#Edit: Per Nick Gammon's answer, I now have this.

Edit: Per Nick Gammon's answer, I now have this.

#Edit: Per Nick Gammon's answer, I now have this.

MotorEncoderMaster.ino

#include </home/marcel/dev/Arduino/encoders/MotorEncoderMaster/Encoders.h>
Encoders * Encoders::instances [2] = { NULL, NULL };


// instances of our class
Encoders right;
Encoders left;

void setup (){
    right.begin (2);   // pin D2
    left.begin (3);   // pin D3
}  // end of setup

void loop (){
    // whatever
}  // end of loop

Encoders.h:

class Encoders{
    volatile bool switchChanged;

    static Encoders * instances [2];

    static void switchPressedExt0(){
        if(Encoders::instances [0] != NULL){
            Encoders::instances [0]->switchPressed();
        }
    }

    static void switchPressedExt1 (){
        if (Encoders::instances [1] != NULL){
            Encoders::instances [1]->switchPressed();
        }
    }


    public:
        void begin (const int whichPin){
            pinMode (whichPin, INPUT_PULLUP);
            switch (whichPin){
                case 2:
                    attachInterrupt(0, switchPressedExt0, CHANGE);
                    instances [0] = this;
                    break;

                case 3:
                    attachInterrupt(1, switchPressedExt1, CHANGE);
                    instances [1] = this;
                    break;

            }
        }

        void switchPressed(){
            switchChanged = true;
        }

};

When I try to compile this, I get :

In file included from MotorEncoderMaster.ino:1:0:
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt0()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:7:34: error: ‘NULL’ was not declared in this scope
    if(Encoders::instances [0] != NULL){
                                  ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt1()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:13:35: error: ‘NULL’ was not declared in this scope
    if (Encoders::instances [1] != NULL){
                                   ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In member function ‘void Encoders::begin(int)’:
/home/marcel/dev/Arduino/encoders/Encoders.h:21:24: error: ‘INPUT_PULLUP’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                        ^
/home/marcel/dev/Arduino/encoders/Encoders.h:21:36: error: ‘pinMode’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                                    ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:45: error: ‘CHANGE’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                             ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:51: error: ‘attachInterrupt’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                                   ^

I am trying to write an library for encoders.

I tried answers on other threads. Most said to make the functions passed to attachInterrupt static. That threw different errors:

The arduino in question is a Mega but I doubt that matters.

I am trying to write a library for encoders.

I tried answers on other threads. Most said to make the functions passed to attachInterrupt() static. That threw different errors:

The Arduino in question is a Mega but I doubt that matters.