Programming With Classes and Objects on the Arduino

A class is a collection of functions and variables that can be used to perform specific programming tasks. In Arduino programming, a class is basically the same thing as a library.

A class is like a template that lets you reuse code without having to type it over and over. For example, say you needed to make 20 documents that all had the same font, header, and margins. It would be too time consuming to change the formatting of all 20 documents individually. Instead you could make a template document with the right font, header, and margins, then add the text and save it as a separate file.

Classes are like that template document. Classes are templates of code for what are called “objects”. Think of a class as the template document, and the object as the individual document. Objects let you access the functions and variables inside of a class. An object is sometimes called an “instance” of a class.

Watch the video for this tutorial here:

Blinking LEDs Without Classes and Objects

To demonstrate how useful classes and objects can be, take a look at an example project that blinks two LEDs at different rates without using classes and objects.

Here are the parts you will need for this project:

• Arduino Uno
• Jumper wires
• Breadboard
• Two 220 Ohm resistors
• Two LEDs

The wiring diagram for the project is shown below:

This sketch will make the green LED blink on and off for two seconds and the red LED blink on and off for one second:

int redLED = 10;
int greenLED = 11;

void setup() {
  pinMode(redLED, OUTPUT);
  pinMode(greenLED, OUTPUT);
}

void loop() {
  digitalWrite(greenLED, HIGH);
  delay(2000);
  digitalWrite(greenLED, LOW);
  delay(2000);

  digitalWrite(redLED, HIGH);
  delay(1000);
  digitalWrite(redLED, LOW);
  delay(1000);
}

Explanation of the Code

At the top of the sketch we declare variables for the redLED and greenLED pins.

In the setup() section, we set the pin mode for each LED pin as an OUTPUT.

In the loop() section, we digital write the greenLED high, delay for 2,000 milliseconds, digital write it low, then delay for another 2,000 milliseconds.

Next we digital write the redLED high, delay for 1,000 milliseconds, digital write it low, then delay for another 1,000 milliseconds.

This is a lot of code for a task as simple as blinking a couple LEDs on and off. We can make this program a lot smaller with classes and objects.

Blinking LEDs With Classes and Objects

Let’s see how to blink the LEDs in the sketch above with classes and objects. We’ll start by writing the code for the class, and later we will create objects in the sketch to access the code inside the class.

To make an Arduino class, we need to create two files – a header file and a source file. We’ll write the sketch that uses this class later, but now let’s start writing the header file.

The Header File

The header file is where the class is declared. It is also where the functions and variables used inside the class are declared. Let’s walk step by step through how to write a header file.

The first piece of code needed in the header file is called an include guard. The include guard prevents program errors if the library is accidentally included more than once in the sketch:

#ifndef MyClass_h
#define MyClass_h
// code goes here
#endif

You can name the header file anything you want but in this article it will be named MyClass. The header file needs to be saved with .h extension. In #ifndef and #define, the name of the header file needs to be followed by _h.

Next we need to include the Arduino.h library. Arduino.h is the core library that contains all of the Arduino’s built-in functions. Including it in the header file makes those functions available for use in the class:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
#endif

Now we can define the class. To define a class, write class followed by the name of the class:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
// class code goes here
};
#endif

The name of the class in this example is MyClass. The code for the class goes inside the curly brackets.

The functions and variables inside a class can be public or private:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
private:
};
#endif

The public and private keywords are known as access specifiers. Public functions and variables can be used inside the sketch, but private functions and variables can only be used inside the class.

Every class needs a special function called a constructor. The constructor defines what will happen when an object is created. When we write the sketch for this project, we’re going to create an object for the green LED, and another object for the red LED. The constructor will tell the Arduino what to do when we create those LED objects.

