Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


There are, at the moment, two back-ends. msm::back is using C++03, msm::back11 is using C++11 and should be used by default if possible as it removes some pre-C++11 limitations (mpl::vector of limited size). This back-end contains the library engine and defines the performance and functionality trade-offs. The currently available back-end implements most of the functionality defined by the UML 2.0 standard at very high runtime speed, in exchange for longer compile-time. The runtime speed is due to a constant-time double-dispatch and self-adapting capabilities allowing the framework to adapt itself to the features used by a given concrete state machine. All unneeded features either disable themselves or can be manually disabled. See section 5.1 for a complete description of the run-to-completion algorithm.


MSM being divided between front and back-end, one needs to first define a front-end. Then, to create a real state machine, the back-end must be declared:

typedef msm::back11::state_machine<my_front_end> my_fsm;

We now have a fully functional state machine type. The next sections will describe what can be done with it.

Starting and stopping a state machine

The start() method starts the state machine, meaning it will activate the initial state, which means in turn that the initial state's entry behavior will be called. We need the start method because you do not always want the entry behavior of the initial state to be called immediately but only when your state machine is ready to process events. A good example of this is when you use a state machine to write an algorithm and each loop back to the initial state is an algorithm call. Each call to start will make the algorithm run once. The iPodSearch example uses this possibility.

The stop() method works the same way. It will cause the exit actions of the currently active states(s) to be called.

Both methods are actually not an absolute need. Not calling them will simply cause your first entry or your last exit action not to be called.

Event dispatching

The main reason to exist for a state machine is to dispatch events. For MSM, events are objects of a given event type. The object itself can contain data, but the event type is what decides of the transition to be taken. For MSM, if some_event is a given type (a simple struct for example) and e1 and e2 concrete instances of some_event, e1 and e2 are equivalent, from a transition perspective. Of course, e1 and e2 can have different values and you can use them inside actions. Events are dispatched as const reference, so actions cannot modify events for obvious side-effect reasons. To dispatch an event of type some_event, you can simply create one on the fly or instantiate if before processing:

my_fsm fsm; fsm.process_event(some_event());
some_event e1; fsm.process_event(e1)

Creating an event on the fly will be optimized by the compiler so the performance will not degrade.

Active state(s)

The backend also offers a way to know which state is active, though you will normally only need this for debugging purposes. If what you need simply is doing something with the active state, internal transitions or visitors are a better alternative. If you need to know what state is active, const int* current_state() will return an array of state ids. Please refer to the internals section to know how state ids are generated.

Upper State Machine (msm::back11 only)

The FSM template argument passed to functors or entry/exit actions is the current state machine, which might not be what is wanted as th upper state machine makes more sense. The back-end provides a get_upper() function returning a pointer to the upper state machine, which is usually what you want to call process_event on.


A common need is the ability to save a state machine and restore it at a different time. MSM supports this feature for the basic and functor front-ends, and in a more limited manner for eUML. MSM supports boost::serialization out of the box (by offering a serialize function). Actually, for basic serialization, you need not do much, a MSM state machine is serializable almost like any other type. Without any special work, you can make a state machine remember its state, for example:

MyFsm fsm;
// write to archive
std::ofstream ofs("fsm.txt");
// save fsm to archive
    boost::archive::text_oarchive oa(ofs);
    // write class instance to archive
    oa << fsm;

Loading back is very similar:

MyFsm fsm;
    // create and open an archive for input
    std::ifstream ifs("fsm.txt");
    boost::archive::text_iarchive ia(ifs);
    // read class state from archive
    ia >> fsm;

This will (de)serialize the state machine itself but not the concrete states' data. This can be done on a per-state basis to reduce the amount of typing necessary. To allow serialization of a concrete state, provide a do_serialize typedef and implement the serialize function:

struct Empty : public msm::front::state<> 
    // we want Empty to be serialized. First provide the typedef
    typedef int do_serialize;
    // then implement serialize
    template<class Archive>
    void serialize(Archive & ar, const unsigned int /* version */)
        ar & some_dummy_data;
    int some_dummy_data;

You can also serialize data contained in the front-end class. Again, you need to provide the typedef and implement serialize:

struct player_ : public msm::front::state_machine_def<player_>
    //we might want to serialize some data contained by the front-end
    int front_end_data;
    // to achieve this, provide the typedef
    typedef int do_serialize;
    // and implement serialize
    template<class Archive>
    void serialize(Archive & ar, const unsigned int )
        ar & front_end_data;

The saving of the back-end data (the current state(s)) is valid for all front-ends, so a front-end written using eUML can be serialized. However, to serialize a concrete state, the macros like BOOST_MSM_EUML_STATE cannot be used, so the state will have to be implemented by directly inheriting from front::euml::euml_state.

