Invalid use of void expression (how to use function pointer with input)

Question

I'm defining an attachinterrupt and I'm running into an error. The line in question is:

void setup() {
   attachInterrupt(digitalPinToInterrupt(stallsense1), changedirection(1), RISING);
   attachInterrupt(digitalPinToInterrupt(stallsense2), changedirection(2), RISING);
}

and the error I'm getting is:

error: invalid use of void expression
attachInterrupt(digitalPinToInterrupt(stallsense2), changedirection(2), RISING);
                                                                                ^
exit status 1
invalid use of void expression

The idea is I have one function changedirection and two possible inputs, either 1 or 2. I believe the issue is where I've called the function changedirection(1). I want to simply tell attachinterrupt to call the function changedirection with an input of either 1 or 2. How would I do this?

timemage · Accepted Answer · 2021-06-17 23:17:32Z

1

attachInterrupt expects a simple pointer to function taking no arguments and returning nothing (void).

So, really there's no way to directly do what you're talking about. There's no currying ability builtin. You can define your own functions like

void proxy_changedirection_1() {
    changedirection(1);
}

And then attachInterrupt specifying proxy_changedirection_1, or you can use a lambda with no captures for the same purpose:

attachInterrupt(
    digitalPinToInterrupt(stallsense1),
    [] () {
        changedirection(1);
    },
    RISING
);

answered Jun 17, 2021 at 23:17

timemage

5,7391 gold badge15 silver badges27 bronze badges

Thank you very much. The lambda notation is closer to what I had envisioned coming from python.

Jules
– Jules

2021-06-17 23:30:27 +00:00
Commented Jun 17, 2021 at 23:30

Add a comment |

JRobert · Accepted Answer · 2021-06-17 23:34:20Z

What happened is you wrote a void expression where an expression of type 'pointer to function of void returning void' (i.e., no arguments and no return value), was expected. The name of a function with no parentheses for an argument list is an expression of type 'pointer to function' taking whatever arguments & types that named function was declared to expect, and returning whatever type that named function was declared to return.

But if you write the parentheses with it, now you've written an expression that 1) would (if it were valid) get evaluated during the call to attachInterrupt() - much earlier that you want it to be! - and return a value of that return type rather than the type you expected.

@timemage has offered two ways to solve this.

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