We’ll write the actual code for the constructor when we define it in the source file, but we still need to declare it in the header file. Since the objects will be created in the sketch, the constructor declaration needs to be public, and have the same name as the class. The class in this example is called MyClass, so the constructor declaration will be MyClass():

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
	MyClass();
private:
};
#endif

In this example project we will have two LEDs connected to the Arduino so we need to set the pin mode of each LED pin as an OUTPUT. When we write the code for the constructor in the source file we will put the pinMode() function inside it. The pinMode() function takes a pin number as the first parameter. We need to pass that pin number along to the constructor so that the pinMode() function can use it. To do that, we declare an int variable called pin inside the parentheses of the constructor declaration:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
	MyClass(int pin);
private:
};
#endif

Now we need to declare the function that will make the LEDs blink. Functions inside classes are declared the same way they’re declared in sketches. First write the return type. This function isn’t going to return any values, so the return type is void. Then write the name of the function. Let’s call it myFunction():

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
	MyClass(int pin);
	void myFunction();
private:
};
#endif

Since this function will blink LEDs it would be nice to have control over how fast the blinking happens. We can do this by passing a variable to the myFunction() function that will define the length of the delays. When the function is used in the sketch it will have a parameter that will set the length of the delay.

Let’s declare a variable in myFunction() to store that parameter and call it blinkRate:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
	MyClass(int pin);
	void myFunction(int blinkRate);
private:
};
#endif

Now, whatever argument is used in the myFunction() function in the sketch will be passed to the function’s code in the source file.

The myFunction() function will need to know the pin numbers the LEDs are connected to, so we need to declare a variable to store them. The pin number variable will only be used inside the function so it will be declared as a private variable. The naming convention for private variables is with an underscore before the variable name. So let’s declare declare a private variable called _pin:

#ifndef MyClass_h
#define MyClass_h
#include "Arduino.h" 
class MyClass {
public:
	MyClass(int pin);
	void myFunction(int blinkRate);
private:
	int _pin;
};
#endif

This is all the code we need for the header file.

To save the header file, navigate to Documents > Arduino > Libraries. Create a new folder with the same name as the class (MyClass in this example). Save the header file in this folder. The header file needs to be saved with a .h extension. In this example, the header file would be named MyClass.h.

The Source File

The source file contains the actual code for the functions, and defines what will happen when you create an object. Let’s walk through the process of writing a source file.

The source file needs to have the same name as the header file but with a “.cpp” extension.

The first thing to do is include the Arduino.h library so we can use the Arduino’s built in functions like pinMode(), digitalWrite(), and delay():

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass() {
}

Then we need to include the header file we just wrote. We do that the same way we include all classes, with a #include followed by the name of the header file in quotation marks:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass() {
}

Now we can write the constructor definition. This code will be executed every time an object is created in the sketch. The constructor definition starts with the name of the class followed by two colons. In this example, the class is MyClass. The two colons are called a scope resolution operator. After the two colons is the name of the constructor that we declared in the header file. The constructor needs to have the same name as the class. So the constructor’s name is MyClass():

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass() {
}

The objects we create with this class are going to represent Arduino pins connected to green and red LEDs. Each time an LED object is created, the pinMode() of each pin will need to be set as an output. To do that, we need to pass the Arduino pin number connected to the LED to the constructor by declaring a variable called pin inside the MyClass() constructor:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
}

Inside the constructor all we need to do is set the pin modes of the LED pins. The pin number stored in the pin variable will be passed to the constructor when the object is created, so we set the pin mode with pin, OUTPUT:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
	pinMode(pin, OUTPUT);
}

Remember the private _pin variable we declared in the header file? Now we set that equal to the public pin variable we declared inside the MyClass() constructor. We’ll see why we want to do this in a minute:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
	pinMode(pin, OUTPUT);
	_pin = pin;
}

But now let’s write the function that will make the LEDs blink. To define a function, first write the return type of the function. The function in this example will not return any values so it’s return type is void.

