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eUML (experimental)

Important note: eUML requires a compiler supporting Boost.Typeof. More generally, eUML has experimental status because some compilers will start crashing when a state machine becomes too big (usually when you write huge actions).

The previous front-ends are simple to write but still force an amount of noise, mostly MPL types, so it would be nice to write code looking like C++ (with a C++ action language) directly inside the transition table, like UML designers like to do on their state machine diagrams. If it were functional programming, it would be even better. This is what eUML is for.

eUML is a Boost.Proto and Boost.Typeof-based compile-time domain specific embedded language. It provides grammars which allow the definition of actions/guards directly inside the transition table or entry/exit in the state definition. There are grammars for actions, guards, flags, attributes, deferred events, initial states.

It also relies on Boost.Typeof as a wrapper around the new decltype C++0x feature to provide a compile-time evaluation of all the grammars. Unfortunately, all the underlying Boost libraries are not Typeof-enabled, so for the moment, you will need a compiler where Typeof is supported (like VC9-10, g++ >= 4.3).

Examples will be provided in the next paragraphs. You need to include eUML basic features:

#include <msm/front/euml/euml.hpp>

To add STL support (at possible cost of longer compilation times), include:

#include <msm/front/euml/stl.hpp>

eUML is defined in the namespace msm::front::euml.

Transition table

A transition can be defined using eUML as:

source + event [guard] / action == target

or as

target == source + event [guard] / action

The first version looks like a drawn transition in a diagram, the second one seems natural to a C++ developer.

The simple transition table written with the functor front-end can now be written as:

Stopped + play [some_guard] / (some_action , start_playback)  == Playing ,
Stopped + open_close/ open_drawer                             == Open    ,
Stopped + stop                                                == Stopped ,
Open    + open_close / close_drawer                           == Empty   ,
Empty   + open_close / open_drawer                            == Open    ,
Empty   + cd_detected [good_disk_format] / store_cd_info      == Stopped

Or, using the alternative notation, it can be:

Playing  == Stopped + play [some_guard] / (some_action , start_playback) ,
Open     == Stopped + open_close/ open_drawer                            ,
Stopped  == Stopped + stop                                               ,
Empty    == Open    + open_close / close_drawer                          ,
Open     == Empty   + open_close / open_drawer                           ,
Stopped  == Empty   + cd_detected [good_disk_format] / store_cd_info

The transition table now looks like a list of (readable) rules with little noise.

UML defines guards between “[ ]” and actions after a “/”, so the chosen syntax is already more readable for UML designers. UML also allows designers to define several actions sequentially (our previous ActionSequence_) separated by a comma. The first transition does just this: two actions separated by a comma and enclosed inside parenthesis to respect C++ operator precedence.

If this seems to you like it will cost you run-time performance, don't worry, eUML is based on typeof (or decltype) which only evaluates the parameters to BOOST_MSM_EUML_TRANSITION_TABLE and no run-time cost occurs. Actually, eUML is only a metaprogramming layer on top of "standard" MSM metaprogramming and this first layer generates the previously-introduced functor front-end.

UML also allows designers to define more complicated guards, like [good_disk_format && (some_condition || some_other_condition)]. This was possible with our previously defined functors, but using a complicated template syntax. This syntax is now possible exactly as written, which means without any syntactic noise at all.

A simple example: rewriting only our transition table

As an introduction to eUML, we will rewrite our tutorial's transition table using eUML. This will require two or three changes, depending on the compiler:

  • events must inherit from msm::front::euml::euml_event< event_name >

  • states must inherit from msm::front::euml::euml_state< state_name >

  • with VC, states must be declared before the front-end

We now can write the transition table like just shown, using BOOST_MSM_EUML_DECLARE_TRANSITION_TABLE instead of BOOST_MSM_EUML_TRANSITION_TABLE. The implementation is pretty straightforward.

The composite implementation is slightly trickier because the submachine has to be a msm::back::state_machine and a msm::front::euml::state. For example:

// front-end like always
struct front_end : public boost::msm::front::state_machine_def<front_end>
// back-end like always
typedef boost::msm::back::state_machine<front_end> back_end;
// this is new: make the submachine a eUML type
struct submachine : public back_end,
                    public boost::msm::front::euml::euml_state<back_end>

Unfortunately, there is a bug with VC, which appears from time to time and causes in a stack overflow. If you get a warning that the program is recursive on all paths, revert to either standard eUML or another front-end as Microsoft doesn't seem to intend to fix it.

