Why does my use of context().method() viola

Question

I've developed some concept code for a project that I will be working on shortly. The project lends itself to a state machine design and I think boost::statechart will do a good job. I hit a roadblock when I tried to use context() however. Here's a sample (I'm happy to put more code up, but I think this is the relevant part):

struct Wait : fsm::simple_state< Wait, Active > {

  typedef mpl::list<fsm::transition< UnderflowEvent, Exec> > reactions;

 public:
  Wait()
    : m_wait_op() {
    std::cout << "entering wait state." << std::endl;
    wait();
  }
  ~Wait() { std::cout << "exiting wait state." << std::endl; }

 private:
  default_wait m_wait_op;
  fsm::result wait() {
    if(context<Active>().underflow_condition()) {
      m_wait_op();
      return transit<Wait>();
    }
    else if(context<Active>().overflow_condition()) {
      return transit<Exec>();
    }
    else {
      // undefined - keep waiting                                                                                                                                                
    }
  }

};

The state Active has methods called "[over|under]flow_condition" which just return true at this point. Problems with my design aside, I am getting the following assertion failure when I instantiate thusly:

int main(void) {

  Throttler my_throttler;

  my_throttler.initiate();

  return 0;

}

and here's the assertion:

assertion "get_pointer( stt.pContext_ ) != 0" failed

I looked this assertion up in file "/usr/include/boost/statechart/simple_state.hpp", line 689 (boost 1.45) and the comments say that it is there to prevent simple_state from using contexts. This puzzled me when I revisited the stopwatch example and saw that the example was doing the very thing I was trying to do. So I compiled it and this assertion is not violated by the stopwatch code unsurprisingly. Am I missing something? Maybe there's something elsewhere in the code that I missed? Here's the entire header (please remember it's concept code... I'm not releasing this into the wild until it's been thoroughly genericized):

    #ifndef _THROTTLER_H_
#define _THROTTLER_H_

#include<queue>
#include<vector>
#include<ctime>

#include<boost/statechart/event.hpp>
#include<boost/statechart/transition.hpp>
#include<boost/statechart/state_machine.hpp>
#include<boost/statechart/simple_state.hpp>

#include<boost/mpl/list.hpp>

#include<iostream>


namespace mpl = boost::mpl;
namespace fsm = boost::statechart;

namespace {

  unsigned int DEFAULT_WAIT_TIME(1000);

}

struct noop {
public:

  noop() { m_priority = (1 << sizeof(int)); }
  noop(unsigned int priority) { m_priority = priority; }
  virtual ~noop() {}

  bool operator()(void) {
    return true;
  }
  friend bool operator<(noop a, noop b);

private:
  unsigned int m_priority;
};



bool operator<(noop a, noop b) {
  return a.m_priority < b.m_priority;
}

struct compare_noops {
  bool operator()(noop a, noop b) {
  }
};


struct default_wait {
  void operator()(unsigned long msecs = DEFAULT_WAIT_TIME) {
    std::clock_t endtime =
      std::clock() + (msecs*1000*CLOCKS_PER_SEC);
    while(clock() < endtime);
  }
};



struct OverflowEvent : fsm::event< OverflowEvent > {};
struct UnderflowEvent : fsm::event< UnderflowEvent > {};
struct ResetEvent : fsm::event< ResetEvent > {};

struct Active;
struct Throttler : fsm::state_machine< Throttler, Active > {};

struct Wait;
struct Active : fsm::simple_state< Active, Throttler, Wait > {

 public:

  typedef mpl::list<fsm::transition< ResetEvent, Active> > reactions;

  bool overflow_condition(void) { return true; }
  bool underflow_condition(void) { return true; }

  void queue_operation(noop op) {
    m_operation_queue.push(op);
  }
  void perform_operation(void) {
    noop op(m_operation_queue.top());
    if(op())
      m_operation_queue.pop();
    else
      throw;
  }

 private:

  std::priority_queue<noop, std::vector<noop>, compare_noops > m_operation_queue;
 private:

  std::priority_queue<noop, std::vector<noop>, compare_noops > m_operation_queue;

};

struct Exec : fsm::simple_state< Exec, Active > {

  typedef mpl::list<fsm::transition< OverflowEvent, Wait> > reactions;

  Exec() { std::cout << "entering exec state." << std::endl; }
  ~Exec() { std::cout << "exiting exec state." << std::endl; }

};

struct Wait : fsm::simple_state< Wait, Active > {

  typedef mpl::list<fsm::transition< UnderflowEvent, Exec> > reactions;

 public:
  Wait()
    : m_wait_op() {
    std::cout << "entering wait state." << std::endl;
    wait();
  }
  ~Wait() { std::cout << "exiting wait state." << std::endl; }

 private:
  default_wait m_wait_op;
  fsm::result wait() {
    if(context<Active>().underflow_condition()) {
      m_wait_op();
      return transit<Wait>();
    }
    else if(context<Active>().overflow_condition()) {
      return transit<Exec>();
    }
    else {
      // undefined - keep waiting                                                                                                                                                
    }
  }

};


#endif

Answer 1

As you've noted in your comment, it's related to attempting to access the outer context from within the constructor, which is not allowed for a simple_state.

From simple_state.hpp:

  // This assert fails when an attempt is made to access the state machine
  // from a constructor of a state that is *not* a subtype of state<>.
  // To correct this, derive from state<> instead of simple_state<>.
  BOOST_ASSERT( get_pointer( pContext_ ) != 0 );

So you should be able to access the outer context from the constructor if you base your states on the state class (rather than a simple_state).

That said, I'm not sure what impacts or implications this may have for your states. If this question gets an answer it may be helpful to you as well (:

From what I understand, you'll need to change Wait to derive from state:

 struct Wait : fsm::state< Wait, Active > {

and then change the Wait() constructor to something like

typedef fsm::state< Wait, Active > my_base;
Wait( my_context ctx ) : my_base( ctx )
// and any other pre-constructor initialisation...

The my_context type is defined (as protected) within state<>, and needs to be passed in from the derived class's constructor.