The Boost C++ Libraries

Chapter 68. Boost.MetaStateMachine

Boost.MetaStateMachine is used to define state machines. State machines describe objects through their states. They describe what states exist and what transitions between states are possible.

Boost.MetaStateMachine provides three different ways to define state machines. The code you need to write to create a state machine depends on the front-end.

If you go with the basic front-end or the function front-end, you define state machines in the conventional way: you create classes, derive them from other classes provided by Boost.MetaStateMachine, define required member variables, and write the required C++ code yourself. The fundamental difference between the basic front-end and the function front-end is that the basic front-end expects function pointers, while the function front-end lets you use function objects.

The third front-end is called eUML and is based on a domain-specific language. This front-end makes it possible to define state machines by reusing definitions of a UML state machine. Developers familiar with UML can copy definitions from a UML behavior diagram to C++ code. You don’t need to translate UML definitions to C++ code.

eUML is based on a set of macros that you must use with this front-end. The advantage of the macros is that you don’t need to work directly with many of the classes provided by Boost.MetaStateMachine. You just need to know which macros to use. This means you can’t forget to derive your state machine from a class, which can happen with the basic front-end or the function front-end. This chapter introduces Boost.MetaStateMachine with eUML.

Example 68.1. Simple state machine with eUML
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_STATE((), Off)
BOOST_MSM_EUML_STATE((), On)

BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press == On,
  On + press == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off),
light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  std::cout << *light.current_state() << '\n';
  light.process_event(press);
  std::cout << *light.current_state() << '\n';
  light.process_event(press);
  std::cout << *light.current_state() << '\n';
}

Example 68.1 uses the simplest state machine possible: A lamp has exactly two states. It is either on or off. If it is off, it can be switched on. If it is on, it can be switched off. It is possible to switch from every state to every other state.

Example 68.1 uses the eUML front-end to describe the state machine of a lamp. Boost.MetaStateMachine doesn’t have a master header file. Therefore, the required header files have to be included one by one. boost/msm/front/euml/euml.hpp and boost/msm/front/euml/state_grammar.hpp provide access to eUML macros. boost/msm/back/state_machine.hpp is required to link a state machine from the front-end to a state-machine from the back-end. While front-ends provide various possibilities to define state machines, the actual implementation of a state machine is found in the back-end. Since Boost.MetaStateMachine contains only one back-end, you don’t need to select an implementation.

All of the definitions from Boost.MetaStateMachine are in the namespace boost::msm. Unfortunately, many eUML macros don’t refer explicitly to classes in this namespace. They use either the namespace msm or no namespace at all. That’s why Example 68.1 creates an alias for the namespace boost::msm and makes the definitions in boost::msm::front::euml available with a using directive. Otherwise the eUML macros lead to compiler errors.

To use the state machine of a lamp, first define the states for off and on. States are defined with the macro BOOST_MSM_EUML_STATE, which expects the name of the state as its second parameter. The first parameter describes the state. You’ll see later how these descriptions look like. The two states defined in Example 68.1 are called Off and On.

To switch between states, events are required. Events are defined with the macro BOOST_MSM_EUML_EVENT, which expects the name of the event as its sole parameter. Example 68.1 defines an event called press, which represents the action of pressing the light switch. Since the same event switches a light on and off, only one event is defined.

When the required states and events are defined, the macro BOOST_MSM_EUML_TRANSITION_TABLE is used to create a transition table. This tables defines valid transitions between states and which events trigger which state transitions.

BOOST_MSM_EUML_TRANSITION_TABLE expects two parameters. The first parameter defines the transition table, and the second is the name of the transition table. The syntax of the first parameter is designed to make it easy to recognize how states and events relate to each other. For example, Off + press == On means that the machine in the state Off switches to the state On with the event press. The intuitive and self-explanatory syntax of a transition table definition is one of the strengths of the eUML front-end.

After the transition table has been created, the state machine is defined with the macro BOOST_MSM_EUML_DECLARE_STATE_MACHINE. The second parameter is again the simpler one: it sets the name of the state machine. The state machine in Example 68.1 is named light_state_machine.

