The Boost Statechart Library - FAQ

###  | # The Boost Statechart Library ## Frequently Asked Questions (FAQs)

What's so cool about state-local storage?
How can I hide the inner workings of a state machine from its clients?
Is it possible to inherit from a given state machine and modify its layout in the subclass?
What about UML2.0 features?
Why do I get an assert when I access the state machine from a state destructor?
Is Boost.Statechart suitable for embedded applications?
Is your library suitable for applications with hard real-time requirements?
With templated states I get an error that 'inner_context_type' is not defined. What's wrong?
My compiler reports an error in the library code. Is this a bug in Boost.Statechart?
Is it possible to disable history for a state at runtime?
How can I compile a state machine into a dynamic link library (DLL)?
Does Boost.Statechart support polymorphic events?
Why are exit-actions called in the wrong order when I use multiple inheritance?

## What's so cool about state-local storage?

This is best explained with an example:

    struct Active;
    struct Stopped;
    struct Running;
    struct StopWatch : sc::state_machine< StopWatch, Active >
    {
      // 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< const Stopped * >() != 0 )
        {
          return elapsedTime_;
        }
        else if ( state_cast< const Running * >() != 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< Active, StopWatch, Stopped >
    {
      typedef sc::transition< EvReset, Active > reactions;

      Active( my_context ctx ) : my_base( ctx )
      {
        outermost_context().elapsedTime_ = 0.0;
      }
    };

      struct Running : sc::state< Running, Active >
      {
        typedef sc::transition< EvStartStop, Stopped > 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< Stopped, Active >
      {
        typedef sc::transition< EvStartStop, Running > reactions;
      };

This StopWatch does not make any use of state-local storage while implementing the same behavior as the tutorial StopWatch. 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:

- 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 `StopWatch::ElapsedTime()`) 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
- 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 `state_machine<>` subtype will therefore be a change hotspot, which is a pretty sure indicator for a bad design

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.

## How can I hide the inner workings of a state machine from its clients?

To see why and how this is possible it is important to recall the following facts:

- 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
- The `InitialState` template parameter of `sc::state_machine` can be an incomplete type (i.e. forward declared)

The class template member function `state_machine<>::initiate()` creates an object of the initial state. So, the definition of this state must be known before the compiler reaches the point where `initiate()` 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 `initiate()`. Instead, we do so in the .cpp file, at a point where the full definition of the initial state is known.

Example:

StopWatch.hpp:

    // define events ...

    struct Active; // the only visible forward
    struct StopWatch : sc::state_machine< StopWatch, Active >
    {
      StopWatch();
    };

StopWatch.cpp:

    struct Stopped;
    struct Active : sc::simple_state< Active, StopWatch, Stopped >
    {
      typedef sc::transition< EvReset, Active > reactions;
    };

      struct Running : sc::simple_state< Running, Active >
      {
        typedef sc::transition< EvStartStop, Stopped > reactions;
      };

      struct Stopped : sc::simple_state< Stopped, Active >
      {
        typedef sc::transition< EvStartStop, Running > 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.
      initiate();
    }

The PingPong example demonstrates how the inner workings of an asynchronous_state_machine<> subclass can be hidden.

## Is it possible to inherit from a given state machine and modify its layout in the subclass?

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:

    struct EvStart : sc::event< EvStart > {};

    struct Idle;
    struct PumpBase : sc::state_machine< PumpBase, Idle >
    {
      virtual sc::result react(
        Idle & idle, const EvStart & ) const;
    };

    struct Idle : sc::simple_state< Idle, PumpBase >
    {
      typedef sc::custom_reaction< EvStart > reactions;

      sc::result react( const EvStart & evt )
      {
        return context< PumpBase >().react( *this, evt );
      }
    };

    struct Running : sc::simple_state< Running, PumpBase > {};

    sc::result PumpBase::react(
      Idle & idle, const EvStart & ) const
    {
      return idle.transit< Running >();
    }

    struct MyRunning : sc::simple_state< MyRunning, PumpBase > {};

    struct MyPump : PumpBase
    {
      virtual sc::result react(
        Idle & idle, const EvStart & ) const
      {
        return idle.transit< MyRunning >();
      }
    };

