C/C++用状态转移表联合函数指针数组实现状态机FSM

状态机在工程中使用非常的频繁，有如下常见的三种实现方法：
1. switch-case 实现，适合简单的状态机；
2. 二维状态表state-event实现，逻辑清晰，但是矩阵通常比较稀疏，而且维护麻烦；
3. 用状态转移表stateTransfer Table实现，数组大小等于状体转移边个数，易扩展；
下面用一个例子来进行详细说明，描述的如下场景：

描述对象：门
状态：开着、关着、锁着 (这里的关着指关了但未锁的状态)
事件：开门、关门、上锁、解锁

代码实现用枚举来定义状态和事件，操作数据节点转移到目的状态用函数实现。枚举本身默认是从0开始的int类型，利用这个特点将状态转移函数放到函数指针数组中与状态对应起来，方便操作。
核心数据结构如下：

状态：枚举类型
事件：枚举类型
状态转移结构体：{当前状态、事件、下个状态}，定义一个全局数组来使用
状态变更函数：到下个状态(放到数组中与状态枚举对应起来)

此种实现方法容易扩展，增加状态和事件都比较容易。如果存在一个状态通过对应事件可以转移到多个状态的情形，则可以扩展状态转移函数，或者在状态转移结构中增加一个判断函数字段。
代码实现如下：

#include 
using namespace std;

typedef enum{
    OPENED,
    CLOSED,
    LOCKED,
}  State;

typedef enum{
    OPEN,
    CLOSE,
    LOCK,
    UNLOCK
} Event;

typedef struct{
    State currentState;
    Event event;
    State NextState;
} StateTransfer;

typedef struct{
    State state;
    int transferTimes;
}Door;

StateTransfer g_stateTransferTable[]{
    {OPENED, CLOSE,  CLOSED},
    {CLOSED, OPEN,   OPENED},
    {CLOSED, LOCK,   LOCKED},
    {LOCKED, UNLOCK, CLOSED},
};

void toOpen(Door& door);
void toClose(Door& door);
void toLock(Door& door);
typedef void (*pfToState)(Door& door);
pfToState g_pFun[] = {toOpen, toClose, toLock}; //状态枚举值对应下标

void toOpen(Door& door){
    door.state = OPENED;
    cout << "open the door!\n";
}

void toClose(Door& door){
    door.state = CLOSED;
    cout << "close the door!\n";
}

void toLock(Door& door){
    door.state = LOCKED;
    cout << "lock the door!\n";
}

void transfer(Door& door,const Event event){
    for (int i = 0; i < sizeof(g_stateTransferTable)/sizeof(StateTransfer); ++i) {
        if(door.state == g_stateTransferTable[i].currentState &&
           event == g_stateTransferTable[i].event){
            g_pFun[g_stateTransferTable[i].NextState](door);
            door.transferTimes++;
            cout << "transfer ok!\n";
            return;
        }
    }
    cout << "This event cannot transfer current state!!\n";
    return;
}

void printDoor(const Door& door){
    string stateNote[] = {"opened","closed","locked"}; // 下标正好对应状态枚举值
    cout << "the door's state is: " << stateNote[door.state] << endl;
    cout << "the door transfer times is: " << door.transferTimes << endl;
}

int main(){
    Door door = {CLOSED, 0};
    printDoor(door);
    transfer(door, OPEN);
    printDoor(door);
    transfer(door, LOCK);
    printDoor(door);
    transfer(door, CLOSE);
    printDoor(door);
    return 0;
}

运行结果如下：

the door’s state is: closed
the door transfer times is: 0
open the door!
transfer ok!
the door’s state is: opened
the door transfer times is: 1
This event cannot transfer current state!!
the door’s state is: opened
the door transfer times is: 1
close the door!
transfer ok!
the door’s state is: closed
the door transfer times is: 2