We now have a new, more readable transition table with few changes to our example. eUML can do much more so please follow the guide.

Defining events, actions and states with entry/exit actions


Events must be proto-enabled. To achieve this, they must inherit from a proto terminal (euml_event<event-name>). eUML also provides a macro to make this easier:


This declares an event type and an instance of this type called play, which is now ready to use in state or transition behaviors.

There is a second macro, BOOST_MSM_EUML_EVENT_WITH_ATTRIBUTES, which takes as second parameter the attributes an event will contain, using the attribute syntax.

Note: as we now have events defined as instances instead of just types, can we still process an event by creating one on the fly, like: fsm.process_event(play()); or do we have to write: fsm.process_event(play);

The answer is you can do both. The second one is easier but unlike other front-ends, the second uses a defined operator(), which creates an event on the fly.


Actions (returning void) and guards (returning a bool) are defined like previous functors, with the difference that they also must be proto-enabled. This can be done by inheriting from euml_action< functor-name >. eUML also provides a macro:

    template <class Fsm,class Evt,class SourceState,class TargetState>
    bool operator()(Evt const& ,Fsm& ,SourceState&,TargetState& ) 
    { return true; }

Like for events, this macro declares a functor type and an instance for use in transition or state behaviors.

It is possible to use the same action grammar from the transition table to define state entry and exit behaviors. So (action1,action2) is a valid entry or exit behavior executing both actions in turn.

The state functors have a slightly different signature as there is no source and target state but only a current state (entry/exit actions are transition-independent), for example:

    template <class Evt,class Fsm,class State>
    void operator()(Evt const& ,Fsm& ,State& ) { ... }                           

It is also possible to reuse the functors from the functor front-end. The syntax is however slightly less comfortable as we need to pretend creating one on the fly for typeof. For example:

struct start_playback 
        template <class Fsm,class Evt,class SourceState,class TargetState>
        void operator()(Evt const& ,Fsm&,SourceState& ,TargetState& )
Playing   == Stopped  + play        / start_playback() ,


There is also a macro for states. This macro has 2 arguments, first the expression defining the state, then the state (instance) name:


This defines a simple state without entry or exit action. You can provide in the expression parameter the state behaviors (entry and exit) using the action grammar, like in the transition table:

BOOST_MSM_EUML_STATE(((Empty_Entry,Dummy_Entry)/*2 entryactions*/,
                       Empty_Exit/*1 exit action*/ ),

This means that Empty is defined as a state with an entry action made of two sub-actions, Empty_Entry and Dummy_Entry (enclosed inside parenthesis), and an exit action, Empty_Exit.

There are several possibilitites for the expression syntax:

  • (): state without entry or exit action.

  • (Expr1): state with entry but no exit action.

  • (Expr1,Expr2): state with entry and exit action.

  • (Expr1,Expr2,Attributes): state with entry and exit action, defining some attributes (read further on).

  • (Expr1,Expr2,Attributes,Configure): state with entry and exit action, defining some attributes (read further on) and flags (standard MSM flags) or deferred events (standard MSM deferred events).

  • (Expr1,Expr2,Attributes,Configure,Base): state with entry and exit action, defining some attributes (read further on), flags and deferred events (plain msm deferred events) and a non-default base state (as defined in standard MSM).

no_action is also defined, which does, well, nothing except being a placeholder (needed for example as entry action if we have no entry but an exit). Expr1 and Expr2 are a sequence of actions, obeying the same action grammar as in the transition table (following the “/” symbol).

