как задать значение статическому элементу класса

#ifndef SAVINGSACCOUNT_H
#define SAVINGSACCOUNT_H
class sav
{
public:
    sav(int);
    void CalculateMounthlyInterest();
    static void ModifyInterestRate(int);
private:
    static int annualInterestRate;
    int SavingsBalance;
};
#endif
#include <iostream>
using namespace std;
sav::sav(int a)
{
    if (a)
        SavingsBalance = a;
    else
        cout << "Error the variable SavingsBalance is not defined";
}
void sav::CalculateMounthlyInterest()
{
    SavingsBalance += (SavingsBalance * annualInterestRate) /12;
 
}
void sav::ModifyInterestRate(int x )
{
    int sav::annualInterestRate = x;
}

class sav
{
public:
    sav(const int);
    void CalculateMounthlyInterest();
    void ModifyInterestRate(const int);
    int annuBal() const {return annualInterestRate;}
private:
    static  int annualInterestRate;
    int SavingsBalance;
};
 
sav::sav(const int a)
{
    if (a)
        SavingsBalance = a;
    else
        cout << "Error the variable SavingsBalance is not defined";
}
void sav::CalculateMounthlyInterest()
{
    SavingsBalance += (SavingsBalance * annualInterestRate) /12;
 
}
void sav::ModifyInterestRate(int x )
{
    annualInterestRate = x;
}
 
 
int sav::annualInterestRate = 0;
 
int main()
{
    sav s(10);
    s.ModifyInterestRate(100);
    std::cout<<"static = "<<s.annuBal()<<"\n";
    
    std::cout<<"\n";
    system("pause");
    return 0;
}

static const int annualInterestRate = 0;

#ifndef SAVINGSACCOUNT_H
#define SAVINGSACCOUNT_H
class sav
{
public:
    sav(int);
    void CalculateMounthlyInterest();
    static void ModifyInterestRate(int x);
private:
    static int annualInterestRate;
    int SavingsBalance;
};
#endif

#include <iostream>
#include <savingaccount.h>
using namespace std;
sav::sav(int a)
{
    if (a)
        SavingsBalance = a;
    else
        cout << "Error the variable SavingsBalance is not defined";
}
void sav::CalculateMounthlyInterest()
{
    SavingsBalance += (SavingsBalance * annualInterestRate) /12;
 
}
static void sav::ModifyInterestRate(int x )
{
    sav::annualInterestRate = x;
}
int sav::annualInterestRate = 0;

#include <iostream>
#include <savingaccount.h>
using namespace std;
int main()
{
    sav a(12000);   
    system("pause");
    return 0;
}

static void sav::ModifyInterestRate(int x )
{
    sav::annualInterestRate = x;
}