[msm] how to write a generic class for all state machine back-end.

Jerry Chen

22 Jun 2011 22 Jun '11

11:55 p.m.

In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine. struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc } struct SMBase { virtual void start() = 0; ... // how to design the rest ??? } // a wrapper implement SMBase template < typename SM > class SMWrapper : public SMBase{ SMWrapper(SM & s): sm(s){}; virtual void start() { sm.start(); } ... SM sm; } main() { typedef boost::msm::back::state_machine SM1; typedef boost::msm::back::state_machine SM2; User a1(new SMWrapper(SM1()) ); // Wrap the concrete state machine and pass it to User. User a2(new SMWrapper(SM2()) ); // Pass a different state machine. ... } The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. But it also needs to be a template according to state machine interface. But we cannot have a virtual template function. How can we achieve/design this generic state machine interface? Many thanks. Jerry

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Christophe Henry

23 Jun 23 Jun

8:04 p.m.

New subject: [msm] how to write a generic class for all statemachine back-end.

...

In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want

...

to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine.

...

struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc }

...

struct SMBase { virtual void start() = 0; ... // how to design the rest ??? }

...

The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. >But it also needs to be a template according to state machine interface. But we cannot have a virtual template function.

...

How can we achieve/design this generic state machine interface?

Hi Jerry, Yes, virtual template methods are not possible in C++ :( (Note to C++ Standard Comittee: I'd consider donating a kidney for this one :) ) This leaves you with few possibilies. I see 2 at a first glance (suggestions welcome): - virtual void process_event_event1(); virtual void process_event_event2(); ...etc (not fun but quite in the OO spirit) - virtual void process_event(boost::any) followed by any_casts until you find the correct one (not fun either) All considered, solution 1 looks better. If you can define a common interface for state machines, then they are bound to expect common events. HTH, Christophe

Juraj Ivančić

27 Jun 27 Jun

12:36 p.m.

New subject: [msm] how to write a generic class for all statemachine back-end.

On 23.6.2011. 22:04, Christophe Henry wrote:

...

...
In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want

...
to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine.

...
struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc }

...
struct SMBase { virtual void start() = 0; ... // how to design the rest ??? }

...
The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. >But it also needs to be a template according to state machine interface. But we cannot have a virtual template function.

...
How can we achieve/design this generic state machine interface?

Hi Jerry,

Yes, virtual template methods are not possible in C++ :(

(Note to C++ Standard Comittee: I'd consider donating a kidney for this one :) )

This leaves you with few possibilies. I see 2 at a first glance (suggestions welcome):

- virtual void process_event_event1(); virtual void process_event_event2(); ...etc (not fun but quite in the OO spirit)

- virtual void process_event(boost::any) followed by any_casts until you find the correct one (not fun either)

All considered, solution 1 looks better. If you can define a common interface for state machines, then they are bound to expect common events.

I have a combination of the above two which is quite fun :) Use boost.variant on all event types: typedef boost::variant event_variant virtual void process_event( event_variant e ) = 0 and to use its apply_visitor function in derived class to force MSM to instantiate all needed process_event templates (untested): template<typename SM> struct ProcessEvent { ProcessEvent( SM & sm ) : sm_( sm ) {} template<typename event> void operator( event & e ) { sm_.process_event( e ); } SM & sm_; }; template <typename SM> class SMWrapper : public SMBase { virtual void process_event( event_variant ev ) { boost::variant::apply_visitor( ev, ProcessEvent<SM>( sm_ ) ); } }; HTH

Christophe Henry

7:06 p.m.

New subject: [msm] how to write a generic class for allstatemachine back-end.

...

On 23.6.2011. 22:04, Christophe Henry wrote:

...
...
In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want

...
to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine.

...
struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc }

...
struct SMBase { virtual void start() = 0; ... // how to design the rest ??? }

...
The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. >But it also needs to be a template according to state machine interface. But we cannot have a virtual template function.

...
How can we achieve/design this generic state machine interface?

Hi Jerry,

Yes, virtual template methods are not possible in C++ :(

(Note to C++ Standard Comittee: I'd consider donating a kidney for this one :) )

This leaves you with few possibilies. I see 2 at a first glance (suggestions welcome):

- virtual void process_event_event1(); virtual void process_event_event2(); ...etc (not fun but quite in the OO spirit)

- virtual void process_event(boost::any) followed by any_casts until you find the correct one (not fun either)

All considered, solution 1 looks better. If you can define a common interface for state machines, then they are bound to expect common events.

I have a combination of the above two which is quite fun :) Use boost.variant on all event types:

typedef boost::variant event_variant

virtual void process_event( event_variant e ) = 0

and to use its apply_visitor function in derived class to force MSM to instantiate all needed process_event templates (untested):

template<typename SM> struct ProcessEvent { ProcessEvent( SM & sm ) : sm_( sm ) {} template<typename event> void operator( event & e ) { sm_.process_event( e ); }

SM & sm_; };

template <typename SM> class SMWrapper : public SMBase { virtual void process_event( event_variant ev ) { boost::variant::apply_visitor( ev, ProcessEvent<SM>( sm_ ) ); } };

Ah yes I didn't think about this solution, it's clearly more elegant. I need to give it a try. Good one :) Thanks, Christophe

Michael Caisse

7:10 p.m.

New subject: [msm] how to write a generic class for allstatemachine back-end.

On 06/27/2011 12:06 PM, Christophe Henry wrote:

...

...
On 23.6.2011. 22:04, Christophe Henry wrote:

...
...
In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want

...
to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine.

...
struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc }

...
struct SMBase { virtual void start() = 0; ... // how to design the rest ??? }

...
The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. But it also needs to be a template according to state machine interface. But we cannot have a virtual template function.

...
How can we achieve/design this generic state machine interface?

Hi Jerry,

Yes, virtual template methods are not possible in C++ :(

(Note to C++ Standard Comittee: I'd consider donating a kidney for this one :) )

This leaves you with few possibilies. I see 2 at a first glance (suggestions welcome):

- virtual void process_event_event1(); virtual void process_event_event2(); ...etc (not fun but quite in the OO spirit)

- virtual void process_event(boost::any) followed by any_casts until you find the correct one (not fun either)

All considered, solution 1 looks better. If you can define a common interface for state machines, then they are bound to expect common events.

I have a combination of the above two which is quite fun :) Use boost.variant on all event types:

typedef boost::variant event_variant

virtual void process_event( event_variant e ) = 0

and to use its apply_visitor function in derived class to force MSM to instantiate all needed process_event templates (untested):

template<typename SM> struct ProcessEvent { ProcessEvent( SM & sm ) : sm_( sm ) {} template<typename event> void operator( event & e ) { sm_.process_event( e ); }

SM & sm_; };

template <typename SM> class SMWrapper : public SMBase { virtual void process_event( event_variant ev ) { boost::variant::apply_visitor( ev, ProcessEvent<SM>( sm_ ) ); } };

Ah yes I didn't think about this solution, it's clearly more elegant. I need to give it a try. Good one :)

Thanks,

Christophe

FWIW ... this is basically what I do also. It works nicely. -- Michael Caisse Object Modeling Designs www.objectmodelingdesigns.com

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Christophe Henry
Jerry Chen
Juraj Ivančić
Michael Caisse