The BOOST_MSM_EUML_STATE macro will allow you to define most common states, but sometimes you will need more, for example provide in your states some special behavior. In this case, you will have to do the macro's job by hand, which is not very complicated. The state will need to inherit from msm::front::state<>, like any state, and from euml_state<state-name> to be proto-enabled. You will then need to declare an instance for use in the transition table. For example:

struct Empty_impl : public msm::front::state<> , public euml_state<Empty_impl> 
   void activate_empty() {std::cout << "switching to Empty " << std::endl;}
   template <class Event,class Fsm>
   void on_entry(Event const& evt,Fsm&fsm){...}
   template <class Event,class Fsm>
   void on_exit(Event const& evt,Fsm&fsm){...}
//instance for use in the transition table
Empty_impl const Empty;

Notice also that we defined a method named activate_empty. We would like to call it inside a behavior. This can be done using the BOOST_MSM_EUML_METHOD macro.


The first parameter is the name of the underlying functor, which you could use with the functor front-end, the second is the state method name, the third is the eUML-generated function, the fourth and fifth the return value when used inside a transition or a state behavior. You can now use this inside a transition:

Empty == Open + open_close / (close_drawer,activate_empty_(target_))

Wrapping up a simple state machine and first complete examples

You can reuse the state machine definition method from the standard front-end and simply replace the transition table by this new one. You can also use eUML to define a state machine "on the fly" (if, for example, you need to provide an on_entry/on_exit for this state machine as a functor). For this, there is also a macro, BOOST_MSM_EUML_DECLARE_STATE_MACHINE, which has 2 arguments, an expression describing the state machine and the state machine name. The expression has up to 8 arguments:

  • (Stt, Init): simplest state machine where only the transition table and initial state(s) are defined.

  • (Stt, Init, Expr1): state machine where the transition table, initial state and entry action are defined.

  • (Stt, Init, Expr1, Expr2): state machine where the transition table, initial state, entry and exit actions are defined.

  • (Stt, Init, Expr1, Expr2, Attributes): state machine where the transition table, initial state, entry and exit actions are defined. Furthermore, some attributes are added (read further on).

  • (Stt, Init, Expr1, Expr2, Attributes, Configure): state machine where the transition table, initial state, entry and exit actions are defined. Furthermore, some attributes (read further on), flags, deferred events and configuration capabilities (no message queue / no exception catching) are added.

  • (Stt, Init, Expr1, Expr2, Attributes, Flags, Deferred , Base): state machine where the transition table, initial state, entry and exit actions are defined. Furthermore, attributes (read further on), flags , deferred events and configuration capabilities (no message queue / no exception catching) are added and a non-default base state (see the back-end description) is defined.

For example, a minimum state machine could be defined as:

BOOST_MSM_EUML_DECLARE_STATE_MACHINE((transition_table,init_ << Empty ),

Please have a look at the player tutorial written using eUML's first syntax and second syntax. The BOOST_MSM_EUML_DECLARE_ATTRIBUTE macro, to which we will get back shortly, declares attributes given to an eUML type (state or event) using the attribute syntax.

Defining a submachine

Defining a submachine (see tutorial) with other front-ends simply means using a state which is a state machine in the transition table of another state machine. This is the same with eUML. One only needs define a second state machine and reference it in the transition table of the containing state machine.

Unlike the state or event definition macros, BOOST_MSM_EUML_DECLARE_STATE_MACHINE defines a type, not an instance because a type is what the back-end requires. This means that you will need to declare yourself an instance to reference your submachine into another state machine, for example:

typedef msm::back::state_machine<Playing_> Playing_type;
Playing_type const Playing;

We can now use this instance inside the transition table of the containing state machine:

Paused == Playing + pause / pause_playback

Attributes / Function call

We now want to make our grammar more useful. Very often, one needs only very simple action methods, for example ++Counter or Counter > 5 where Counter is usually defined as some attribute of the class containing the state machine. It seems like a waste to write a functor for such a simple action. Furthermore, states within MSM are also classes so they can have attributes, and we would also like to provide them with attributes.