The first parameter of BOOST_MSM_EUML_DECLARE_STATE_MACHINE is a tuple. The first value is the name of the transition table. The second value is an expression using init_, which is an attribute provided by Boost.MetaStateMachine. You’ll learn more about attributes later. The expression init_ << Off is required to set the initial state of the state machine to Off.

The state machine light_state_machine, defined with BOOST_MSM_EUML_DECLARE_STATE_MACHINE, is a class. You use this class to instantiate a state machine from the back-end. In Example 68.1 this is done by passing light_state_machine to the class template boost::msm::back::state_machine as a parameter. This creates a state machine called light.

State machines provide a member function process_event() to process events. If you pass an event to process_event(), the state machines changes its state depending on its transition table.

To make it easier to see what happens in Example 68.1 when process_event() is called multiple times, current_state() is called. This member function should only be used for debugging purposes. It returns a pointer to an int. Every state is an int value assigned in the order the states have been accessed in BOOST_MSM_EUML_TRANSITION_TABLE. In Example 68.1 Off is assigned the value 0 and On is assigned the value 1. The example writes 0, 1, and 0 to standard output. The light switch is pressed two times, which switches the light on and off.

Example 68.2. State machine with state, event, and action
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_STATE((), Off)
BOOST_MSM_EUML_STATE((), On)

BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_ACTION(switch_light)
{
  template <class Event, class Fsm>
  void operator()(const Event &ev, Fsm &fsm,
    BOOST_MSM_EUML_STATE_NAME(Off) &sourceState,
    BOOST_MSM_EUML_STATE_NAME(On) &targetState) const
  {
    std::cout << "Switching on\n";
  }

  template <class Event, class Fsm>
  void operator()(const Event &ev, Fsm &fsm,
    decltype(On) &sourceState,
    decltype(Off) &targetState) const
  {
    std::cout << "Switching off\n";
  }
};

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press / switch_light == On,
  On + press / switch_light == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off),
light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  light.process_event(press);
  light.process_event(press);
}

Example 68.2 extends the state machine for the lamp by an action. An action is executed by an event triggering a state transition. Because actions are optional, a state machine could be defined without them.

Actions are defined with BOOST_MSM_EUML_ACTION. Strictly speaking, a function object is defined. You must overload the operator operator(). The operator must accept four parameters. The parameters reference an event, a state machine and two states. You are free to define a template or use concrete types for all of the parameters. In Example 68.2, concrete types are only set for the last two parameters. Because these parameters describe the beginning and ending states, you can overload operator() so that different member functions are executed for different switches.

Please note that the states On and Off are objects. Boost.MetaStateMachine provides a macro BOOST_MSM_EUML_STATE_NAME to get the type of a state. If you use C++11, you can use the operator decltype instead of the macro.

The action switch_light, which has been defined with BOOST_MSM_EUML_ACTION, is executed when the light switch is pressed. The transition table has been changed accordingly. The first transition is now Off + press / switch_light == On. You pass actions after a slash after the event. This transition means that the operator operator() of switch_light is called if the current state is Off and the event press happens. After the action has been executed, the new state is On.

Example 68.2 writes Switching on and then Switching off to standard output.

Example 68.3. State machine with state, event, guard, and action
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_STATE((), Off)
BOOST_MSM_EUML_STATE((), On)

BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_ACTION(is_broken)
{
  template <class Event, class Fsm, class Source, class Target>
  bool operator()(const Event &ev, Fsm &fsm, Source &src, Target &trg) const
  {
    return true;
  }
};

BOOST_MSM_EUML_ACTION(switch_light)
{
  template <class Event, class Fsm, class Source, class Target>
  void operator()(const Event &ev, Fsm &fsm, Source &src, Target &trg) const
  {
    std::cout << "Switching\n";
  }
};

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press [!is_broken] / switch_light == On,
  On + press / switch_light == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off),
light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  light.process_event(press);
  light.process_event(press);
}

Example 68.3 uses a guard in the transition table. The definition of the first transition is Off + press [!is_broken] / switch_light == On. Passing is_broken in brackets means that the state machine checks before the action switch_light is called whether the transition may occur. This is called a guard. A guard must return a result of type bool.

A guard like is_broken is defined with BOOST_MSM_EUML_ACTION in the same way as actions. Thus, the operator operator() has to be overloaded for the same four parameters. operator() must have a return value of type bool to be used as a guard.

Please note that you can use logical operators like operator! on guards inside brackets.

If you run the example, you’ll notice that nothing is written to standard output. The action switch_light is not executed – the light stays off. The guard is_broken returns true. However, because the operator operator! is used, the expression in brackets evaluates to false.

You can use guards to check whether a state transition can occur. Example 68.3 uses is_broken to check whether the lamp is broken. While a transition from off to on is usually possible and the transition table describes lamps correctly, in this example, the lamp cannot be switched on. Despite two calls to process_event(), the state of light is Off.

Example 68.4. State machine with state, event, entry action, and exit action
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_ACTION(state_entry)
{
  template <class Event, class Fsm, class State>
  void operator()(const Event &ev, Fsm &fsm, State &state) const
  {
    std::cout << "Entering\n";
  }
};

BOOST_MSM_EUML_ACTION(state_exit)
{
  template <class Event, class Fsm, class State>
  void operator()(const Event &ev, Fsm &fsm, State &state) const
  {
    std::cout << "Exiting\n";
  }
};

BOOST_MSM_EUML_STATE((state_entry, state_exit), Off)
BOOST_MSM_EUML_STATE((state_entry, state_exit), On)

BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press == On,
  On + press == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off),
light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  light.process_event(press);
  light.process_event(press);
}

In Example 68.4, the first parameter passed to BOOST_MSM_EUML_STATE is a tuple consisting of state_entry and state_exit. state_entry is an entry action, and state_exit is an exit action. These actions are executed when a state is entered or exited.

Like actions, entry and exit actions are defined with BOOST_MSM_EUML_ACTION. However, the overloaded operator operator() expects only three parameters: references to an event, a state machine, and a state. Transitions between states don’t matter for entry and exit actions, so only one state needs to be passed to operator(). For entry actions, this state is entered. For exit actions, this state is exited.

In Example 68.4, both states Off and On have entry and exit actions. Because the event press occurs twice, Entering and Exiting is displayed twice. Please note that Exiting is displayed first and Entering afterwards because the first action executed is an exit action.

The first event press triggers a transition from Off to On, and Exiting and Entering are each displayed once. The second event press switches the state to Off. Again Exiting and Entering are each displayed once. Thus, state transitions execute the exit action first, then the entry action of the new state.

Example 68.5. Attributes in a state machine
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_DECLARE_ATTRIBUTE(int, switched_on)

BOOST_MSM_EUML_ACTION(state_entry)
{
  template <class Event, class Fsm, class State>
  void operator()(const Event &ev, Fsm &fsm, State &state) const
  {
    std::cout << "Switched on\n";
    ++fsm.get_attribute(switched_on);
  }
};
BOOST_MSM_EUML_ACTION(is_broken)
{
  template <class Event, class Fsm, class Source, class Target>
  bool operator()(const Event &ev, Fsm &fsm, Source &src, Target &trg) const
  {
    return fsm.get_attribute(switched_on) > 1;
  }
};

BOOST_MSM_EUML_STATE((), Off)
BOOST_MSM_EUML_STATE((state_entry), On)
BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press [!is_broken] == On,
  On + press == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off, no_action, no_action,
attributes_ << switched_on), light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
}

Example 68.5 uses the guard is_broken to check whether a state transition from Off to On is possible. This time the return value of is_broken depends on how often the light switch has been pressed. It is possible to switch the light on two times before the lamp is broken. To count how often the light has been switched on, an attribute is used.

Attributes are variables that can be attached to objects. They let you adapt the behavior of state machines at run time. Because data such as how often the light has been switched on has to be stored somewhere, it makes sense to store it directly in the state machine, in a state, or in an event.

Before an attribute can be used, it has to be defined. This is done with the macro BOOST_MSM_EUML_DECLARE_ATTRIBUTE. The first parameter passed to BOOST_MSM_EUML_DECLARE_ATTRIBUTE is the type, and the second is the name of the attribute. Example 68.5 defines the attribute switched_on of type int.

