compile. Moreover, for the tutorial StopWatch there's a much higher
chance that a programmer will get a change correct the first time since
the code that calculates the elapsed time is located close to the code
that updates the variables</li>
<li>We need to change the StopWatch class whenever we want to introduce a
new variable or change the type of an already existing variable. That is,
many changes in the FSM will likely lead to a change in the StopWatch
class. In all FSMs that do not employ state-local storage, the
<code>state_machine&lt;&gt;</code> subtype will therefore be a change
hotspot, which is a pretty sure indicator for a bad design</li>
</ul>
<p>Both points are not much of a problem in a small example like this,
which can easily be implemented in a single translation unit by a single
programmer. However, they quickly become a major problem for a big complex
machine spread over multiple translation units, which are possibly even
maintained by different programmers.</p>
<h2><a name="HideInnerWorkings" id="HideInnerWorkings">How can I hide the
inner workings of a state machine from its clients?</a></h2>
<p>To see why and how this is possible it is important to recall the
following facts:</p>
<ul>
<li>Member functions of a C++ class template are instantiated at the
point where they're actually called. If the function is never called, it
will not be instantiated and not a single assembly instruction will ever
be generated</li>
<li>The <code>InitialState</code> template parameter of
<code>sc::state_machine</code> can be an incomplete type (i.e. forward
declared)</li>
</ul>
<p>The class template member function
<code>state_machine&lt;&gt;::initiate()</code> creates an object of the
initial state. So, the definition of this state must be known before the
compiler reaches the point where <code>initiate()</code> is called. To be
able to hide the initial state of a state machine in a .cpp file we must
therefore no longer let clients call <code>initiate()</code>. Instead, we
do so in the .cpp file, at a point where the full definition of the initial
state is known.</p>
<p>Example:</p>
<p>StopWatch.hpp:</p>
<pre>
// define events ...
struct Active; // the only visible forward
struct StopWatch : sc::state_machine&lt; StopWatch, Active &gt;
{
StopWatch();
};
</pre>
<p>StopWatch.cpp:</p>
<pre>
struct Stopped;
struct Active : sc::simple_state&lt; Active, StopWatch, Stopped &gt;
{
typedef sc::transition&lt; EvReset, Active &gt; reactions;
};
struct Running : sc::simple_state&lt; Running, Active &gt;
{
typedef sc::transition&lt; EvStartStop, Stopped &gt; reactions;
};
struct Stopped : sc::simple_state&lt; Stopped, Active &gt;
{
typedef sc::transition&lt; EvStartStop, Running &gt; reactions;
};
StopWatch::StopWatch()
{
// For example, we might want to ensure that the state
// machine is already started after construction.
// Alternatively, we could add our own initiate() function
// to StopWatch and call the base class initiate() in the
// implementation.
<b>initiate();</b>
}
</pre>
<p>The PingPong example demonstrates how the inner workings of an
asynchronous_state_machine<> subclass can be hidden.</p>
<h2><a name="MachineInheritance" id="MachineInheritance">Is it possible to
inherit from a given state machine and modify its layout in the
subclass?</a></h2>
<p>Yes, but contrary to what some FSM code generators allow,
Boost.Statechart machines can do so only in a way that was foreseen by the
designer of the base state machine:</p>
<pre>
struct EvStart : sc::event&lt; EvStart &gt; {};
struct Idle;
struct PumpBase : sc::state_machine&lt; PumpBase, Idle &gt;
{
<b>virtual sc::result react(
