C编程查找表

Question

经过一些帮助，因为有一些代码可以工作，但试图完全理解一小部分，它使用查找表作为状态机的一部分。我理解并一直用于我正在编写的教程的状态机可在此处找到 http://coder-tronics.com/state-machine-tutorial-pt1有人建议使用我喜欢但不完全理解的这种方法，而不是在 UponExit、UponEnter 和 ActionWhileInState 中使用更多的 switch case 语句。

现在我不确定的部分是这些与 typedef 和查找表有关的行

typedef void (* const voidFunc)(void);

voidFunc UponEnter[S_MAX] =          {hEnter_OFF,   hEnter_ON,   hEnter_PROCESS};
voidFunc ActionWhileInState[S_MAX] = {hInState_OFF, hInState_ON, hInState_PROCESS};
voidFunc UponExit[S_MAX] =           {hExit_OFF,    hExit_ON,    hExit_PROCESS};

我查阅了 typedef 和查找表并有了基本的了解，但希望有人能简要介绍一下在这种情况下这是如何工作的？

为了完整起见，下面列出了与此相关的完整代码部分。

enum states { S_OFF, S_ON, S_PROCESS, S_MAX };
enum events { E_OFF, E_ON, E_START, E_MAX};
typedef void (* const voidFunc)(void);
void hEnter_OFF(void);   void hEnter_ON(void);   void hEnter_PROCESS(void);
void hInState_OFF(void); void hInState_ON(void); void hInState_PROCESS(void);
void hExit_OFF(void);    void hExit_ON(void);    void hExit_PROCESS(void);

voidFunc UponEnter[S_MAX] =          {hEnter_OFF,   hEnter_ON,   hEnter_PROCESS};
voidFunc ActionWhileInState[S_MAX] = {hInState_OFF, hInState_ON, hInState_PROCESS};
voidFunc UponExit[S_MAX] =           {hExit_OFF,    hExit_ON,    hExit_PROCESS};

enum states StateMachine(enum events event, enum states Current_State)
{
    int Next_State = Current_State;

    switch ( Current_State )
    {
            case S_OFF:
       switch (event )
       {
           // A transition to the next state will occur here
           case E_ON:
            Next_State = S_ON;
               break;
           default:     // Default case placed here to avoid Eclipse warnings as Eclipse expects
               break;   //to handle all enumerated values
       }
       break;
   case S_ON:
       switch (event )
       {
           // A transition to the next state will occur here
           case E_OFF:
            Next_State = S_OFF;
               break;
           case E_START:
            Next_State = S_PROCESS;
               break;
           default:     // Default case placed here to avoid Eclipse warnings as Eclipse expects
               break;   //to handle all enumerated values
       }
       break;
   case S_PROCESS:
       switch (event )
       {
           // A transition to the next state will occur here
       case E_OFF:
        Next_State = S_OFF;
           break;
       default:     // Default case placed here to avoid Eclipse warnings as Eclipse expects
           break;   //to handle all enumerated values
       }
       break;
       // The program should never arrive here
    default:
       break;
    }

    if (Next_State != Current_State)
    {
        // Function call for Upon Exit function, it can be omitted but allows extra functionality
        UponExit[Current_State]();
        // Function call for Upon Enter function, it can be omitted but allows extra functionality
        if (Next_State != S_MAX) UponEnter[Next_State]();
    }
    else // I think ActionWhileInState should only be called when NOT doing a transition!
     {
        if ( event != E_MAX) ActionWhileInState[Current_State]();
    }
    return Next_State;
}

谢谢，

Ant

Answer 1

typedef void (* const voidFunc)(void);

是函数指针的 typedef(对于指向不需要参数且不返回任何内容的函数的指针)。所以你的数组

voidFunc UponEnter[S_MAX] =          {hEnter_OFF,   hEnter_ON,   hEnter_PROCESS};
voidFunc ActionWhileInState[S_MAX] = {hInState_OFF, hInState_ON, hInState_PROCESS}; 
voidFunc UponExit[S_MAX] =           {hExit_OFF,    hExit_ON,    hExit_PROCESS};

每个都有三个不同的函数指针，每个状态一个。所以像这样的一行

UponExit[Current_State]();

调用指针UponExit[Current_State] 指向的函数，如果Current_State 为0，则hExit_ON 指向hExit_OFF > 如果 Current_State 为 1 或 hExit_PROCESS 如果 Current_State 为 2。你也可以这样写:

(*UponExit[Current_State])();

看起来更复杂但清楚地表明 UponExit[Current_State] 是一个函数指针，它被“取消引用”到对其指向的函数的调用。

编辑:

你的状态有一个enum:

enum states { S_OFF, S_ON, S_PROCESS, S_MAX };

因此，S_OFF == 0、S_ON == 1、S_PROCESS == 2 和 S_MAX == 3(看here)