The only limitiation is that the event queues cannot be serialized so serializing must be done in a stable state, when no event is being processed. You can serialize during event processing only if using no queue (deferred or event queue).

This example shows a state machine which we serialize after processing an event. The Empty state also has some data to serialize.

Base state type

Sometimes, one needs to customize states to avoid repetition and provide a common functionality, for example in the form of a virtual method. You might also want to make your states polymorphic so that you can call typeid on them for logging or debugging. It is also useful if you need a visitor, like the next section will show. You will notice that all front-ends offer the possibility of adding a base type. Note that all states and state machines must have the same base state, so this could reduce reuse. For example, using the basic front end, you need to:

  • Add the non-default base state in your msm::front::state<> definition, as first template argument (except for interrupt_states for which it is the second argument, the first one being the event ending the interrupt), for example, my_base_state being your new base state for all states in a given state machine:

    struct Empty : public msm::front::state<my_base_state>

    Now, my_base_state is your new base state. If it has a virtual function, your states become polymorphic. MSM also provides a default polymorphic base type, msm::front::polymorphic_state

  • Add the user-defined base state in the state machine frontend definition, as a second template argument, for example:

    struct player_ : public msm::front::state_machine<player_,my_base_state>             

You can also ask for a state with a given id (which you might have gotten from current_state()) using const base_state* get_state_by_id(int id) const where base_state is the one you just defined. You can now do something polymorphically.


In some cases, having a pointer-to-base of the currently active states is not enough. You might want to call non-virtually a method of the currently active states. It will not be said that MSM forces the virtual keyword down your throat!

To achieve this goal, MSM provides its own variation of a visitor pattern using the previously described user-defined state technique. If you add to your user-defined base state an accept_sig typedef giving the return value (unused for the moment) and parameters and provide an accept method with this signature, calling visit_current_states will cause accept to be called on the currently active states. Typically, you will also want to provide an empty default accept in your base state in order in order not to force all your states to implement accept. For example your base state could be:

struct my_visitable_state
   // signature of the accept function
   typedef args<void> accept_sig;
   // we also want polymorphic states
   virtual ~my_visitable_state() {}
   // default implementation for states who do not need to be visited
   void accept() const {}

This makes your states polymorphic and visitable. In this case, accept is made const and takes no argument. It could also be:

struct SomeVisitor {…};
struct my_visitable_state
    // signature of the accept function
    typedef args<void,SomeVisitor&> accept_sig;
    // we also want polymorphic states
    virtual ~my_visitable_state() {}
    // default implementation for states who do not need to be visited
    void accept(SomeVisitor&) const {}

And now, accept will take one argument (it could also be non-const). By default, accept takes up to 2 arguments. To get more, set #define BOOST_MSM_VISITOR_ARG_SIZE to another value before including state_machine.hpp. For example:

#include <boost/msm/back/state_machine.hpp>

Note that accept will be called on ALL active states and also automatically on sub-states of a submachine.

Important warning: The method visit_current_states takes its parameter by value, so if the signature of the accept function is to contain a parameter passed by reference, pass this parameter with a boost:ref/cref to avoid undesired copies or slicing. So, for example, in the above case, call:

SomeVisitor vis; sm.visit_current_states(boost::ref(vis));

This example uses a visiting function with 2 arguments.


Flags is a MSM-only concept, supported by all front-ends, which base themselves on the functions:

template <class Flag> bool is_flag_active()
template <class Flag,class BinaryOp> bool is_flag_active()

These functions return true if the currently active state(s) support the Flag property. The first variant ORs the result if there are several orthogonal regions, the second one expects OR or AND, for example:


Please refer to the front-ends sections for usage examples.