If you look back at our examples using the first and second syntaxes, you will find a BOOST_MSM_EUML_DECLARE_ATTRIBUTE and a BOOST_MSM_EUML_ATTRIBUTES macro. The first one declares possible attributes:


This declares two attributes: cd_name of type std::string and cd_type of type DiskTypeEnum. These attributes are not part of any event or state in particular, we just declared a name and a type. Now, we can add attributes to our cd_detected event using the second one:

BOOST_MSM_EUML_ATTRIBUTES((attributes_ << cd_name << cd_type ), 

This declares an attribute list which is not linked to anything in particular yet. It can be attached to a state or an event. For example, if we want the event cd_detected to have these defined attributes we write:


For states, we use the BOOST_MSM_EUML_STATE macro, which has an expression form where one can provide attributes. For example:

BOOST_MSM_EUML_STATE((no_action /*entry*/,no_action/*exit*/,
                      attributes_ << cd_detected_attributes),

OK, great, we now have a way to add attributes to a class, which we could have done more easily, so what is the point? The point is that we can now reference these attributes directly, at compile-time, in the transition table. For example, in the example, you will find this transition:


Read event_(cd_type) as event_->cd_type with event_ a type generic for events, whatever the concrete event is (in this particular case, it happens to be a cd_detected as the transition shows).

The main advantage of this feature is that you do not need to define a new functor and you do not need to look inside the functor to know what it does, you have all at hand.

MSM provides more generic objects for state machine types:

  • event_ : used inside any action, the event triggering the transition

  • state_: used inside entry and exit actions, the entered / exited state

  • source_: used inside a transition action, the source state

  • target_: used inside a transition action, the target state

  • fsm_: used inside any action, the (lowest-level) state machine processing the transition

  • Int_<int value>: a functor representing an int

  • Char_<value>: a functor representing a char

  • Size_t_<value>: a functor representing a size_t

  • String_<mpl::string> (boost >= 1.40): a functor representing a string.

These helpers can be used in two different ways:

  • helper(attribute_name) returns the attribute with name attribute_name

  • helper returns the state / event type itself.

The second form is helpful if you want to provide your states with their own methods, which you also want to use inside the transition table. In the above tutorial, we provide Empty with an activate_empty method. We would like to create a eUML functor and call it from inside the transition table. This is done using the MSM_EUML_METHOD / MSM_EUML_FUNCTION macros. The first creates a functor to a method, the second to a free function. In the tutorial, we write:


The first parameter is the functor name, for use with the functor front-end. The second is the name of the method to call. The third is the function name for use with eUML, the fourth is the return type of the function if used in the context of a transition action, the fifth is the result type if used in the context of a state entry / exit action (usually fourth and fifth are the same). We now have a new eUML function calling a method of "something", and this "something" is one of the five previously shown generic helpers. We can now use this in a transition, for example:

Empty == Open + open_close / (close_drawer,activate_empty_(target_))

The action is now defined as a sequence of two actions: close_drawer and activate_empty, which is called on the target itself. The target being Empty (the state defined left), this really will call Empty::activate_empty(). This method could also have an (or several) argument(s), for example the event, we could then call activate_empty_(target_ , event_).

More examples can be found in the terrible compiler stress test, the timer example or in the iPodSearch with eUML (for String_ and more).

Orthogonal regions, flags, event deferring

Defining orthogonal regions really means providing more initial states. To add more initial states, “shift left” some, for example, if we had another initial state named AllOk :

                                     init_ << Empty << AllOk ),

You remember from the BOOST_MSM_EUML_STATE and BOOST_MSM_EUML_DECLARE_STATE_MACHINE signatures that just after attributes, we can define flags, like in the basic MSM front-end. To do this, we have another "shift-left" grammar, for example:

BOOST_MSM_EUML_STATE((no_action,no_action, attributes_ <<no_attributes_, 
                      /* flags */ configure_<< PlayingPaused << CDLoaded), 

We now defined that Paused will get two flags, PlayingPaused and CDLoaded, defined, with another macro:


This corresponds to the following basic front-end definition of Paused:

struct Paused : public msm::front::state<>
   typedef mpl::vector2<PlayingPaused,CDLoaded> flag_list; 

Under the hood, what you get really is a mpl::vector2.

Note: As we use the version of BOOST_MSM_EUML_STATE's expression with 4 arguments, we need to tell eUML that we need no attributes. Similarly to a cout << endl, we need a attributes_ << no_attributes_ syntax.