Then write the name of the class to which the function belongs. In this example, the function will belong to the MyClass class. Next, include the :: scope resolution operator followed by the name of the function. We already declared the myFunction() function in the header file, so we can use that:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
	pinMode(pin, OUTPUT);
	_pin = pin;
}
void MyClass::myFunction(){
}  

The myFunction() function will have a parameter that sets the duration of the LED blinks. We declared a variable in the header file called blinkRate to store that value, but we also need to declare it here in the source file inside the parentheses of myFunction():

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
	pinMode(pin, OUTPUT);
	_pin = pin;
}
void MyClass::myFunction(int blinkRate){
}  

This will create a parameter for the myFunction() function. Now when we call the function in the sketch we can pass it an argument. In this example, the argument will be the blinkRate variable.

Now we can include the code to make the LEDs blink. It’s the same basic code that is usually used to blink LEDs:

#include "Arduino.h”
#include "MyClass.h"
MyClass::MyClass(int pin) {
	pinMode(pin, OUTPUT);
	_pin = pin;
}
void MyClass::myFunction(int blinkRate){
digitalWrite(_pin, HIGH);
delay(blinkRate);
digitalWrite(_pin, LOW);
delay(blinkRate);
}

First we digital write the private _pin variable high. In the constructor, the private _pin variable was set equal to the public pin variable. So when the LED object is created, the pin number will be stored in the public pin variable and passed to the constructor. That pin number will also be stored in the private _pin variable, and be passed on to the digitalWrite() function.

The delay duration will be set in the sketch and stored in the blinkRate variable, so we use blinkRate as the argument of the delay() function. Then we digital write the private _pin variable low and delay again by the blinkRate.

Ok, that’s all we need for the source file.

To save the source file, navigate to Documents > Arduino > Libraries. Save the source file in the folder that was created for MyClass. The source file needs to have the same name as the header file, but with a .cpp extension. In this example, the header file would be named MyClass.cpp.

Now let’s write the sketch.

The Sketch

In Arduino programming, classes are treated just like libraries, so we can include the MyClass class in the sketch the same way libraries are included:

#include <MyClass.h>

To use the functions that were defined in the class, we will need to create some objects. We are blinking two LEDs, so we will have one object for the red LED and one object for the green LED.

To create an object, write the name of the class, then the name of the object. Objects can be named however you want. In this example we are creating a redLED() object for the red LED, and a greenLED() object for the green LED. Each object is called through the MyClass class:

#include <MyClass.h>
MyClass redLED(10);
MyClass greenLED(11);

When we defined the MyClass constructor in the source file, we passed the public pin variable to it. So now we can use the Arduino pin number as the argument of the object and that pin number will be passed to the constructor. In this example, the constructor will then set the pin mode of pins 10 and 11 as outputs.

Now we can write the loop() section.

We are going to blink the red LED and green LED with the myFunction() function that was defined in the source file. To use functions from a class or library, write the name of the object, a period, then the name of the function you want to call:

#include <MyClass.h>
MyClass redLED(10);
MyClass greenLED(11);
void setup(){
}
void loop(){
	greenLED.myFunction(2000);
}

This will call the myFunction() function from the MyClass class and apply it to the greenLED object.

When we defined the myFunction() function we passed it the blinkRate variable to it to control how fast the LEDs will blink. With 2000 as the argument, the delay() inside the myFunction() function will delay for 2,000 milliseconds.

Now we just need to call the myFunction() function on the redLED object. The red LED will blink on and off for 1,000 milliseconds:

#include <MyClass.h>
MyClass redLED(10);
MyClass greenLED(11);
void setup() {
}
void loop() {
  greenLED.myFunction(2000);
  redLED.myFunction(1000);
}

The great thing about using classes and objects is that you can re-use the same functions on multiple objects. This makes your code simpler, more efficient, and easier for others to use as well.

After you upload the sketch, you should see the red LED flashing on and off once per second and the green LED flashing on and off every two seconds.

Hope this article makes it easy for you to use classes and objects on the Arduino. If you have any questions, be sure to leave them in the comment section below…