## What about UML 2.0 features?

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 & Boost.Statechart semantics:

- 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 here for more information
- 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)

## Why do I get an assert when I access the state machine from a state destructor?

When compiled with `NDEBUG` undefined, running the following program results in a failed assert:

    #include <boost/statechart/state_machine.hpp>
    #include <boost/statechart/simple_state.hpp>
    #include <iostream>

    struct Initial;
    struct Machine : boost::statechart::state_machine< Machine, Initial >
    {
      Machine() { someMember_ = 42; }
      int someMember_;
    };

    struct Initial : boost::statechart::simple_state< Initial, Machine >
    {
      ~Initial() { std::cout << outermost_context().someMember_; }
    };

    int main()
    {
      Machine().initiate();
      return 0;
    }

The problem arises because `state_machine<>::~state_machine` inevitably destructs all remaining active states. At this time, `Machine::~Machine` has already been run, making it illegal to access any of the `Machine` members. This problem can be avoided by defining the following destructor:

    ~Machine() { terminate(); }

## Is Boost.Statechart suitable for embedded applications?

It depends. As explained under Speed versus scalability tradeoffs 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 very rough estimates:

- 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 `state_machine<>::process_event()`. 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
- A single state machine object uses typically less than 1KB of memory, even if it implements a very complex machine
- 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).
  Moreover, the library can be compiled with C++ RTTI and exception handling turned off, resulting in significant savings on most platforms

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.

## Is your library suitable for applications with hard real-time requirements?

Yes. Out of the box, the only operations taking potentially non-deterministic time that the library performs are calls to `std::allocator<>` member functions and `dynamic_cast`s. `std::allocator<>` member function calls can be avoided by passing a custom allocator to `event<>`, `state_machine<>`, `asynchronous_state_machine<>`, `fifo_scheduler<>` and `fifo_worker<>`. `dynamic_cast`s can be avoided by not calling the `state_cast<>` member functions of `state_machine<>`, `simple_state<>` and `state<>` but using the deterministic variant `state_downcast<>` instead.

## With templated states I get an error that 'inner_context_type' is not defined. What's wrong?

The following code generates such an error:

    #include <boost/statechart/state_machine.hpp>
    #include <boost/statechart/simple_state.hpp>

    namespace sc = boost::statechart;

    template< typename X > struct A;
    struct Machine : sc::state_machine< Machine, A< int > > {};

    template< typename X > struct B;
    template< typename X >
    struct A : sc::simple_state< A< X >, Machine, B< X > > {};

      template< typename X >
      struct B : sc::simple_state< B< X >, A< X > > {};

    int main()
    {
      Machine machine;
      machine.initiate();
      return 0;
    }

If the templates `A` and `B` 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 `B< X >` 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 `mpl::list<>`, as follows:

    struct A : sc::simple_state<
      A< X >, Machine, mpl::list< B< X > > > {};

See this post for technical details.

## My compiler reports an error in the library code. Is this a bug in Boost.Statechart?

Probably not. There are several possible reasons for such compile-time errors:

1. Your compiler is too buggy to compile the library, see here 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.
2. The error is reported on a line similar to the following:

       BOOST_STATIC_ASSERT( ( mpl::less<
         orthogonal_position,
         typename context_type::no_of_orthogonal_regions >::value ) );

   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:

       ...\boost\statechart\shallow_history.hpp(34) : error C2027: use of undefined type 'boost::STATIC_ASSERTION_FAILURE<x>'
         with
         [
           x=false
         ]
         ...\boost\statechart\shallow_history.hpp(34) : see reference to class template instantiation 'boost::STATIC_ASSERTION_FAILURE<x>' being compiled
         with
         [
           x=false
         ]
         ...\boost\statechart\simple_state.hpp(861) : see reference to class template instantiation 'boost::statechart::shallow_history<DefaultState>' being compiled
         with
         [
           DefaultState=B
         ]
         ...\boost\statechart\simple_state.hpp(599) : see reference to function template instantiation 'void boost::statechart::simple_state<MostDerived,Context,InnerInitial>::deep_construct_inner_impl_non_empty::deep_construct_inner_impl<InnerList>(const boost::statechart::simple_state<MostDerived,Context,InnerInitial>::inner_context_ptr_type &,boost::statechart::simple_state<MostDerived,Context,InnerInitial>::outermost_context_base_type &)' being compiled
         with
         [
           MostDerived=A,
           Context=InconsistentHistoryTest,
           InnerInitial=boost::mpl::list<boost::statechart::shallow_history<B>>,
           InnerList=boost::statechart::simple_state<A,InconsistentHistoryTest,boost::mpl::list<boost::statechart::shallow_history<B>>>::inner_initial_list
         ]
         ...\boost\statechart\simple_state.hpp(567) : see reference to function template instantiation 'void boost::statechart::simple_state<MostDerived,Context,InnerInitial>::deep_construct_inner<boost::statechart::simple_state<MostDerived,Context,InnerInitial>::inner_initial_list>(const boost::statechart::simple_state<MostDerived,Context,InnerInitial>::inner_context_ptr_type &,boost::statechart::simple_state<MostDerived,Context,InnerInitial>::outermost_context_base_type &)' being compiled
         with
         [
           MostDerived=A,
           Context=InconsistentHistoryTest,
           InnerInitial=boost::mpl::list<boost::statechart::shallow_history<B>>
         ]
         ...\boost\statechart\simple_state.hpp(563) : while compiling class-template member function 'void boost::statechart::simple_state<MostDerived,Context,InnerInitial>::deep_construct(const boost::statechart::simple_state<MostDerived,Context,InnerInitial>::context_ptr_type & ,boost::statechart::simple_state<MostDerived,Context,InnerInitial>::outermost_context_base_type &)'
         with
         [
           MostDerived=A,
           Context=InconsistentHistoryTest,
           InnerInitial=boost::mpl::list<boost::statechart::shallow_history<B>>
         ]
         ...\libs\statechart\test\InconsistentHistoryTest1.cpp(29) : see reference to class template instantiation 'boost::statechart::simple_state<MostDerived,Context,InnerInitial>' being compiled
         with
         [
           MostDerived=A,
           Context=InconsistentHistoryTest,
           InnerInitial=boost::mpl::list<boost::statechart::shallow_history<B>>
         ]

   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.
3. 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 send me a small but complete program showing the problem. Thank you!

## Is it possible to disable history for a state at runtime?

Yes, see simple_state::clear_shallow_history() and simple_state::clear_deep_history(). Calling these functions is often preferable to introducting additional normal transitions when ...

- a state with history is the target of many transitions, and/or
- the decision to ignore history is made in a different place than the transition to a state with history

## How can I compile a state machine into a dynamic link library (DLL)?

Invisible to the user, the library uses static data members to implement its own speed-optimized RTTI-mechanism for `event<>` and `simple_state<>` 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 statically linked into the same binary. It is a lot more complex when DLLs are involved. The TuTest*.?pp files illustrate this:

- TuTest.hpp: Instantiates a class template containing a static data member
- TuTest.cpp: Includes TuTest.hpp and is compiled into a DLL
- TuTestMain.cpp: Includes TuTest.hpp and is compiled into an executable

Without any precautions (e.g. `__declspec(dllexport)` on MSVC compatible compilers), on most platforms both binaries (exe & 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:

- 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 BOOST_STATECHART_USE_NATIVE_RTTI in all TUs will usually work around the problem
- MSVC-compatible compilers support `__declspec(dllimport)` and `__declspec(dllexport)`, 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 `event<>` and `simple_state<>` subtypes defined in headers that are compiled into more than one binary. Alternatively, of course BOOST_STATECHART_USE_NATIVE_RTTI can also be used to save the work of importing and exporting

## Does Boost.Statechart support polymorphic events?

No. Although events can be derived from each other to write common code only once, reactions can only be defined for most-derived events.

