问题描述
我正在研究使用 C 的状态机,我从 this site 中遇到了这段代码。
有一些我以前从未见过的 typedef
声明:
typedef eSystemState (*const afEventHandler[last_State][last_Event])(void);
在这里使用:
// Table to define valid states and event of finite state machine
static afEventHandler StateMachine =
{
[Idle_State] ={[Card_Insert_Event]= InsertCardHandler },[Card_Inserted_State] ={[Pin_Enter_Event] = EnterPinHandler },[Pin_Eentered_State] ={[Option_Selection_Event] = OptionSelectionHandler},[Option_Selected_State] ={[Amount_Enter_Event] = EnteramountHandler},[Amount_Entered_State] ={[Amount_dispatch_Event] = AmountdispatchHandler},};
谁能解释一下这是什么?这是完整的代码
#include <stdio.h>
//Different state of ATM machine
typedef enum
{
Idle_State,Card_Inserted_State,Pin_Eentered_State,Option_Selected_State,Amount_Entered_State,last_State
} eSystemState;
//Different type events
typedef enum
{
Card_Insert_Event,Pin_Enter_Event,Option_Selection_Event,Amount_Enter_Event,Amount_dispatch_Event,last_Event
} eSystemEvent;
//typedef of 2d array
typedef eSystemState (*const afEventHandler[last_State][last_Event])(void);
//typedef of function pointer
typedef eSystemState (*pfEventHandler)(void);
//function call to dispatch the amount and return the ideal state
eSystemState AmountdispatchHandler(void)
{
return Idle_State;
}
//function call to Enter amount and return amount enetered state
eSystemState EnteramountHandler(void)
{
return Amount_Entered_State;
}
//function call to option select and return the option selected state
eSystemState OptionSelectionHandler(void)
{
return Option_Selected_State;
}
//function call to enter the pin and return pin entered state
eSystemState EnterPinHandler(void)
{
return Pin_Eentered_State;
}
//function call to processing track data and return card inserted state
eSystemState InsertCardHandler(void)
{
return Card_Inserted_State;
}
int main(int argc,char *argv[])
{
eSystemState eNextState = Idle_State;
eSystemEvent eNewEvent;
// Table to define valid states and event of finite state machine
static afEventHandler StateMachine =
{
[Idle_State] ={[Card_Insert_Event]= InsertCardHandler },};
while(1)
{
// assume api to read the next event
eSystemEvent eNewEvent = ReadEvent();
//Check NULL pointer and array boundary
if( ( eNextState < last_State) && (eNewEvent < last_Event) && StateMachine[eNextState][eNewEvent]!= NULL)
{
// function call as per the state and event and return the next state of the finite state machine
eNextState = (*StateMachine[eNextState][eNewEvent])();
}
else
{
//Invalid
}
}
return 0;
}
解决方法
让我们考虑 typedef
typedef eSystemState (*const afEventHandler[last_State][last_Event])(void);
循序渐进。
这个
afEventHandler[last_State][last_Event]
声明一个二维数组。考虑到枚举数 last_State
和 lastEvent
等于 5
那么上面的记录等价于
afEventHandler[5][5]
这个
*const afEventHandler[last_State][last_Event]
声明一个二维常量指针数组(指针本身是常量)。
最后这个
typedef eSystemState (*const afEventHandler[last_State][last_Event])(void);
声明一个二维数组,其中包含指向 eSystemState ( void )
类型函数的常量指针。即该函数具有返回类型 eSystemState
和参数列表 void
。
因此名称 afEventHandler
是指向 eSystemState ( void )
类型函数的二维常量指针数组类型的同义词
至于本声明
// Table to define valid states and event of finite state machine
static afEventHandler StateMachine =
{
[Idle_State] ={[Card_Insert_Event]= InsertCardHandler },[Card_Inserted_State] ={[Pin_Enter_Event] = EnterPinHandler },[Pin_Eentered_State] ={[Option_Selection_Event] = OptionSelectionHandler},[Option_Selected_State] ={[Amount_Enter_Event] = EnterAmountHandler},[Amount_Entered_State] ={[Amount_Dispatch_Event] = AmountDispatchHandler},};
然后使用指定的初始化器声明和初始化上述类型的数组。
这是一个使用枚举的一维函数指针数组的类似 typedef 声明的更简化示例。
#include <stdio.h>
typedef enum { Add,Subtract,Multiply,Divide,TotalOperations } Operation;
typedef int ( * const Action[TotalOperations] )( int,int );
int add( int x,int y ) { return x + y; }
int subtract( int x,int y ) { return x - y; }
int multiply( int x,int y ) { return x * y; }
int divide( int x,int y ) { return x / y; }
int main(void)
{
Action action =
{
[Add] = add,[Subtract] = subtract,[Multiply] = multiply,[Divide] = divide
};
int x = 100,y = 5;
for ( int i = 0; i < TotalOperations; i++ )
{
printf( "%d\n",action[i]( x,y ) );
}
return 0;
}
程序输出为
105
95
500
20
,
typedef eSystemState (*const afEventHandler[last_State][last_Event])(void);
将 afEventHandler
定义为没有参数并返回 eSystemState
的二维函数指针数组。
static afEventHandler StateMachine =
{
[Idle_State] ={[Card_Insert_Event]= InsertCardHandler },};
是定义类型的变量 StateMachine
的初始化。
StateMachine[IdleState]
然后被初始化为指向函数的一维指针数组,没有参数,返回eSystemState
:
{[Card_Insert_Event]= InsertCardHandler }
一个一维数组,其索引 Card_Insert_Event
的元素对应于函数 InsertCardHandler
。