You can use the flag with the is_flag_active method of a state machine. You can also use the provided helper function is_flag_ (returning a bool) for state and transition behaviors. For example, in the iPod implementation with eUML, you find the following transition:

ForwardPressed == NoForward + EastPressed[!is_flag_(NoFastFwd)]

The function also has an optional second parameter which is the state machine on which the function is called. By default, fsm_ is used (the current state machine) but you could provide a functor returning a reference to another state machine.

eUML also supports defining deferred events in the state (state machine) definition. To this aim, we can reuse the flag grammar. For example:

BOOST_MSM_EUML_STATE((Empty_Entry,Empty_Exit, attributes_ << no_attributes_,
                      /* deferred */ configure_<< play ),Empty) 

The configure_ left shift is also responsible for deferring events. Shift inside configure_ a flag and the state will get a flag, shift an event and it will get a deferred event. This replaces the basic front-end definition:

typedef mpl::vector<play> deferred_events;

In this tutorial, player is defining a second orthogonal region with AllOk as initial state. The Empty and Open states also defer the event play. Open, Stopped and Pause also support the flag CDLoaded using the same left shift into configure_.

In the functor front-end, we also had the possibility to defer an event inside a transition, which makes possible conditional deferring. This is also possible with eUML through the use of the defer_ order, as shown in this tutorial. You will find the following transition:

Open + play / defer_

This is an internal transition. Ignore it for the moment. Interesting is, that when the event play is fired and Open is active, the event will be deferred. Now add a guard and you can conditionally defer the event, for example:

Open + play [ some_condition ] / defer_

This is similar to what we did with the functor front-end. This means that we have the same constraints. Using defer_ instead of a state declaration, we need to tell MSM that we have deferred events in this state machine. We do this (again) using a configure_ declaration in the state machine definition in which we shift the deferred_events configuration flag:

                                      init_ << Empty << AllOk,
                                      attributes_ << no_attributes_,
                                      configure_<< deferred_events ),

A tutorial illustrates this possibility.

Customizing a state machine / Getting more speed

We just saw how to use configure_ to define deferred events or flags. We can also use it to configure our state machine like we did with the other front-ends:

  • configure_ << no_exception: disables exception handling

  • configure_ << no_msg_queue deactivates the message queue

  • configure_ << deferred_events manually enables event deferring

Deactivating the first two features and not activating the third if not needed greatly improves the event dispatching speed of your state machine. Our speed testing example with eUML does this for the best performance.

Important note: As exit pseudo states are using the message queue to forward events out of a submachine, the no_message_queue option cannot be used with state machines containing an exit pseudo state.

Completion / Anonymous transitions

Anonymous transitions (See UML tutorial) are transitions without a named event, which are therefore triggered immediately when the source state becomes active, provided a guard allows it. As there is no event, to define such a transition, simply omit the “+” part of the transition (the event), for example:

State3 == State4 [always_true] / State3ToState4
State4 [always_true] / State3ToState4 == State3

Please have a look at this example, which implements the previously defined state machine with eUML.

Internal transitions

Like both other front-ends, eUML supports two ways of defining internal transitions:

  • in the state machine's transition table. In this case, you need to specify a source state, event, actions and guards but no target state, which eUML will interpret as an internal transition, for example this defines a transition internal to Open, on the event open_close:

    Open + open_close [internal_guard1] / internal_action1

    A full example is also provided.

  • in a state's internal_transition_table. For example:

    struct Open_impl : public Open_def
            open_close [internal_guard1] / internal_action1

    Notice how we do not need to repeat that the transition originates from Open as we already are in Open's context.

    The implementation also shows the added bonus offered for submachines, which can have both the standard transition_table and an internal_transition_table (which has higher priority). This makes it easier if you decide to make a full submachine from a state. It is also slightly faster than the standard alternative, adding orthogonal regions, because event dispatching will, if accepted by the internal table, not continue to the subregions. This gives you a O(1) dispatch instead of O(number of regions).

Other state types

We saw the build_state function, which creates a simple state. Likewise, eUML provides other state-building macros for other types of states:

  • BOOST_MSM_EUML_TERMINATE_STATE takes the same arguments as BOOST_MSM_EUML_STATE and defines, well, a terminate state.

