static member

sm.cpp

#include <iostream> 

class Account
{
    public:
    Account () = default;
    Account (std::string n, int id, double m) :
               name (n) , id_no (id), money (m) { }

    void modify_prime (double);
    void update ();
    Account &display (Account &);

    private:
        static double prime_rate;
        std::string name;
        int id_no = 0;
        double money = 0;

};

double Account::prime_rate = 2.3;

void Account::modify_prime (double mp) {
    prime_rate = mp;
    update ();
}

void Account::update () {
    money = money * prime_rate;
}

Account &Account::display (Account &obj) {
    std::cout << "name: " << obj.name << std::endl
              << "id no: " << obj.id_no << std::endl
              << "prime rate: " << obj.prime_rate << std::endl
              << "money: " << obj.money << std::endl;
    return obj;
}

int main ()
{
    Account account1("kryssel", 223334, 22);

    account1.modify_prime(3.4);
    account1.display(account1);



    return 0;
}