Using C++ classes in Arduino

Question

I need to use classes in Arduino to be able to multitask. I've found a class example in here but I wanted to keep my main code clean, so I've decided to put the class in .h and .cpp files. After a bit of googling this is what I've came up with: Work.h file:

/*
 * Work.h
 *
 * Created on: 2016年05月09日
 * Author: Secret
 */
#ifndef WORK_H_
#define WORK_H_
int ledPin; // the number of the LED pin
unsigned long OnTime; // milliseconds of on-time
unsigned long OffTime; // milliseconds of off-time
int ledState; // ledState used to set the LED
unsigned long previousMillis; // will store last time LED was updated
class Work {
public:
 Work(int pin, long on, long off);
 void Update();
};
#endif /* WORK_H_ */

Work.cpp file:

/*
 * Work.cpp
 *
 * Created on: 2016年05月09日
 * Author: Secret
 */
#include "Work.h"
#include <Arduino.h>
Work::Work(int pin, long on, long off) {
 ledPin = pin;
 pinMode(ledPin, OUTPUT);
 OnTime = on;
 OffTime = off;
 ledState = LOW;
 previousMillis = 0;
}
void Update() {
 // check to see if it's time to change the state of the LED
 unsigned long currentMillis = millis();
 if ((ledState == HIGH) && (currentMillis - previousMillis >= OnTime)) {
 ledState = LOW; // Turn it off
 previousMillis = currentMillis; // Remember the time
 digitalWrite(ledPin, ledState); // Update the actual LED
 } else if ((ledState == LOW)
 && (currentMillis - previousMillis >= OffTime)) {
 ledState = HIGH; // turn it on
 previousMillis = currentMillis; // Remember the time
 digitalWrite(ledPin, ledState); // Update the actual LED
 }
}

When I tried compiling, I got errors that in Work.cpp, method Update() I've got multiple definitions of OnTime, OffTime, ledState, previousMillis.
What am I doing wrong here and how to solve this?

• Some programmer dude: Why are you defining those variables in the header file globally? Why aren't those variables member variables in the class?
• Stack Exchange Broke The Law: A) you don't need classes to multitask, B) your problem is not related to classes.
• Justin: A) as far as I've read, I do need classes to multitask properly. Check the article I've linked. B) How is it not related to classes if I'm writing a C++ class for Arduino?

Answer 1

That error means you have those variables appearing in multiple translation units. By default, non-const global variables have external linkage.

To make it internal, you can qualify it with static. Or you can simply move the definitions to your translation unit (.cpp file)

---

Qualify with static in Work.h

static int ledPin; // the number of the LED pin
static unsigned long OnTime; // milliseconds of on-time
static unsigned long OffTime; // milliseconds of off-time
static int ledState; // ledState used to set the LED
static unsigned long previousMillis; // will store last time LED was updated
class Work {
public:
 Work(int pin, long on, long off);
 void Update();
};

Preferably, move them to your .cpp files since you aren't using them in your Work.h file.

EDIT

The error resulted from your Update() function in the .cpp file.

void Update() { ....

^^ that defined a new function, and is using those global variables... I believe you wanted to do

void Work::Update() {....

• Justin: Thank you for such fast response. I've moved the variables to Work.cpp file. Also made changes to the definition of Update() method as @Saad_Qureshi suggested. Everything works fine now!

Answer 2

Since the method Update() is a part of class Work, it should be defined within the scope of Work class like this:

void Work :: Update () {....