  • BOOST_MSM_EUML_INTERRUPT_STATE takes the same arguments as BOOST_MSM_EUML_STATE and defines an interrupt state. However, the expression argument must contain as first element the event ending the interruption, for example: BOOST_MSM_EUML_INTERRUPT_STATE(( end_error /*end interrupt event*/,ErrorMode_Entry,ErrorMode_Exit ),ErrorMode)

  • BOOST_MSM_EUML_EXIT_STATE takes the same arguments as BOOST_MSM_EUML_STATE and defines an exit pseudo state. However, the expression argument must contain as first element the event propagated from the exit point: BOOST_MSM_EUML_EXIT_STATE(( event6 /*propagated event*/,PseudoExit1_Entry,PseudoExit1_Exit ),PseudoExit1)

  • BOOST_MSM_EUML_EXPLICIT_ENTRY_STATE defines an entry pseudo state. It takes 3 parameters: the region index to be entered is defined as an int argument, followed by the configuration expression like BOOST_MSM_EUML_STATE and the state name, so that BOOST_MSM_EUML_EXPLICIT_ENTRY_STATE(0 /*region index*/,( SubState2_Entry,SubState2_Exit ),SubState2) defines an entry state into the first region of a submachine.

  • BOOST_MSM_EUML_ENTRY_STATE defines an entry pseudo state. It takes 3 parameters: the region index to be entered is defined as an int argument, followed by the configuration expression like BOOST_MSM_EUML_STATE and the state name, so that BOOST_MSM_EUML_ENTRY_STATE(0,( PseudoEntry1_Entry,PseudoEntry1_Exit ),PseudoEntry1) defines a pseudo entry state into the first region of a submachine.

To use these states in the transition table, eUML offers the functions explicit_, exit_pt_ and entry_pt_. For example, a direct entry into the substate SubState2 from SubFsm2 could be:

explicit_(SubFsm2,SubState2) == State1 + event2

Forks being a list on direct entries, eUML supports a logical syntax (state1, state2, ...), for example:

 explicit_(SubFsm2,SubState2c)) == State1 + event3 

An entry point is entered using the same syntax as explicit entries:

entry_pt_(SubFsm2,PseudoEntry1) == State1 + event4

For exit points, it is again the same syntax except that exit points are used as source of the transition:

State2 == exit_pt_(SubFsm2,PseudoExit1) + event6 

The entry tutorial is also available with eUML.

Helper functions

We saw a few helpers but there are more, so let us have a more complete description:

  • event_ : used inside any action, the event triggering the transition

  • state_: used inside entry and exit actions, the entered / exited state

  • source_: used inside a transition action, the source state

  • target_: used inside a transition action, the target state

  • fsm_: used inside any action, the (deepest-level) state machine processing the transition

  • These objects can also be used as a function and return an attribute, for example event_(cd_name)

  • Int_<int value>: a functor representing an int

  • Char_<value>: a functor representing a char

  • Size_t_<value>: a functor representing a size_t

  • True_ and False_ functors returning true and false respectively

  • String_<mpl::string> (boost >= 1.40): a functor representing a string.

  • if_then_else_(guard, action, action) where action can be an action sequence

  • if_then_(guard, action) where action can be an action sequence

  • while_(guard, action) where action can be an action sequence

  • do_while_(guard, action) where action can be an action sequence

  • for_(action, guard, action, action) where action can be an action sequence

  • process_(some_event [, some state machine] [, some state machine] [, some state machine] [, some state machine]) will call process_event (some_event) on the current state machine or on the one(s) passed as 2nd , 3rd, 4th, 5th argument. This allow sending events to several external machines

  • process_(event_): reprocesses the event which triggered the transition

  • reprocess_(): same as above but shorter to write

  • process2_(some_event,Value [, some state machine] [, some state machine] [, some state machine]) will call process_event (some_event(Value)) on the current state machine or on the one(s) passed as 3rd, 4th, 5th argument

  • is_ flag_(some_flag[, some state machine]) will call is_flag_active on the current state machine or on the one passed as 2nd argument

  • Predicate_<some predicate>: Used in STL algorithms. Wraps unary/binary functions to make them eUML-compatible so that they can be used in STL algorithms

This can be quite fun. For example,

/( if_then_else_(--fsm_(m_SongIndex) > Int_<0>(),/*if clause*/
                 show_playing_song, /*then clause*/
                 (fsm_(m_SongIndex)=Int_<1>(),process_(EndPlay))/*else clause*/

means: if (fsm.SongIndex > 0, call show_playing_song else {fsm.SongIndex=1; process EndPlay on fsm;}

A few examples are using these features:

  • the iPod example introduced at the BoostCon09 has been rewritten with eUML (weak compilers please move on...)

