C++ Boost状态图：返回到以前的状态

我有一个状态机，如果我进入一个特定的状态，有时我需要定期转换到另一个状态，有时我需要返回到以前的状态

例如，有状态A B C，假设转换S将状态A移动到C，并从B移动到C。我需要转换T在状态A中发生时将C移动到A，在状态B中发生时将C移动到B

在下面的代码中，转换S发生在状态B中，因此我希望转换T返回到状态B（而当前，它返回到状态A）

我想看到的是：

entered B
entered C
entered B
entered C
entered D

---

使用Boost:：Statechart有什么干净的方法可以做到这一点吗？

我找到了一种~okay~方法，为状态创建枚举映射，将以前的状态存储在最外层的上下文（顶级fsm）中，然后对T事件使用自定义反应：

#include <boost/mpl/list.hpp>

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

// states
struct A;
struct B;
struct C;
struct D;

// state enum mapping
enum class state_mapping {
    A = 0,
    B,
    C,
    D
};

// events
struct S : boost::statechart::event<S> {};
struct T : boost::statechart::event<T> {};
struct U : boost::statechart::event<U> {};

// fsm
struct FSM : boost::statechart::state_machine<FSM, B> {
    state_mapping previous_state = state_mapping::B;
};

// fully defined states/transitions
struct A : boost::statechart::simple_state<A, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    A() { std::cout << "entered A" << std::endl; }
    virtual ~A() { outermost_context().previous_state = state_mapping::A; }
};

struct B : boost::statechart::simple_state<B, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    B() { std::cout << "entered B" << std::endl; }
    virtual ~B() { outermost_context().previous_state = state_mapping::B; }
};

struct C : boost::statechart::simple_state<C, FSM> {
    typedef boost::mpl::list<
                boost::statechart::custom_reaction<T>,
                boost::statechart::transition<U, D> > reactions;
                
    C() { std::cout << "entered C" << std::endl; }
    
    boost::statechart::result react(const T&) {
        
        switch(outermost_context().previous_state) {
            
        case state_mapping::A:
            return transit<A>();
        case state_mapping::B:
            return  transit<B>();
        default:
            return discard_event();
            
        }
        
    }
};

struct D : boost::statechart::simple_state<D, FSM> {
    D() { std::cout << "entered D" << std::endl; }
};

int main() {
    FSM fsm;
    
    fsm.initiate();
    
    fsm.process_event(S());
    fsm.process_event(T());
    fsm.process_event(S());
    fsm.process_event(U());

    return 0;
}

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//州
结构A；
结构B；
结构C；
结构D；
//状态枚举映射
枚举类状态映射{
A=0，
B
C
D
};
//事件
结构S:boost:：statechart:：event{}；
struct:boost:：statechart:：event{}；
结构U:boost:：statechart:：event{}；
//密克罗尼西亚联邦
struct FSM:boost:：statechart:：state\u机器{
状态映射前一个状态=状态映射：：B；
};
//完全定义的状态/转换
结构A:boost:：statechart:：simple\u state{
typedef boost:：statechart:：过渡反应；
A（）{std:：cout请提供一个
entered B
entered C
entered B
entered C
entered D

#include <boost/mpl/list.hpp>

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

// states
struct A;
struct B;
struct C;
struct D;

// state enum mapping
enum class state_mapping {
    A = 0,
    B,
    C,
    D
};

// events
struct S : boost::statechart::event<S> {};
struct T : boost::statechart::event<T> {};
struct U : boost::statechart::event<U> {};

// fsm
struct FSM : boost::statechart::state_machine<FSM, B> {
    state_mapping previous_state = state_mapping::B;
};

// fully defined states/transitions
struct A : boost::statechart::simple_state<A, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    A() { std::cout << "entered A" << std::endl; }
    virtual ~A() { outermost_context().previous_state = state_mapping::A; }
};

struct B : boost::statechart::simple_state<B, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    B() { std::cout << "entered B" << std::endl; }
    virtual ~B() { outermost_context().previous_state = state_mapping::B; }
};

struct C : boost::statechart::simple_state<C, FSM> {
    typedef boost::mpl::list<
                boost::statechart::custom_reaction<T>,
                boost::statechart::transition<U, D> > reactions;
                
    C() { std::cout << "entered C" << std::endl; }
    
    boost::statechart::result react(const T&) {
        
        switch(outermost_context().previous_state) {
            
        case state_mapping::A:
            return transit<A>();
        case state_mapping::B:
            return  transit<B>();
        default:
            return discard_event();
            
        }
        
    }
};

struct D : boost::statechart::simple_state<D, FSM> {
    D() { std::cout << "entered D" << std::endl; }
};

int main() {
    FSM fsm;
    
    fsm.initiate();
    
    fsm.process_event(S());
    fsm.process_event(T());
    fsm.process_event(S());
    fsm.process_event(U());

    return 0;
}