Arduino Programming: Static Variables

Here we are with another tutorial on Arduino programming. This time, we will look at static variables and how you can use them to your advantage.

What is a Static Variable?

A static variable is a variable that stores its data as long as the program is running. Consider a function that rotates a servo motor:

rotateServo(){
 int angle = 90; 
 servo.write(angle);
 Serial.println(angle);
 angle += 10; 
}

What would happen if you called this function five times? Since the variable angle is initialized at 90 inside this function, the +10 increment will not hold effect with every call.

void loop(){
 rotateServo();
 rotateServo();
 rotateServo();
 rotateServo();
 rotateServo();
}

Output:
90
90
90
90
90

By adding the modifier static to the angle variable, the value is retained.

rotateServo(){
  static int angle = 90;
  servo.write(angle);
  Serial.println(angle);
  angle += 10;
}

This time, the angle will be incremented by 10 for every call of the rotateServo() function.

void loop(){
 rotateServo();
 rotateServo();
 rotateServo();
 rotateServo();
 rotateServo();
}

Output:
90
100
110
120
130

Static vs. Global Variables

While it may seem that making a variable static makes it looks like a global variable, that is not the case.

For example, we can make angle global:

int angle = 90;

rotateServo(){
  servo.write(angle);
  angle += 10;
}

Calling rotateServo() will still increment angle by 10. But the difference is that with the above code, you can use angle in another function. Whereas in this code:

rotateServo(){
  static int angle = 90;
  servo.write(angle);
  angle += 10;
}

angle is usable only within rotateServo(). Using it outside will cause a compiler error.

Static Members in a Class

If a class has a static member, then there would be only one copy of that member and will be shared among all instances of the class.

Consider this example class that controls an LED:

class LED{
  public:
    LED (int p){
      pinMode(p, OUTPUT);
      pin = p;
    }
    void toggle(void){
      digitalWrite(pin,!digitalRead(pin));
      delay(1000);
    }
  private:
    int pin;
};

We want to improve this class by allowing the user to specify the delay for the toggle. Hence, instead of the constant, we add a variable, _delay:

class LED{
  public:
    int _delay;
      LED (int p){
        pinMode(p, OUTPUT);
        pin = p;
      }
    void toggle(void){
      digitalWrite(pin,!digitalRead(pin));
      delay(_delay);
    }
  private:
    int pin;
};

To use this class, we create an instance:

LED led1(9);

Then we specify the value of _delay is through:

int LED::_delay = 1000;

To toggle the LED, we use the class function toggle() inside loop:

void loop() {
  led1.toggle();
}

However, this code will have a compile error:

19:10: error: 'int LED::_delay' is not a static data member of 'class LED'
exit status 1

To fix this, we simply need to declare _delay as static:

class LED{
  public:
    static int _delay;
    LED (int p){
      pinMode(p, OUTPUT);
      pin = p;
    }
    void toggle(void){
      digitalWrite(pin,!digitalRead(pin));
      delay(_delay);
    }
  private:
    int pin;
};

Also, we can now create multiple instances of the class but only be declaring _delay once:

LED led1(9);
LED led2(10);

int LED::_delay = 1000;

void setup() {}

void loop() {
  led1.toggle();
  led2.toggle();
}