</b> <b>Idle &amp; idle, const EvStart &amp; ) const;
</b>};
struct Idle : sc::simple_state&lt; Idle, PumpBase &gt;
{
typedef sc::custom_reaction&lt; EvStart &gt; reactions;
sc::result react( const EvStart &amp; evt )
{
<b>return context&lt; PumpBase &gt;().react( *this, evt );</b>
}
};
struct Running : sc::simple_state&lt; Running, PumpBase &gt; {};
sc::result PumpBase::react(
Idle &amp; idle, const EvStart &amp; ) const
{
<b>return idle.transit&lt; Running &gt;();
</b>}
struct MyRunning : sc::simple_state&lt; MyRunning, PumpBase &gt; {};
struct MyPump : PumpBase
{
virtual sc::result react(
Idle &amp; idle, const EvStart &amp; ) const
{
<b>return idle.transit&lt; MyRunning &gt;();
</b> }
};
</pre>
<h2><a name="Uml2" id="Uml2">What about UML 2.0 features?</a></h2>
<p>The library was designed before 2.0 came along. Therefore, if not
explicitly noted otherwise, the library implements the behavior mandated by
the UML1.5 standard. Here's an incomplete list of differences between the
2.0 semantics &amp; Boost.Statechart semantics:</p>
<ul>
<li>All transitions are always external. Local transitions are not
supported at all. Unfortunately, the UML2.0 specifications are not
entirely clear how local transitions are supposed to work, see <a href=
"http://thread.gmane.org/gmane.comp.lib.boost.user/18641">here</a> for
more information</li>
<li>There is no direct support for the UML2.0 elements entry point and
exit point. However, both can easily be simulated, the former with a
typedef and the latter with a state that is a template (with the
transition destination as a template parameter)</li>
</ul>
<h2><a name="AssertInStateDestructor" id="AssertInStateDestructor">Why do I
get an assert when I access the state machine from a state destructor?</a>
</h2>
<p>When compiled with <code>NDEBUG</code> undefined, running the following
program results in a failed assert:</p>
<pre>#include &lt;boost/statechart/state_machine.hpp&gt;
#include &lt;boost/statechart/simple_state.hpp&gt;
#include &lt;iostream&gt;
struct Initial;
struct Machine : boost::statechart::state_machine&lt; Machine, Initial &gt;
{
Machine() { someMember_ = 42; }
int someMember_;
};
struct Initial : boost::statechart::simple_state&lt; Initial, Machine &gt;
{
~Initial() { std::cout &lt;&lt; outermost_context().someMember_; }
};
int main()
{
Machine().initiate();
return 0;
}</pre>
<p>The problem arises because <code>state_machine&lt;&gt;::~state_machine</code>
inevitably destructs all remaining active states. At this time,
<code>Machine::~Machine</code> has already been run, making it illegal to
access any of the <code>Machine</code> members. This problem can be avoided
by defining the following destructor:</p>
<pre>~Machine() { terminate(); }</pre>
<h2><a name="EmbeddedApplications" id="EmbeddedApplications">Is
Boost.Statechart suitable for embedded applications?</a></h2>
<p>It depends. As explained under <a href=
"performance.html#SpeedVersusScalabilityTradeoffs">Speed versus scalability
tradeoffs</a> on the Performance page, the virtually limitless scalability
offered by this library does have its price. Especially small and simple
FSMs can easily be implemented so that they consume fewer cycles and less
memory and occupy less code space in the executable. Here are some
obviously <b>very rough</b> estimates:</p>
<ul>
<li>For a state machine with at most one simultaneously active state
(that is, the machine is flat and does not have orthogonal regions) with
trivial actions, customized memory management and compiled with a good
optimizing compiler, a Pentium 4 class CPU should not spend more than
1000 cycles inside <code>state_machine&lt;&gt;::process_event()</code>.
