How to attachInterrupt to a function inside a class

Projects General Guidance
1

Im making a class for my encoder that uses interrupts and in begin() function i need it to attach to a function of the class. but it throws me an error

error: invalid use of non-static member function 'void ENCODER::recordTime()'

Minimal Code that throws error

Class ENCODER{
private:
   void recordTime(){
   }

public:
   void begin(){
      attachInterrupt(digitalPinToInterrupt(5), this->recordTime, RISING)
   }
};

I have tried and they all met the same error.

attachInterrupt(digitalPinToInterrupt(5), recordTime, RISING)
attachInterrupt(digitalPinToInterrupt(5), this.recordTime, RISING)
attachInterrupt(digitalPinToInterrupt(5), this->recordTime, RISING)
2

Did you see this clue in the error message?

3

What makes You think You should use class?
Interrupt is a hardware dependant function. You can't "class", like clone, one piece of hardware inte several instances.

4

although true that you cant clone one piece hardware to several instance, it is possible to pass a pointer to the class , just like how you can pass the SPI or Wire pointer to multiple class instance.

but im not sure though that attachInterrupt is a class or just an ordinary function

5

I apologize but im going to need a bit more explination...

6

You can't attach the interrupt to the class method.

You should create an intermediate non-class static function, from which you will be able to call a class method for specific class instance:

class Example_class
{
  public:
   void foo() {};
};

// pointer to the our class instance
Example_class* E;

// interrupt function, calling the class method
static void irq_func() {
  E->foo();
}

Example_class A;

void setup() {
  E = &A; 
  attachInterrupt(2, FALLING, irq_func);
}

void loop() {}
7

Think about it for a moment.
A class is designed to have multiple instances.
But an interrupt is bound to a single hardware resource.
If you had an ISR that was a class member, then every instance of the class would contend for the same hardware resource.

8

That's not necessarily true. Consider multiple sensors, all tied to the same external interrupt signal with a big "or" gate. However, ISRs are pure C functions, and do not know how to set up "this" for calling a class function.

b707's suggestion of an intermediate non-class function is the way to go.

You can look at the way Serial is handled in the core (on boards with multiple serial ports, like a Zero or Nano Every), for an example.

9

I meant it in the sense of "it's bound to a single processor hardware resource" - like a falling or rising edge on a particular pin, or a receiver register full condition

10

I dunno. Quibbling over semantics. There are CPUs without the concept of "vectored" interrupts, with on

11

How would a static member function get the right context? Can they access class variables using this. to this-> perhaps?

12

Not if each class instance deals with interrupts from a distinct pin. I do it in one of my projects it works flawlessly.

13

If we didn't, we wouldn't be programmers, would we?

14

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