Example:

    template< class MostDerived >
    struct EvButtonPressed : sc::event< MostDerived >
    {
      // common code
    };

    struct EvPlayButtonPressed :
      EvButtonPressed< EvPlayButtonPressed > {};
    struct EvStopButtonPressed :
      EvButtonPressed< EvStopButtonPressed > {};
    struct EvForwardButtonPressed :
      EvButtonPressed< EvForwardButtonPressed > {};

    /* ... */

    // 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< Off, Mp3Player >
    {
      typedef mpl::list<
        sc::custom_reaction< EvPlayButtonPressed >,
        sc::custom_reaction< EvStopButtonPressed >,
        sc::custom_reaction< EvForwardButtonPressed >
      > reactions;

      template< class MostDerived >
      sc::result react( const EvButtonPressed< MostDerived > & )
      {
        // ...
      }
    };

## Why are exit-actions called in the wrong order when I use multiple inheritance?

Update: 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 and 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.

They definitely aren't for the `simple_state<>` and `state<>` subtypes, but the destructors of additional bases might be called in construction order (rather than the reverse construction order):

    #include <boost/statechart/state_machine.hpp>
    #include <boost/statechart/simple_state.hpp>

    namespace sc = boost::statechart;

    class EntryExitDisplayer
    {
      protected:
        EntryExitDisplayer( const char * pName ) :
          pName_( pName )
        {
          std::cout << pName_ << " entered\n";
        }

        ~EntryExitDisplayer()
        {
          std::cout << pName_ << " exited\n";
        }

      private:
        const char * const pName_;
    };

    struct Outer;
    struct Machine : sc::state_machine< Machine, Outer > {};
    struct Inner;
    struct Outer : EntryExitDisplayer, sc::simple_state<
      Outer, Machine, Inner >
    {
      Outer() : EntryExitDisplayer( "Outer" ) {}
    };

    struct Inner : EntryExitDisplayer,
      sc::simple_state< Inner, Outer >
    {
      Inner() : EntryExitDisplayer( "Inner" ) {}
    };

    int main()
    {
      Machine myMachine;
      myMachine.initiate();
      return 0;
    }

This program will produce the following output:

    Outer entered
    Inner entered
    Outer exited
    Inner exited

That is, the `EntryExitDisplayer` base class portion of `Outer` is destructed before the one of `Inner` although `Inner::~Inner()` is called before `Outer::~Outer()`. This somewhat counter-intuitive behavior is caused by the following facts:

- The `simple_state<>` base class portion of `Inner` is responsible to destruct `Outer`
- Destructors of base class portions are called in the reverse order of construction

So, when the `Outer` destructor is called the call stack looks as follows:

    Outer::~Outer()
    simple_state< Inner, ... >::~simple_state()
    Inner::~Inner()

Note that `Inner::~Inner()` did not yet have a chance to destroy its `EntryExitDisplayer` base class portion, as it first has to call the destructor of the second base class. Now `Outer::~Outer()` will first destruct its `simple_state< Outer, ... >` base class portion and then do the same with its `EntryExitDisplayer` base class portion. The stack then unwinds back to `Inner::~Inner()`, which can then finally finish by calling `EntryExitDisplayer::~EntryExitDisplayer()`.

Luckily, there is an easy work-around: Always let `simple_state<>` and `state<>` 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.

Revised 05 January, 2008

Copyright © 2003-2008 Andreas Huber Dönni

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)