Unresolved external symbol error in c++
- by Crystal
I am trying to do a simple hw problem involving namespace, static data members and functions.  I am getting an unresolved external symbol error 
Error   1   error LNK2001: unresolved external symbol "private: static double JWong::SavingsAccount::annualInterestRate" (?annualInterestRate@SavingsAccount@JWong@@0NA)    SavingsAccount.obj  SavingsAccount
And I don't see why I am getting this error.  Maybe I don't know something about static variables compared to regular data members that is causing this error.  Here is my code:
SavingsAccount.h file
#ifndef JWONG_SAVINGSACCOUNT_H
#define JWONG_SAVINGSACCOUNT_H
namespace JWong
{
    class SavingsAccount
    {
    public: 
        // default constructor
        SavingsAccount();
        // constructor
        SavingsAccount(double savingsBalance);
        double getSavingsBalance();
        void setSavingsBalance(double savingsBalance);
        double calculateMonthlyInterest();
        // static functions
        static void modifyInterestRate(double newInterestRate);
        static double getAnnualInterestRest();
    private:
        double savingsBalance;
        // static members
        static double annualInterestRate; 
    };
}
#endif
SavingsAccount.cpp file
#include <iostream>
#include "SavingsAccount.h"
// default constructor, set savingsBalance to 0
JWong::SavingsAccount::SavingsAccount() : savingsBalance(0)
{}
// constructor
JWong::SavingsAccount::SavingsAccount(double savingsBalance) : savingsBalance(savingsBalance)
{}
double JWong::SavingsAccount::getSavingsBalance()
{
    return savingsBalance;
}
void JWong::SavingsAccount::setSavingsBalance(double savingsBalance)
{
    this->savingsBalance = savingsBalance;
}
// returns monthly interest and sets savingsBalance to new amount
double JWong::SavingsAccount::calculateMonthlyInterest()
{
    double monthlyInterest = savingsBalance * SavingsAccount::annualInterestRate / 12; 
    setSavingsBalance(savingsBalance + monthlyInterest);
    return monthlyInterest; 
}
void JWong::SavingsAccount::modifyInterestRate(double newInterestRate)
{
    SavingsAccount::annualInterestRate = newInterestRate;
}
double JWong::SavingsAccount::getAnnualInterestRest()
{
    return SavingsAccount::annualInterestRate;
}