This worst-case time to process one event scales more or less linearly
with the number of simultaneously active states for more complex state
machines, with the typical average being much lower than that. So, a
fairly complex machine with at most 10 simultaneously active states
running on a 100MHz CPU should be able to process more than 10'000 events
per second</li>
<li>A single state machine object uses typically less than 1KB of memory,
even if it implements a very complex machine</li>
<li>For code size, it is difficult to give a concrete guideline but tests
with the BitMachine example suggest that code size scales more or less
linearly with the number of states (transitions seem to have only little
impact). When compiled with MSVC7.1 on Windows, 32 states and 224
transitions seem to fit in ~108KB executable code (with all optimizations
turned on).<br>
Moreover, the library can be compiled with C++ RTTI and exception
handling turned off, resulting in significant savings on most
platforms</li>
</ul>
<p>As mentioned above, these are very rough estimates derived from the use
of the library on a desktop PC, so they should only be used to decide
whether there is a point in making your own performance tests on your
target platform.</p>
<h2><a name="HardRealtime" id="HardRealtime">Is your library suitable for
applications with hard real-time requirements?</a></h2>
<p>Yes. Out of the box, the only operations taking potentially
non-deterministic time that the library performs are calls to
<code>std::allocator&lt;&gt;</code> member functions and
<code>dynamic_cast</code>s. <code>std::allocator&lt;&gt;</code> member
function calls can be avoided by passing a custom allocator to
<code>event&lt;&gt;</code>, <code>state_machine&lt;&gt;</code>,
<code>asynchronous_state_machine&lt;&gt;</code>,
<code>fifo_scheduler&lt;&gt;</code> and <code>fifo_worker&lt;&gt;</code>.
<code>dynamic_cast</code>s can be avoided by not calling the
<code>state_cast&lt;&gt;</code> member functions of
<code>state_machine&lt;&gt;</code>, <code>simple_state&lt;&gt;</code> and
<code>state&lt;&gt;</code> but using the deterministic variant
<code>state_downcast&lt;&gt;</code> instead.</p>
<h2><a name="TemplatedStates" id="TemplatedStates">With templated states I
get an error that 'inner_context_type' is not defined. What's
wrong?</a></h2>
<p>The following code generates such an error:</p>
<pre>
#include &lt;boost/statechart/state_machine.hpp&gt;
#include &lt;boost/statechart/simple_state.hpp&gt;
namespace sc = boost::statechart;
template&lt; typename X &gt; struct A;
struct Machine : sc::state_machine&lt; Machine, A&lt; int &gt; &gt; {};
template&lt; typename X &gt; struct B;
template&lt; typename X &gt;
struct A : sc::simple_state&lt; A&lt; X &gt;, Machine, B&lt; X &gt; &gt; {};
template&lt; typename X &gt;
struct B : sc::simple_state&lt; B&lt; X &gt;, A&lt; X &gt; &gt; {};
int main()
{
Machine machine;
machine.initiate();
return 0;
}
</pre>
<p>If the templates <code>A</code> and <code>B</code> are replaced with
normal types, the above code compiles without errors. This is rooted in the
fact that C++ treats forward-declared templates differently than
forward-declared types. Namely, the compiler tries to access member
typedefs of <code>B&lt; X &gt;</code> at a point where the template has not
yet been defined. Luckily, this can easily be avoided by putting all inner
initial state arguments in an <code>mpl::list&lt;&gt;</code>, as
follows:</p>
<pre>
struct A : sc::simple_state&lt;
A&lt; X &gt;, Machine, mpl::list&lt; B&lt; X &gt; &gt; &gt; {};
</pre>
<p>See <a href=
"http://article.gmane.org/gmane.comp.lib.boost.devel/128741">this post</a>
for technical details.</p>
<h2><a name="CompilerError" id="CompilerError">My compiler reports an error
in the library code. Is this a bug in Boost.Statechart?</a></h2>
<p>Probably not. There are several possible reasons for such compile-time
errors:</p>
<ol>
<li>Your compiler is too buggy to compile the library, see <a href=
"index.html#SupportedPlatforms">here</a> for information on the status of
your compiler. If you absolutely must use such a compiler for your
project, I'm afraid Boost.Statechart is not for you.</li>
<li>The error is reported on a line similar to the following:
<pre>
BOOST_STATIC_ASSERT( ( mpl::less&lt;
orthogonal_position,
typename context_type::no_of_orthogonal_regions &gt;::value ) );
</pre>Most probably, there is an error in your code. The library has many
such compile-time assertions to ensure that invalid state machines cannot be
compiled (for an idea what kinds of errors are reported at compile time, see
the compile-fail tests). Above each of these assertions there is a comment
explaining the problem. On almost all current compilers an error in template
code is accompanied by the current "instantiation stack". Very much like the
call stack you see in the debugger, this "instantiation stack" allows you to
trace the error back through instantiations of library code until you hit the
line of your code that causes the problem. As an example, here's the MSVC7.1
error message for the code in InconsistentHistoryTest1.cpp:
<pre>
...\boost\statechart\shallow_history.hpp(34) : error C2027: use of undefined type 'boost::STATIC_ASSERTION_FAILURE&lt;x&gt;'
with
[
x=false
]
...\boost\statechart\shallow_history.hpp(34) : see reference to class template instantiation 'boost::STATIC_ASSERTION_FAILURE&lt;x&gt;' being compiled
with
[
x=false
]
...\boost\statechart\simple_state.hpp(861) : see reference to class template instantiation 'boost::statechart::shallow_history&lt;DefaultState&gt;' being compiled
with
[
DefaultState=B
]
...\boost\statechart\simple_state.hpp(599) : see reference to function template instantiation 'void boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::deep_construct_inner_impl_non_empty::deep_construct_inner_impl&lt;InnerList&gt;(const boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::inner_context_ptr_type &amp;,boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::outermost_context_base_type &amp;)' being compiled
with
[
MostDerived=A,
Context=InconsistentHistoryTest,
InnerInitial=boost::mpl::list&lt;boost::statechart::shallow_history&lt;B&gt;&gt;,
InnerList=boost::statechart::simple_state&lt;A,InconsistentHistoryTest,boost::mpl::list&lt;boost::statechart::shallow_history&lt;B&gt;&gt;&gt;::inner_initial_list
]
...\boost\statechart\simple_state.hpp(567) : see reference to function template instantiation 'void boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::deep_construct_inner&lt;boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::inner_initial_list&gt;(const boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::inner_context_ptr_type &amp;,boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::outermost_context_base_type &amp;)' being compiled
with
[
MostDerived=A,
Context=InconsistentHistoryTest,
InnerInitial=boost::mpl::list&lt;boost::statechart::shallow_history&lt;B&gt;&gt;
]
...\boost\statechart\simple_state.hpp(563) : while compiling class-template member function 'void boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::deep_construct(const boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::context_ptr_type &amp; ,boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;::outermost_context_base_type &amp;)'
with
[
MostDerived=A,
Context=InconsistentHistoryTest,
InnerInitial=boost::mpl::list&lt;boost::statechart::shallow_history&lt;B&gt;&gt;
]
...\libs\statechart\test\InconsistentHistoryTest1.cpp(29) : see reference to class template instantiation 'boost::statechart::simple_state&lt;MostDerived,Context,InnerInitial&gt;' being compiled
with
[
MostDerived=A,
Context=InconsistentHistoryTest,
InnerInitial=boost::mpl::list&lt;boost::statechart::shallow_history&lt;B&gt;&gt;
]
</pre>Depending on the IDE you use, it is possible that you need to switch to
another window to see this full error message (e.g. for Visual Studio 2003,
you need to switch to the Output window). Starting at the top and going down
the list of instantiations you see that each of them is accompanied by a file
name and a line number. Ignoring all files belonging to the library, we find
the culprit close to the bottom in file InconsistentHistoryTest1.cpp on line
29.
</li>
<li>The error is reported on a line nowhere near a BOOST_STATIC_ASSERT.
Use the technique described under point 2 to see what line of your code
causes the problem. If your code is correct then you've found a bug in
either the compiler or Boost.Statechart. Please <a href=
"contact.html">send me</a> a small but complete program showing the
problem. Thank you!</li>
</ol>
<h2><a name="DisableHistory" id="DisableHistory">Is it possible to disable
history for a state at runtime?</a></h2>
<p>Yes, see <a href=
"reference.html#clear_shallow_history">simple_state::clear_shallow_history()</a>
and <a href=
"reference.html#clear_deep_history">simple_state::clear_deep_history()</a>.
Calling these functions is often preferable to introducting additional
normal transitions when ...</p>
<ul>
<li>a state with history is the target of many transitions,
<b>and/or</b></li>
<li>the decision to ignore history is made in a different place than
the transition to a state with history</li>
</ul>
<h2><a name="Dll" id="Dll">How can I compile a state machine into a dynamic
link library (DLL)?</a></h2>
<p>Invisible to the user, the library uses static data members to implement
its own speed-optimized RTTI-mechanism for <code>event&lt;&gt;</code> and
<code>simple_state&lt;&gt;</code> subtypes. Whenever such a subtype is
defined in a header file and then included in multiple TUs, the linker
later needs to eliminate the duplicate definitions of static data members.
This usually works flawlessly as long as all these TUs are
<b>statically</b> linked into the same binary. It is a lot more complex
when DLLs are involved. The TuTest*.?pp files illustrate this:</p>
<ul>
<li><a href="../test/TuTest.hpp">TuTest.hpp</a>: Instantiates a class
template containing a static data member</li>
<li><a href="../test/TuTest.cpp">TuTest.cpp</a>: Includes TuTest.hpp and
is compiled into a DLL</li>
<li><a href="../test/TuTestMain.cpp">TuTestMain.cpp</a>: Includes
TuTest.hpp and is compiled into an executable</li>
</ul>
<p>Without any precautions (e.g. <code>__declspec(dllexport)</code> on MSVC
compatible compilers), on most platforms both binaries (exe &amp; dll) now
contain their own instance of the static data member. Since the RTTI
mechanism assumes that there is exactly one object of that member at
runtime, the mechanism fails spectacularly when the process running the exe
also loads the dll. Different platforms deal differently with this
problem:</p>
<ul>
<li>On some platforms (e.g. MinGW) there simply doesn't seem to be a way
to enforce that such a member only exists once at runtime. Therefore, the
internal RTTI mechanism cannot be used reliably in conjunction with DLLs.
Disabling it by defining <a href=
"configuration.html#ApplicationDefinedMacros">BOOST_STATECHART_USE_NATIVE_RTTI</a>
in all TUs will <b>usually</b> work around the problem</li>
<li>MSVC-compatible compilers support <code>__declspec(dllimport)</code>
and <code>__declspec(dllexport)</code>, which allow to define exactly
what needs to be loaded from a DLL (see TuTest for an example how to do
this). Therefore, the internal RTTI mechanism can be used but care must
be taken to correctly export and import all <code>event&lt;&gt;</code>
and <code>simple_state&lt;&gt;</code> subtypes defined in headers that
are compiled into more than one binary. Alternatively, of course <a href=
"configuration.html#ApplicationDefinedMacros">BOOST_STATECHART_USE_NATIVE_RTTI</a>
can also be used to save the work of importing and exporting</li>
</ul>
<h2><a name="PolymorphicEvents" id="PolymorphicEvents">Does
Boost.Statechart support polymorphic events?</a></h2>
<p>No. Although events can be derived from each other to write common code
only once, <a href="definitions.html#Reaction">reactions</a> can only be
defined for most-derived events.</p>
<p>Example:</p>
<pre>
template&lt; class MostDerived &gt;
struct EvButtonPressed : sc::event&lt; MostDerived &gt;
{
// common code
};
struct EvPlayButtonPressed :
EvButtonPressed&lt; EvPlayButtonPressed &gt; {};
struct EvStopButtonPressed :
EvButtonPressed&lt; EvStopButtonPressed &gt; {};
struct EvForwardButtonPressed :
EvButtonPressed&lt; EvForwardButtonPressed &gt; {};
/* ... */
// We want to turn the player on, no matter what button we
// press in the Off state. Although we can write the reaction
// code only once, we must mention all most-derived events in
// the reaction list.
struct Off : sc::simple_state&lt; Off, Mp3Player &gt;
{
typedef mpl::list&lt;
sc::custom_reaction&lt; EvPlayButtonPressed &gt;,
sc::custom_reaction&lt; EvStopButtonPressed &gt;,
sc::custom_reaction&lt; EvForwardButtonPressed &gt;
&gt; reactions;
template&lt; class MostDerived &gt;
sc::result react( const EvButtonPressed&lt; MostDerived &gt; &amp; )
{
// ...