  • the iPodSearch example also introduced at the BoostCon09 has been rewritten with eUML. In this example, you will also find some examples of STL functor usage.

  • A simpler timer example is a good starting point.

There is unfortunately a small catch. Defining a functor using MSM_EUML_METHOD or MSM_EUML_FUNCTION will create a correct functor. Your own eUML functors written as described at the beginning of this section will also work well, except, for the moment, with the while_, if_then_, if_then_else_ functions.

Phoenix-like STL support

eUML supports most C++ operators (except address-of). For example it is possible to write event_(some_attribute)++ or [source_(some_bool) && fsm_(some_other_bool)]. But a programmer needs more than operators in his daily programming. The STL is clearly a must have. Therefore, eUML comes in with a lot of functors to further reduce the need for your own functors for the transition table. For almost every algorithm or container method of the STL, a corresponding eUML function is defined. Like Boost.Phoenix, “.” And “->” of call on objects are replaced by a functional programming paradigm, for example:

  • begin_(container), end_(container): return iterators of a container.

  • empty_(container): returns container.empty()

  • clear_(container): container.clear()

  • transform_ : std::transform

In a nutshell, almost every STL method or algorithm is matched by a corresponding functor, which can then be used in the transition table or state actions. The reference lists all eUML functions and the underlying functor (so that this possibility is not reserved to eUML but also to the functor-based front-end). The file structure of this Phoenix-like library matches the one of Boost.Phoenix. All functors for STL algorithms are to be found in:

#include <msm/front/euml/algorithm.hpp>

The algorithms are also divided into sub-headers, matching the phoenix structure for simplicity:

#include < msm/front/euml/iteration.hpp> 
#include < msm/front/euml/transformation.hpp>
#include < msm/front/euml/querying.hpp> 

Container methods can be found in:

#include < msm/front/euml/container.hpp>

Or one can simply include the whole STL support (you will also need to include euml.hpp):

#include < msm/front/euml/stl.hpp>

A few examples (to be found in this tutorial):

  • push_back_(fsm_(m_tgt_container),event_(m_song)): the state machine has an attribute m_tgt_container of type std::vector<OneSong> and the event has an attribute m_song of type OneSong. The line therefore pushes m_song at the end of m_tgt_container

  • if_then_( state_(m_src_it) != end_(fsm_(m_src_container)), process2_(OneSong(),*(state_(m_src_it)++)) ): the current state has an attribute m_src_it (an iterator). If this iterator != fsm.m_src_container.end(), process OneSong on fsm, copy-constructed from state.m_src_it which we post-increment

Writing actions with Boost.Phoenix (in development)

It is also possible to write actions, guards, state entry and exit actions using a reduced set of Boost.Phoenix capabilities. This feature is still in development stage, so you might get here and there some surprise. Simple cases, however, should work well. What will not work will be mixing of eUML and Phoenix functors. Writing guards in one language and actions in another is ok though.

Phoenix also supports a larger syntax than what will ever be possible with eUML, so you can only use a reduced set of phoenix's grammar. This is due to the nature of eUML. The run-time transition table definition is translated to a type using Boost.Typeof. The result is a "normal" MSM transition table made of functor types. As C++ does not allow mixing run-time and compile-time constructs, there will be some limit (trying to instantiate a template class MyTemplateClass<i> where i is an int will give you an idea). This means following valid Phoenix constructs will not work:

  • literals

  • function pointers

  • bind

  • ->*

MSM also provides placeholders which make more sense in its context than arg1.. argn:

  • _event: the event triggering the transition

  • _fsm: the state machine processing the event

  • _source: the source state of the transition

  • _target: the target state of the transition

  • _state: for state entry/exit actions, the entry/exit state

Future versions of MSM will support Phoenix better. You can contribute by finding out cases which do not work but should, so that they can be added.

Phoenix support is not activated by default. To activate it, add before any MSM header: #define BOOST_MSM_EUML_PHOENIX_SUPPORT.

A simple example shows some basic capabilities.