Static members of class

Static data members should be declared inside class body but defined outside of class (constant static member is an exemption).

class Account {
     public:
         // interface functions here
         void applyint() { amount += amount * interestRate; }
         static double rate() { return interestRate; }
         static void rate(double); // sets a new rate
     private:
         std::string owner;
         double amount;
         static double interestRate;
         static double initRate();
};

Define static data members: don't use static keyword. Outside of class body.

double Account::interestRate = initRate();

Define static member function: don't use static keyword.

void Account::rate(double newRate)
     {
         interestRate = newRate;
     }