I cannot use the static variable

Question

I'm trying to print the employee count after each created object but I'm getting an error like this: undefined reference to `Employee::numberofEmployees'. How can I solve this little problem? Any ideas? Btw I'm using a local class.

the purpose of the numberofEmployees variable is to store the information on the number of employee objects created/instantiated so far.

#include <iostream>
#include <string>

using namespace std;

class Employee{

public:
    string name;
    string surname;
    int year;
    double salary;
    static int numberofEmployees;


    Employee(int yil,string isim,string soyisim): year(yil),name(isim),surname(soyisim){
        numberofEmployees++;
    }

    Employee(){
        name = "not-set";
        year = 0;
        surname = "not-set";
        salary = 0.0;
    }

    void calculateSalary(){
        salary =  2310 + 2310 * year * 12 / 100.0;
    }

    void printInfo(){
        cout << name << " " << surname << " " << "(" << year << ")" << " " << salary << "TL/month" << endl;
    }
};

int main()
{
    Employee person1(4,"Berk","Kandemir");
    cout << Employee::numberofEmployees << endl;
    Employee person2(8,"Esat","Kandemir");
    cout << Employee::numberofEmployees << endl;
    Employee person3(20,"Paul","Walker");
    cout << Employee::numberofEmployees << endl;
    person1.calculateSalary();
    person2.calculateSalary();
    person3.calculateSalary();
    person1.printInfo();
    person2.printInfo();
    person3.printInfo();

    return 0;
}

Answer 1

You have to initialize your static data member outside the class using scope resolution operator something like this.

int Employee::numberofEmployees = 0;

This shows that you are accessing a static variable of class Employee and then initializing it.

Answer 2

If you have a static variable within a class, you are supposed to initialise it outside of the class, ie in a .cpp file. From C++17, however, you are allowed to initialise it in-class, in-line, using the inline keyword.

Code:

#include <string>
#include <iostream>

using namespace std;

class Employee
{

public:
    string name;
    string surname;
    int year;
    double salary;
    static inline int numberofEmployees = 0;

    Employee(int yil, string isim, string soyisim) : year(yil), name(isim), surname(soyisim)
    {
        numberofEmployees++;
    }

    Employee()
    {
        name = "not-set";
        year = 0;
        surname = "not-set";
        salary = 0.0;
    }

    void calculateSalary()
    {
        salary = 2310 + 2310 * year * 12 / 100.0;
    }
    void printInfo()
    {

        cout << name << " " << surname << " "
             << "(" << year << ")"
             << " " << salary << "TL/month" << endl;
    }
};

int main()
{
    Employee person1(4, "Berk", "Kandemir");
    cout << Employee::numberofEmployees << endl;
    Employee person2(8, "Esat", "Kandemir");
    cout << Employee::numberofEmployees << endl;
    Employee person3(20, "Paul", "Walker");
    cout << Employee::numberofEmployees << endl;
    person1.calculateSalary();
    person2.calculateSalary();
    person3.calculateSalary();
    person1.printInfo();
    person2.printInfo();
    person3.printInfo();

    return 0;
}

Or keep your code and add:

int Employee::numberofEmployees = 0;

After your class.

Output:

1
2
3
Berk Kandemir (4) 3418.8TL/month
Esat Kandemir (8) 4527.6TL/month
Paul Walker (20) 7854TL/month