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POWhy does my use of context<State>().method() violate statechart assertion?
 

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  1. POWhy does my use of context<State>().method() violate statechart assertion?
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    <p>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):</p> <pre><code>struct Wait : fsm::simple_state&lt; Wait, Active &gt; { typedef mpl::list&lt;fsm::transition&lt; UnderflowEvent, Exec&gt; &gt; reactions; public: Wait() : m_wait_op() { std::cout &lt;&lt; "entering wait state." &lt;&lt; std::endl; wait(); } ~Wait() { std::cout &lt;&lt; "exiting wait state." &lt;&lt; std::endl; } private: default_wait m_wait_op; fsm::result wait() { if(context&lt;Active&gt;().underflow_condition()) { m_wait_op(); return transit&lt;Wait&gt;(); } else if(context&lt;Active&gt;().overflow_condition()) { return transit&lt;Exec&gt;(); } else { // undefined - keep waiting } } }; </code></pre> <p>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:</p> <pre><code>int main(void) { Throttler my_throttler; my_throttler.initiate(); return 0; } </code></pre> <p>and here's the assertion:</p> <blockquote> <p>assertion "get_pointer( stt.pContext_ ) != 0" failed</p> </blockquote> <p>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):</p> <pre><code> #ifndef _THROTTLER_H_ #define _THROTTLER_H_ #include&lt;queue&gt; #include&lt;vector&gt; #include&lt;ctime&gt; #include&lt;boost/statechart/event.hpp&gt; #include&lt;boost/statechart/transition.hpp&gt; #include&lt;boost/statechart/state_machine.hpp&gt; #include&lt;boost/statechart/simple_state.hpp&gt; #include&lt;boost/mpl/list.hpp&gt; #include&lt;iostream&gt; namespace mpl = boost::mpl; namespace fsm = boost::statechart; namespace { unsigned int DEFAULT_WAIT_TIME(1000); } struct noop { public: noop() { m_priority = (1 &lt;&lt; sizeof(int)); } noop(unsigned int priority) { m_priority = priority; } virtual ~noop() {} bool operator()(void) { return true; } friend bool operator&lt;(noop a, noop b); private: unsigned int m_priority; }; bool operator&lt;(noop a, noop b) { return a.m_priority &lt; 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() &lt; endtime); } }; struct OverflowEvent : fsm::event&lt; OverflowEvent &gt; {}; struct UnderflowEvent : fsm::event&lt; UnderflowEvent &gt; {}; struct ResetEvent : fsm::event&lt; ResetEvent &gt; {}; struct Active; struct Throttler : fsm::state_machine&lt; Throttler, Active &gt; {}; struct Wait; struct Active : fsm::simple_state&lt; Active, Throttler, Wait &gt; { public: typedef mpl::list&lt;fsm::transition&lt; ResetEvent, Active&gt; &gt; 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&lt;noop, std::vector&lt;noop&gt;, compare_noops &gt; m_operation_queue; private: std::priority_queue&lt;noop, std::vector&lt;noop&gt;, compare_noops &gt; m_operation_queue; }; struct Exec : fsm::simple_state&lt; Exec, Active &gt; { typedef mpl::list&lt;fsm::transition&lt; OverflowEvent, Wait&gt; &gt; reactions; Exec() { std::cout &lt;&lt; "entering exec state." &lt;&lt; std::endl; } ~Exec() { std::cout &lt;&lt; "exiting exec state." &lt;&lt; std::endl; } }; struct Wait : fsm::simple_state&lt; Wait, Active &gt; { typedef mpl::list&lt;fsm::transition&lt; UnderflowEvent, Exec&gt; &gt; reactions; public: Wait() : m_wait_op() { std::cout &lt;&lt; "entering wait state." &lt;&lt; std::endl; wait(); } ~Wait() { std::cout &lt;&lt; "exiting wait state." &lt;&lt; std::endl; } private: default_wait m_wait_op; fsm::result wait() { if(context&lt;Active&gt;().underflow_condition()) { m_wait_op(); return transit&lt;Wait&gt;(); } else if(context&lt;Active&gt;().overflow_condition()) { return transit&lt;Exec&gt;(); } else { // undefined - keep waiting } } }; #endif </code></pre>
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