Getting a state

It is sometimes necessary to have the client code get access to the states' data. After all, the states are created once for good and hang around as long as the state machine does so why not use it? You simply just need sometimes to get information about any state, even inactive ones. An example is if you want to write a coverage tool and know how many times a state was visited. To get a state, use the get_state method giving the state name, for example:

player::Stopped* tempstate = p.get_state<player::Stopped*>();


player::Stopped& tempstate2 = p.get_state<player::Stopped&>();

depending on your personal taste.

State machine constructor with arguments

You might want to define a state machine with a non-default constructor. For example, you might want to write:

struct player_ : public msm::front::state_machine_def<player_> 
    player_(int some_value){…} 

This is possible, using the back-end as forwarding object:

typedef msm::back11::state_machine<player_ > player; player p(3);

The back-end will call the corresponding front-end constructor upon creation.

You can pass arguments up to the value of the BOOST_MSM_CONSTRUCTOR_ARG_SIZE macro (currently 5) arguments. Change this value before including any header if you need to overwrite the default.

You can also pass arguments by reference (or const-reference) using boost::ref (or boost::cref):

struct player_ : public msm::front::state_machine_def<player_>  
   player_(SomeType& t, int some_value){…}  

typedef msm::back11::state_machine<player_ > player; 
SomeType data;
player p(boost::ref(data),3);

Normally, MSM default-constructs all its states or submachines. There are however cases where you might not want this. An example is when you use a state machine as submachine, and this submachine used the above defined constructors. You can add as first argument of the state machine constructor an expression where existing states are passed and copied:

player p( back::states_ << state_1 << ... << state_n , boost::ref(data),3);

Where state_1..n are instances of some or all of the states of the state machine. Submachines being state machines, this can recurse, for example, if Playing is a submachine containing a state Song1 having itself a constructor where some data is passed:

player p( back::states_ << Playing(back::states_ << Song1(some_Song1_data)) , 

It is also possible to replace a given state by a new instance at any time using set_states() and the same syntax, for example:

p.set_states( back::states_ << state_1 << ... << state_n );

An example making intensive use of this capability is provided.

Trading run-time speed for better compile-time / multi-TU compilation (msm::back only)

MSM is optimized for run-time speed at the cost of longer compile-time. This can become a problem with older compilers and big state machines, especially if you don't really care about run-time speed that much and would be satisfied by a performance roughly the same as most state machine libraries. MSM offers a back-end policy to help there. But before you try it, if you are using a VC compiler, deactivate the /Gm compiler option (default for debug builds). This option can cause builds to be 3 times longer... If the compile-time still is a problem, read further. MSM offers a policy which will speed up compiling in two main cases:

  • many transition conflicts

  • submachines

Important warningmsm::back11 is not supporting this feature yet.

The back-end msm::back::state_machine has a policy argument (first is the front-end, then the history policy) defaulting to favor_runtime_speed. To switch to favor_compile_time, which is declared in <msm/back/favor_compile_time.hpp>, you need to:

  • switch the policy to favor_compile_time for the main state machine (and possibly submachines)

  • move the submachine declarations into their own header which includes <msm/back/favor_compile_time.hpp>

  • add for each submachine a cpp file including your header and calling a macro, which generates helper code, for example:

    #include "mysubmachine.hpp"
  • configure your compiler for multi-core compilation

You will now compile your state machine on as many cores as you have submachines, which will greatly speed up the compilation if you factor your state machine into smaller submachines.

Independently, transition conflicts resolution will also be much faster.

This policy uses boost.any behind the hood, which means that we will lose a feature which MSM offers with the default policy, event hierarchy. The following example takes our iPod example and speeds up compile-time by using this technique. We have:

Compile-time state machine analysis

A MSM state machine being a metaprogram, it is only logical that cheking for the validity of a concrete state machine happens compile-time. To this aim, using the compile-time graph library mpl_graph (delivered at the moment with MSM) from Gordon Woodhull, MSM provides several compile-time checks:

  • Check that orthogonal regions ar truly orthogonal.

