一个简单的有限状态机，

三种状态：

停止状态
运行状态
暂停状态

三个事件

Start
Pause
Stop

状态转换说明：

Stopped状态：通过Start事件转换为Running状态
Running状态：通过Pause事件可转换为Pause状态
Pause状态：通过Start事件可转换为Running状态

实现方式有多种

使用if-else或switch-case

使用if-else或switch-case判断并设置状态之间的转换，比较小儿科，如果写不好会有很多很多的bug，最后当状态变得很多很复杂的时候，bug可能不好找完，尤其是在加入新的状态以后.
有一个明显的优点：就是想怎么改就怎么改，容易上手
有一个明显的缺点：很难扩展新的状态，增加新的事件

不做过多的解释哈。

基于boost的state_machine来实现，如下：

state_machine.h

#pragma once

#include <iostream>

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

//Pre-declarations
class StoppedState;
class RunningState;
class PauseState;

//Three events declaration
class StopEvent : public boost::statechart::event<StopEvent>
{
public:
  StopEvent()
  {
    std::cout << "StopEvent" << std::endl;
  }
  virtual ~StopEvent() = default;
};

class StartEvent : public boost::statechart::event<StartEvent>
{
public:
  StartEvent()
  {
    std::cout << "StartEvent" << std::endl;
  }
  virtual ~StartEvent() = default;
};

class PauseEvent : public boost::statechart::event<PauseEvent>
{
public:
  PauseEvent()
  {
    std::cout << "PauseEvent" << std::endl;
  }
  virtual ~PauseEvent() = default;
};

//enum state declaration
enum class State : std::int8_t
{
  Stopped = 83, //S, S for Stopped status
  Running = 82, //R, R for Running status
  Pause = 80    //P, P for Pause status
};


//StateMachine main class
class StateMachine : public boost::statechart::state_machine<StateMachine, StoppedState> //the initialized status is Stopped
{
public:
  StateMachine() = default;
  virtual ~StateMachine() = default;

  State state_; //should not set it to public, but for simple， so so 
};

//Three states
class StoppedState : public boost::statechart::simple_state<StoppedState, StateMachine>
{
public:
  typedef boost::statechart::custom_reaction<StartEvent> reactions;
  boost::statechart::result react(const StartEvent & event) // StartEvent, will set the status from Stopped to Running
  {
    context<StateMachine>().state_ = State::Running;
    std::cout << "CurrentState = " << static_cast<std::underlying_type<State>::type>(context<StateMachine>().state_) << std::endl << std::endl;
    return transit<RunningState>();
  }
  virtual ~StoppedState() = default;
};

class RunningState : public boost::statechart::simple_state<RunningState, StateMachine>
{
public:
  typedef boost::statechart::custom_reaction<PauseEvent> reactions;
  boost::statechart::result react(const PauseEvent & event)// PauseEvent, will set the status from Running to Paused
  {
    context<StateMachine>().state_ = State::Pause;
    std::cout << "CurrentState = " << static_cast<std::underlying_type<State>::type>(context<StateMachine>().state_) << std::endl << std::endl;
    return transit<PauseState>();
  }
  virtual ~RunningState() = default;
};

class PauseState : public boost::statechart::simple_state<PauseState, StateMachine>
{
public:
  typedef boost::statechart::custom_reaction<StartEvent> reactions;
  boost::statechart::result react(const StartEvent & event) // StartEvent, will set the status from Paused to Running
  {
    context<StateMachine>().state_ = State::Running;
    std::cout << "CurrentState = " << static_cast<std::underlying_type<State>::type>(context<StateMachine>().state_) << std::endl << std::endl;
    return transit<RunningState>();
  }
  virtual ~PauseState() = default;
};

state_machine.cc

#include "state_machine.h"
//It is an empty file, all code is in its header file

main.cc

#include <iostream>

#include "state_machine.h"

int main(int argc, char * argv[])
{

  StateMachine sm;
  sm.initiate();

  //exception testing
  sm.process_event(PauseEvent()); //can't pause

  sm.process_event(StartEvent());
  sm.process_event(PauseEvent());

  sm.process_event(StartEvent());
  sm.process_event(PauseEvent());

  sm.process_event(StopEvent());  //can't stop

  sm.process_event(StartEvent());
  
  sm.process_event(StopEvent()); //can't stop
  
  sm.process_event(StopEvent());  //can't stop

  return 0;
}