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Function pointers questions

Started by CRACK123 Dec 6, 2005 at 1:08 AM 4 replies 900+ views

Original Post

Dec 6, 2005 at 1:08 AM

Hi, I have a FSM type of code in which I am trying to avoid the second call. For example I am calling (*CallAI)(Idle). What I really want to do is just do CallAI = &Idle and then somehow force the function to run. Is this possible ? For eg:- If you see in the main function I have the line CallAI = &Idle. I want to have only a line like this and somehow force the Idle function to execute. Is this possible.

[Source]
void AIFsm(char ch)
{
        switch(ch)
        {
                case 'i' :
                        CallAI = &Idle
                        (*CallAI)(Idle);
                        break;

                case 'w' :
                        CallAI = &Wander
                        (*CallAI)(Wander);
                        break;

                case 'f' :
                        CallAI = &Flee
                        (*CallAI)(Flee);
                        break;

                default :
                        break;
        }

}

int main()
{
        char ch;

        CallAI = &Idle /* Want to do something like this and make sure Idle    
                           function gets called */

        while(1)
        {
                ch = fgetc(stdin);
                if(ch == 'q')
                {
                        break;
                }
                else
                {
                        AIFsm(ch);
                }
        }

        return 0;
}


[/Source]

Thanks

Skeleton_V@T

Dec 6, 2005 at 1:13 AM

Ummm, I don't get it. What will you benefit from having one less line of code ?. You might have misunderstood the purpose of function pointer. Can you state what exactly you're trying to do ?

CRACK123

Author

CRACK123

Dec 6, 2005 at 1:19 AM

Hi,

Maybe I might have misunderstood it. I am trying to eleminate the following line
(*CallAI)(Idle);

The reason is because if I have to call that code then I could just call the function itself.

dave

322

dave

Dec 6, 2005 at 1:26 AM

There is no reason to change it really, just leave it as it is.

Dave

darookie

121

darookie

Dec 6, 2005 at 1:33 AM

I still don't get it. You don't seem to use "CallAI" anyways, so why have it there in the first place?

typedef void ( *ai_function )( void );void AIFsm(char ch, ai_function default_func){        ai_function ai_func = default_func;        switch(ch)        {                case 'i' :                        ai_func = Idle;                        break;                case 'w' :                        ai_func = Wander;                        break;                case 'f' :                        ai_func = Flee;                        break;                default :                        break;        }        ai_func( );}int main(){        char ch;        while(1)        {                ch = fgetc(stdin);                if(ch == 'q')                {                        break;                }                else                {                        AIFsm(ch, Idle);                }        }        return 0;}

The above code does pretty much the same as yours, while the use of "ai_func" is still somewhat superfluous. Instead of a switch statement you could use a look-up table and simply register your AI functions with that. The input character would then be replaced (semantically, at least) by an event or input signal that triggers the registered function.

Skeleton_V@T

Dec 6, 2005 at 1:42 AM

CallAI = &Idle(*CallAI)(Idle);

These lines (and some below) I found quite confusing, what are the declarations (the syntax) of Idle () and CallAI () ?.

You may refer to this site for function pointers: www.newty.de

In short, this is the general use of function pointers:
Implemented with switch:

switch (Player.getState ()){case Moving:    Player.Move () ;    break ;case Standing:    Player.JugglingWaitingForUser () ;    break ;case Dead:    Player.DoNothing () ;    break ;}

Implemented with fp:

//This line will replace all the switch above//Player.DoIt () ;(Player.*(Player.m_pfnFunc)) () ;

Player::m_pfnFunc is the function pointer that may point to Player::Move (),
Player::JugglingWaitingForUser () or Player::DoNothing () and be called through Player.m_pfnFunc.

I noticed you used normal function pointer, but that shouldn't be much differences, also, there're some better alternatives that I'm quite sure will be pointed out by the other folks :). Good luck.

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