CODEWITHSUNDEEP
c++
J Sanson
J Sanson

Reputation: 11

Stateful C++ global void function pointer

Is it possible to create a function with a single parameter, which returns a pointer to a globally scoped void returning, parameterless function?

I ask in the sense of embedded hardware, where I'm trying to define a series of interrupt service routines to attach to digital pins.

An example of what I mean:

#define MAX_BUTTONS 5

int main()
{
    for (int i = 0; i < MAX_BUTTONS; i++) {
        attachInterrupt(i, isrForI(i), RISING);
    }
}

typedef void (*Isr)(void);

Isr isrForI(int i)
{
    // Return a function of type Isr which calls handleInterrupt(i)
}

void handleInterrupt(int i)
{
    // Do something with i
}

The issue here is not knowing how to be generic enough in isrForI, as I need it to be extensible such that MAX_BUTTONS can be any number.

Upvotes: 1

Views: 793

Answers (1)

Holt
Holt

Reputation: 37661

Since you know MAX_BUTTONS at compile time, you could probably use templates to avoid having to create runtime function:

#define MAX_BUTTONS 5

typedef void (*Isr)(void);

template <int N>
void handleInterrupt() { /* int i = N; */ }


template <int N>
Isr isrForI() {
    return handleInterrupt<N>;
}

template <int N>
struct attach_interrupts {
    static void attach() {
        attachInterrupt(N, isrForI<N>(), RISING); 
        attach_interrupts<N - 1>::attach();
    }
};

template <>
struct attach_interrupts<0> {
    static void attach() {
        attachInterrupt(0, isrForI<0>(), RISING); 
    }
};

int main() {
    attach_interrupts<MAX_BUTTONS - 1>::attach();
}

The only difference with your code is that it attaches interrupts from MAX_BUTTONS - 1 to 0 instead of 0 to MAX_BUTTONS - 1 (but you can easily adapt the templates).

As mentioned by @StoryTeller in the comments, if you want to keep handleInterrupt(int), you could simply do:

void handleInterrupt(int i) { /* Your original handleInterrupt... */ }

template <int N>
void handleInterrupt() { 
    // Call the original one:
    handleInterrupt(N);
}

Upvotes: 2

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