Back-end
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## Name

Back-end — The back-end headers

## msm/back/state_machine.hpp

This header provides one type, state_machine, MSM's state machine engine implementation.

     template <class Derived,class HistoryPolicy=NoHistory,class
                                CompilePolicy=favor_runtime_speed> state_machine {
    }

### Template arguments

#### Derived

The name of the front-end state machine definition. All three front-ends are possible.

#### HistoryPolicy

The desired history. This can be: AlwaysHistory, NoHistory, ShallowHistory. Default is NoHistory.

#### CompilePolicy

The trade-off performance / compile-time. There are two predefined policies, favor_runtime_speed and favor_compile_time. Default is favor_runtime_speed, best performance, longer compile-time. See the backend.

### methods

#### start

The start methods must be called before any call to process_event. It activates the entry action of the initial state(s). This allows you to choose when a state machine can start. See backend.

`void start();`

#### process_event

The event processing method implements the double-dispatch. Each call to this function with a new event type instantiates a new dispatch algorithm and increases compile-time.

`template <class Event> HandledEnum process_event(Event const&);`

#### current_state

Returns the ids of currently active states. You will typically need it only for debugging or logging purposes.

`const int* current_state const();`

#### get_state_by_id

Returns the state whose id is given. As all states of a concrete state machine share a common base state, the return value is a base state. If the id corresponds to no state, a null pointer is returned.

`const BaseState* get_state_by_id const(int id);`

#### is_contained

Helper returning true if the state machine is contained as a submachine of another state machine.

`bool is_contained const();`

#### get_state

Returns the required state of the state machine as a pointer. A compile error will occur if the state is not to be found in the state machine.

`template <class State> State* get_state();`

#### get_state

Returns the required state of the state machine as a reference. A compile error will occur if the state is not to be found in the state machine.

`template <class State> State& get_state();`

#### is_flag_active

Returns true if the given flag is currently active. A flag is active if the active state of one region is tagged with this flag (using OR as BinaryOp) or active states of all regions (using AND as BinaryOp)

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

#### is_flag_active

Returns true if the given flag is currently active. A flag is active if the active state of one region is tagged with this flag.

`template <class Flag> bool is_flag_active();`

#### visit_current_states

Visits all active states and their substates. A state is visited using the `accept` method without argument. The base class of all states must provide an `accept_sig` type.

`void visit_current_states();`

#### visit_current_states

Visits all active states and their substates. A state is visited using the `accept` method with arguments. The base class of all states must provide an `accept_sig` type defining the signature and thus the number and type of the parameters.

`void visit_current_states(any-type param1, any-type param2,...);`

#### defer_event

Defers the provided event. This method can be called only if at least one state defers an event or if the state machine provides the `activate_deferred_events`(see example) type either directly or using the deferred_events configuration of eUML (`configure_ << deferred_events`)

`template <class Event> void defer_event(Event const&);`

### Types

#### nr_regions

The number of orthogonal regions contained in the state machine

#### entry_pt

This nested type provides the necessary typedef for entry point pseudostates. `state_machine<...>::entry_pt<state_name>` is a transition's valid target inside the containing state machine's transition table.

     entry_pt {
    }

#### exit_pt

This nested type provides the necessary typedef for exit point pseudostates. `state_machine<...>::exit_pt<state_name>` is a transition's valid source inside the containing state machine's transition table.

     exit_pt {
    }

#### direct

This nested type provides the necessary typedef for an explicit entry inside a submachine. `state_machine<...>::direct<state_name>` is a transition's valid target inside the containing state machine's transition table.

     direct {
    }

#### stt

Calling state_machine<frontend>::stt returns a mpl::vector containing the transition table of the state machine. This type can then be used with generate_state_set or generate_event_set.

## args.hpp

This header provides one type, args. which provides the necessary types for a visitor implementation.

## msm/back/history_policies.hpp

This header provides the out-of-the-box history policies supported by MSM. There are 3 such policies.

### Every history policy must implement the following methods:

#### set_initial_states

This method is called by msm::back::state_machine when constructed. It gives the policy a chance to save the ids of all initial states (passed as array).

`void set_initial_states(` | `)`; |

``(`int* const`)`` ;

#### history_exit

This method is called by msm::back::state_machine when the submachine is exited. It gives the policy a chance to remember the ids of the last active substates of this submachine (passed as array).

`void history_exit(` | `)`; |

``(`int* const`)`` ;

#### history_entry

This method is called by msm::back::state_machine when the submachine is entered. It gives the policy a chance to set the active states according to the policy's aim. The policy gets as parameter the event which activated the submachine and returns an array of active states ids.

`template <class Event> int* const history_exit(` | `)`; |

``(`Event const&`)`` ;

### Out-of-the-box policies:

#### NoHistory

This policy is the default used by state_machine. No active state of a submachine is remembered and at every new activation of the submachine, the initial state(s) are activated.

#### AlwaysHistory

This policy is a non-UML-standard extension. The active state(s) of a submachine is (are) always remembered at every new activation of the submachine.

#### ShallowHistory

This policy activates the active state(s) of a submachine if the event is found in the policy's event list.

## msm/back/default_compile_policy.hpp

This header contains the definition of favor_runtime_speed. This policy has two settings:

- Submachines dispatch faster because their transitions are added into their containing machine's transition table instead of simply forwarding events.

- It solves transition conflicts at compile-time

## msm/back/favor_compile_time.hpp

This header contains the definition of favor_compile_time. This policy has two settings:

- Submachines dispatch is slower because all events, even those with no dispatch chance, are forwarded to submachines. In exchange, no row is added into the containing machine's transition table, which reduces compile-time.

- It solves transition conflicts at run-time.

## msm/back/metafunctions.hpp

This header contains metafunctions for use by the library. Three metafunctions can be useful for the user:

- `generate_state_set< stt >`: generates the list of all states referenced by the transition table stt. If stt is a recursive table (generated by `recursive_get_transition_table`), the metafunction finds recursively all states of the submachines. A non-recursive table can be obtained with some_backend_fsm::stt.

- `generate_event_set< stt>`: generates the list of all events referenced by the transition table stt. If stt is a recursive table (generated by `recursive_get_transition_table`), the metafunction finds recursively all events of the submachines. A non-recursive table can be obtained with some_backend_fsm::stt.

- `recursive_get_transition_table<fsm>`: recursively extends the transition table of the state machine fsm with tables from the submachines.

## msm/back/tools.hpp

This header contains a few metaprogramming tools to get some information out of a state machine.

### fill_state_names

#### attributes

fill_state_names has for attribute:

- `char const** m_names`: an already allocated array of const char* where the typeid-generated names of a state machine states will be witten.

#### constructor

`char const** names_to_fill(char const** names_to_fill);`

#### usage

fill_state_names is made for use in a mpl::for_each iterating on a state list and writing inside a pre-allocated array the state names. Example:

    typedef some_fsm::stt Stt;
    typedef msm::back::generate_state_set<Stt>::type all_states; //states
    static char const* state_names[mpl::size<all_states>::value];
    // array to fill with names
    // fill the names of the states defined in the state machine
    mpl::for_each<all_states,boost::msm::wrap<mpl::placeholders::_1> >
        (msm::back::fill_state_names<Stt>(state_names));
    // display all active states
    for (unsigned int i=0;i<some_fsm::nr_regions::value;++i)
    {
        std::cout << " -> "
                  << state_names[my_fsm_instance.current_state()[i]]
                  << std::endl;
    }

### get_state_name

#### attributes

get_state_name has for attributes:

- std::string& m_name: the return value of the iteration

- int m_state_id: the searched state's id

#### constructor

The constructor takes as argument a reference to the string to fill with the state name and the id which must be searched.

`string& name_to_fill,int state_id(string& name_to_fill,int state_id);`

#### usage

This type is made for the same search as in the previous example, using a mpl::for_each to iterate on states. After the iteration, the state name reference has been set.

    // we need a fsm's table
    typedef player::stt Stt;
    typedef msm::back::generate_state_set<Stt>::type all_states; //all states
    std::string name_of_open; // id of Open is 1
    // fill name_of_open for state of id 1
    boost::mpl::for_each<all_states,boost::msm::wrap<mpl::placeholders::_1> >
              (msm::back::get_state_name<Stt>(name_of_open,1));
    std::cout << "typeid-generated name Open is: " <<  name_of_open << std::endl;

### display_type

#### attributes

none

#### usage

Reusing the state list from the previous example, we can output all state names:

`mpl::for_each<all_states,boost::msm::wrap<mpl::placeholders::_1> >(msm::back::display_type ());`

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