}
};
</pre>
<h2><a name="WrongExitActionOrder" id="WrongExitActionOrder">Why are
exit-actions called in the wrong order when I use multiple
inheritance?</a></h2>
<p><b>Update</b>: The implementation has changed considerably in this area.
It is still possible to get this behavior under rare circumstances (when an
action propagates an exception in a state machine with orthogonal regions
<b>and</b> if the statechart layout satisfies certain conditions), but it
can no longer be demonstrated with the example program below. However, the
described workaround is still valid and ensures that this behavior will
never show up.</p>
<p>They definitely aren't for the <code>simple_state&lt;&gt;</code> and
<code>state&lt;&gt;</code> subtypes, but the destructors of additional
bases might be called in construction order (rather than the reverse
construction order):</p>
<pre>
#include &lt;boost/statechart/state_machine.hpp&gt;
#include &lt;boost/statechart/simple_state.hpp&gt;
namespace sc = boost::statechart;
class EntryExitDisplayer
{
protected:
EntryExitDisplayer( const char * pName ) :
pName_( pName )
{
std::cout &lt;&lt; pName_ &lt;&lt; " entered\n";
}
~EntryExitDisplayer()
{
std::cout &lt;&lt; pName_ &lt;&lt; " exited\n";
}
private:
const char * const pName_;
};
struct Outer;
struct Machine : sc::state_machine&lt; Machine, Outer &gt; {};
struct Inner;
struct Outer : EntryExitDisplayer, sc::simple_state&lt;
Outer, Machine, Inner &gt;
{
Outer() : EntryExitDisplayer( "Outer" ) {}
};
struct Inner : EntryExitDisplayer,
sc::simple_state&lt; Inner, Outer &gt;
{
Inner() : EntryExitDisplayer( "Inner" ) {}
};
int main()
{
Machine myMachine;
myMachine.initiate();
return 0;
}
</pre>
<p>This program will produce the following output:</p>
<pre>
Outer entered
Inner entered
Outer exited
Inner exited
</pre>
<p>That is, the <b><code>EntryExitDisplayer</code> base class portion</b>
of <code>Outer</code> is destructed before the one of <code>Inner</code>
although <code>Inner::~Inner()</code> is called before
<code>Outer::~Outer()</code>. This somewhat counter-intuitive behavior is
caused by the following facts:</p>
<ul>
<li>The <code>simple_state&lt;&gt;</code> base class portion of
<code>Inner</code> is responsible to destruct <code>Outer</code></li>
<li>Destructors of base class portions are called in the reverse order of
construction</li>
</ul>
<p>So, when the <code>Outer</code> destructor is called the call stack
looks as follows:</p>
<pre>
Outer::~Outer()
simple_state&lt; Inner, ... &gt;::~simple_state()
Inner::~Inner()
</pre>
<p>Note that <code>Inner::~Inner()</code> did not yet have a chance to
destroy its <code>EntryExitDisplayer</code> base class portion, as it first
has to call the destructor of the <b>second</b> base class. Now
<code>Outer::~Outer()</code> will first destruct its <code>simple_state&lt;
Outer, ... &gt;</code> base class portion and then do the same with its
<code>EntryExitDisplayer</code> base class portion. The stack then unwinds
back to <code>Inner::~Inner()</code>, which can then finally finish by
calling <code>EntryExitDisplayer::~EntryExitDisplayer()</code>.</p>
<p>Luckily, there is an easy work-around: Always let
<code>simple_state&lt;&gt;</code> and <code>state&lt;&gt;</code> be the
first base class of a state. This ensures that destructors of additional
bases are called before recursion employed by state base destructors can
alter the order of destruction.</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 05 January, 2008</p>
<p><i>Copyright &copy; 2003-2008 <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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