Function returns pointer to function that returns pointer to function

Question

I am trying to create a threaded state machine where each state returns a pointer to the next state. I have a number of state machines running so the next state of all these is stored in an array that is sequenced through to call them in a round robin fashion. So what I would like to do is:

pState[i] = (pState[i])();

I understand how to return a function pointer in general but since I need the returned pointer to be a type of function that returns a pointer to a function the recursion is confusing me. The state would look like:

pSatateFunc StateA(void){
// ... code ...
return StateB;
}

So, how do I define this so I can define the array of pointers?

I know this has been asked before but I can't find the question. But the short answer is no, you can't recursively define function pointers. But there are workarounds (simplest being, have the function return a void* which is cast to a function pointer) — Taylor Brandstetter
– Taylor Brandstetter, Commented Dec 4, 2013 at 21:57
Thanks, I was kind of thinking this but thought maybe I was missing something. — rrritchey
– rrritchey, Commented Dec 4, 2013 at 22:25

caf · Accepted Answer · 2013-12-05 04:22:23Z

1

You can't do this directly, but you can do it indirectly by introducing a struct type whose only member is a function pointer:

struct state
{
    struct state (*func)(void);
};

Then your functions are defined to return this type:

struct state StateA(void)
{
    /* ... code ... */
    return (struct state){ StateB };
}

Your pstate array would be of this type too:

struct state pState[N];

/* ... */

pState[i] = pState[i].func();

answered Dec 5, 2013 at 4:22

caf

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