C++ Namespace/scope problem when trying to attach arduino interrupt

Question

I have defined a c++ class (RpmDriver) and want to use attachInterrupt in the constructor to link an Arduino pin to its ISR (RpmInt) in the class. When I try this, I get:

error: cannot convert 'RpmDriver::RPMInt' from type 'void (RpmDriver::)()' to type 'void (*)()'

This is the cpp file:

#include "RpmDriver.h"
#include <Arduino.h>
int currentRpm;
long lastMillis;
int pin;

RpmDriver::RpmDriver(int Pins[])
{
    pin=Pins[0];
    currentRpm=0;
    pinMode(pin,INPUT);
    lastMillis = millis();
    attachInterrupt(digitalPinToInterrupt(pin),RPMInt, RISING);
}
RpmDriver::~RpmDriver(void)
{}

bool RpmDriver::GetData(char* dest,char* format)
{
    sprintf(dest,format,currentRpm);
}

void RpmDriver::RPMInt()
{
    double RPMConstant=40000;
    unsigned long newMillis=millis();
    unsigned long t = newMillis-lastMillis;
    t=100;//temp
    currentRpm=(t==0)?0:(int)RPMConstant/t;
    lastMillis=newMillis;
}

and the header file is

#pragma once
class RpmDriver
{
public:
    RpmDriver(int pins[]);
    ~RpmDriver(void);
    bool GetData(char*,char*);
private:
    void RPMInt(void);
};

Any help much appreciated!

Answer 1

You can't.

Simply because a class member is not the right format of function for attachInterrupt.

You can only use normal functions with attachInterrupt, so the normal method is to have a normal function outside your class which then calls the member function within your class. How you arrange it so that it knows where your objects are is up to you.

• If you have only one object then assign it to a global pointer in your library using the constructor
• If you have lots of instances then create a linked list within your class in a static context and add each object to it in the constructor, then iterate through that list when the interrupt fires.

Answer 2

You should qualify RpmDriver::RPMInt() as static. Static member functions are very much like plain functions, only using their class as a sort of namespace. Obviously, you will not be able to access any member data, but this should not be a problem as it seems your class is not storing any data anyway.