After the attribute has been defined, it must be attached to an object. The example attaches the attribute switched_on to the state machine. This is done via the fifth value in the tuple, which is passed as the first parameter to BOOST_MSM_EUML_DECLARE_STATE_MACHINE. With attributes_, a keyword from Boost.MetaStateMachine is used to create a lambda function. To attach the attribute switched_on to the state machine, write switched_on to attributes_ as though it were a stream, using operator<<.

The third and fourth values in the tuples are both set to no_action. The attribute is passed as the fifth value in the tuple. The third and fourth values can be used to define entry and exit actions for the state machine. If no entry and exit actions are defined, use no_action.

After the attribute has been attached to the state machine, it can be accessed with get_attribute(). In Example 68.5, this member function is called in the entry action state_entry to increment the value of the attribute. Because state_entry is only linked to the state On, switched_on is only incremented when the light is switched on.

switched_on is also accessed from the guard is_broken, which checks whether the value of the attribute is greater than 1. If it is, the guard returns true. Because attributes are initialized with the default constructor and switched_on is set to 0, is_broken returns true if the light has been switched on two times.

In Example 68.5, the event press occurs five times. The light is switched on and off two times and then switched on again. The first two times the light is switched on, Switched on is displayed. However, the third time the light is switched on there is no output. This happens because is_broken returns true after the light has been switched on two times, and therefore, there is no state transition from Off to On. This means the entry action for the state On is not executed, and the example does not write to standard output.

Example 68.6. Accessing attributes in transition tables
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/euml/state_grammar.hpp>
#include <boost/msm/back/state_machine.hpp>
#include <iostream>

namespace msm = boost::msm;
using namespace boost::msm::front::euml;

BOOST_MSM_EUML_DECLARE_ATTRIBUTE(int, switched_on)

void write_message()
{
  std::cout << "Switched on\n";
}

BOOST_MSM_EUML_FUNCTION(WriteMessage_, write_message, write_message_,
  void, void)

BOOST_MSM_EUML_STATE((), Off)
BOOST_MSM_EUML_STATE((), On)

BOOST_MSM_EUML_EVENT(press)

BOOST_MSM_EUML_TRANSITION_TABLE((
  Off + press [fsm_(switched_on) < Int_<2>()] / (++fsm_(switched_on),
    write_message_()) == On,
  On + press == Off
), light_transition_table)

BOOST_MSM_EUML_DECLARE_STATE_MACHINE(
(light_transition_table, init_ << Off, no_action, no_action,
attributes_ << switched_on), light_state_machine)

int main()
{
  msm::back::state_machine<light_state_machine> light;
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
  light.process_event(press);
}

Example 68.6 does the same thing as Example 68.5: after switching the light on two times, the light is broken and can’t be switched on anymore. While the previous example accessed the attribute switched_on in actions, this example uses attributes in the transition table.

Boost.MetaStateMachine provides the function fsm_() to access an attribute in a state machine. That way a guard is defined that checks whether switched_on is smaller than 2. And an action is defined that increments switched_on every time the state switches from Off to On.

Please note that the smaller-than comparison in the guard is done with Int_<2>(). The number 2 must be passed as a template parameter to Int_ to create an instance of this class. That creates a function object that has the type needed by Boost.MetaStateMachine.

Example 68.6 also uses the macro BOOST_MSM_EUML_FUNCTION to make a function an action. The first parameter passed to BOOST_MSM_EUML_FUNCTION is the name of the action that can be used in the function front-end. The second parameter is the name of the function. The third parameter is the name of the action as it is used in eUML. The fourth and fifth parameters are the return values for the function – one for the case where the action is used for a state transition, and the other for the case where the action describes an entry or exit action. After write_message() has been turned into an action this way, an object of type write_message_ is created and used following ++fsm_(switched_on) in the transition table. In a state transition from Off to On, the attribute switched_on is incremented and then write_message() is called.

Example 68.6 displays Switched on twice, as in Example 68.5.

Boost.MetaStateMachine provides additional functions, such as state_() and event_(), to access attributes attached to other objects. Other classes, such as Char_ and String_, can also be used like Int_.

Tip

As you can see in the examples, the front-end eUML requires you to use many macros. The header file boost/msm/front/euml/common.hpp contains definitions for all of the eUML macros, which makes it a useful reference.