Static variable class member

Projects Programming
1

HI All

I am doing some practice tests with classes. I wrote this simple class:

class Person{

private: 
  String _name;
  int _age;
  int _size;

public:
  Person(){
    _size = 48;
  }
  Person(String name){
    _name = name;
    _size = 48;
  }
    Person(String name, int size){
    _name = name;
    _size = size;
  }


 int setName(String name){
    _name = name;
  }

  int setAge(int age){
    _age = age;
  }

  int setSize(int size){
    _size = size;
  }

  void showData(){
    Serial.print("this person's name is : ");
    Serial.println(_name);
    Serial.print("this person's age is : ");
    Serial.println(_age);
    Serial.print("this person's size is : ");
    Serial.println(_size);
  } 
};


void setup() {
  Serial.begin(9600);
Person john("John"); 
john.setAge(45);
john.showData();

}

void loop() {
}

Now, I would like the private variable _size to be the same for all objects of the class. I have tried to use the keyword static in front of it but it throws errors.

My understanding is that a static variable would act like a global variable for the class.
I may want to use it as a constant in certain scenarios or as a variable in others, but it has to be the same for all the objects of the class.
Any suggestions?
Cheers.

2

Try this:

class Person{

private: 
  String _name;
  int _age;
  static int _size;

public:

  Person(){
    _size = 48;
  }
  Person(String name){
    _name = name;
    _size = 48;
  }
    Person(String name, int size){
    _name = name;
    _size = size;
  }


 int setName(String name){
    _name = name;
  }

  int setAge(int age){
    _age = age;
  }

  int setSize(int size){
    _size = size;
  }

  void showData(){
    Serial.print("this person's name is : ");
    Serial.println(_name);
    Serial.print("this person's age is : ");
    Serial.println(_age);
    Serial.print("this person's size is : ");
    Serial.println(_size);
  } 
};

int Person::_size=0;

void setup() {
  Serial.begin(9600);
Person john("John"); 
john.setAge(45);
john.showData();

}

void loop() {
}
3

please show the code where you use "static"

4

please see belwo:

5

@wildbill this is exactly what I was trying but it throws errors...

6 
inline static int _size;

Also in your Wokvi you have a several errors "no return in function declared non-void" in your calss methods setName, setAge and setSize

8

think it needs to be inline..

add inline before static..

good luck.. ~q

9

@wildbill , apologies, you were right I missed the line "int Person::_size=0;" of your code
@Delta_G Thanks
this works! So static variables HAVE to be defined outside the class ?
I was trying to have it initialized by the constructor. I have noticed that if I add
int Person::_size=0;
then also the constructor initialization works... I will paste the code and output below...

@b707 you are right, strangely it does not given an error about that and the functions work as expected though

class Person{

private: 
  String _name;
  int _age;
  static int _size;

public:

  Person(){
    _size = 48;
  }
  Person(String name){
    _name = name;
    _size = 48;
  }
    Person(String name, int size){
    _name = name;
    _size = size;
  }


 int setName(String name){
    _name = name;
  }

  int setAge(int age){
    _age = age;
  }

  int setSize(int size){
    _size = size;
  }

  void showData(){
    Serial.print("this person's name is : ");
    Serial.println(_name);
    Serial.print("this person's age is : ");
    Serial.println(_age);
    Serial.print("this person's size is : ");
    Serial.println(_size);
  } 
};

int Person::_size=0;

void setup() {
  Serial.begin(9600);
Person john("John"); 
john.setAge(45);
john.showData();

}

void loop() {
}

output
This person's name is : John
this person's age is : 45
this person's size is : 48

11

Thank you all😊

12

It depends on Arduino IDE settings, that are different for a different boards. When I selected a Raspberry Pi RP2040 in order to compile your code - it throw me that errors.

14

@b707
@Delta_G
got it! thanks! :slight_smile: