typedef void (*voidFuncPtr)();
class PWMInput {
  int inputPin;
  public:
    PWMInput(int inputPin, voidFuncPtr interruptDispatch);
    void doChannelA(){}
    volatile int position = 0;
};

// Interrupt dispatch forward declaration
void myMotorDispatch();

// Declare the object and pass in our global dispatch function
PWMInput myMotor = PWMInput(0, &myMotorDispatch);

void setup() {
}

void loop() {
}

void myMotorDispatch() {
  myMotor.doChannelA();
}

PWMInput::PWMInput(int channelA, voidFuncPtr interruptDispatch) {
  pinMode(channelA, INPUT);
  attachInterrupt(channelA, interruptDispatch, RISING);
}

void PWMInput::doChannelA() {
  position++;
}


=================
Arduino: 1.6.7 (Windows 10), Board: "Arduino Pro or Pro Mini, ATmega328 (5V, 16 MHz)"

pwm4:31: error: redefinition of 'void PWMInput::doChannelA()'

 void PWMInput::doChannelA() {

      ^

pwm4:6: error: 'void PWMInput::doChannelA()' previously defined here

     void doChannelA(){}

          ^

exit status 1
redefinition of 'void PWMInput::doChannelA()'

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