  • Check that all states are either reachable from the initial states or are explicit entries / pseudo-entry states.

To make use of this feature, the back-end provides a policy (default is no analysis), msm::back::mpl_graph_fsm_check. For example:

 typedef msm::back11::state_machine< player_,msm::back::mpl_graph_fsm_check> player;           

As MSM is now using Boost.Parameter to declare policies, the policy choice can be made at any position after the front-end type (in this case player_).

In case an error is detected, a compile-time assertion is provoked.

This feature is not enabled by default because it has a non-neglectable compile-time cost. The algorithm is linear if no explicit or pseudo entry states are found in the state machine, unfortunately still O(number of states * number of entry states) otherwise. This will be improved in future versions of MSM.

The same algorithm is also used in case you want to omit providing the region index in the explicit entry / pseudo entry state declaration.

The author's advice is to enable the checks after any state machine structure change and disable it again after sucessful analysis.

The following example provokes an assertion if one of the first two lines of the transition table is used.

Enqueueing events for later processing

Calling process_event(Event const&) will immediately process the event with run-to-completion semantics. You can also enqueue the events and delay their processing by calling enqueue_event(Event const&) instead. Calling execute_queued_events() will then process all enqueued events (in FIFO order). Calling execute_single_queued_event() will execute the oldest enqueued event.

You can query the queue size by calling get_message_queue_size().

Customizing the message queues

MSM uses by default a std::deque for its queues (one message queue for events generated during run-to-completion or with enqueue_event, one for deferred events). Unfortunately, on some STL implementations, it is a very expensive container in size and copying time. Should this be a problem, MSM offers an alternative based on boost::circular_buffer. The policy is msm::back::queue_container_circular. To use it, you need to provide it to the back-end definition:

 typedef msm::back11::state_machine< player_,msm::back::queue_container_circular> player;           

You can access the queues with get_message_queue and get_deferred_queue, both returning a reference or a const reference to the queues themselves. Boost::circular_buffer is outside of the scope of this documentation. What you will however need to define is the queue capacity (initially is 0) to what you think your queue will at most grow, for example (size 1 is common):


Policy definition with Boost.Parameter

MSM uses Boost.Parameter to allow easier definition of back11::state_machine<> policy arguments (all except the front-end). This allows you to define policy arguments (history, compile-time / run-time, state machine analysis, container for the queues) at any position, in any number. For example:

 typedef msm::back11::state_machine< player_,msm::back::mpl_graph_fsm_check> player;  
 typedef msm::back11::state_machine< player_,msm::back::AlwaysHistory> player;  
 typedef msm::back11::state_machine< player_,msm::back::mpl_graph_fsm_check,msm::back::AlwaysHistory> player; 
 typedef msm::back11::state_machine< player_,msm::back::AlwaysHistory,msm::back::mpl_graph_fsm_check> player;      

Choosing when to switch active states

The UML Standard is silent about a very important question: when a transition fires, at which exact point is the target state the new active state of a state machine? At the end of the transition? After the source state has been left? What if an exception is thrown? The Standard considers that run-to-completion means a transition completes in almost no time. But even this can be in some conditions a very very long time. Consider the following example. We have a state machine representing a network connection. We can be Connected and Disconnected. When we move from one state to another, we send a (Boost) Signal to another entity. By default, MSM makes the target state as the new state after the transition is completed. We want to send a signal based on a flag is_connected which is true when in state Connected.

We are in state Disconnected and receive an event connect. The transition action will ask the state machine is_flag_active<is_connected> and will get... false because we are still in Disconnected. Hmm, what to do? We could queue the action and execute it later, but it means an extra queue, more work and higher run-time.

MSM provides the possibility (in form of a policy) for a front-end to decide when the target state becomes active. It can be:

  • before the transition fires, if the guard will allow the transition to fire: active_state_switch_before_transition

  • after calling the exit action of the source state: active_state_switch_after_exit

  • after the transition action is executed: active_state_switch_after_transition_action

  • after the entry action of the target state is executed (default): active_state_switch_after_entry

The problem and the solution is shown for the functor-front-end and eUML. Removing active_state_switch_before_transition will show the default state.